SILICON NANOPARTICLE FORMATION BY ELECTRODEPOSITION FROM SILICATE

TECHNICAL FIELD

The present invention generally concerns the formation of elemental silicon nanoparticles.

BACKGROUND ART

Silicon nanoparticles have properties unlike bulk silicon. Among many interesting applications and developing applications are those applications that leverage the fluorescent nature of silicon nanoparticles. The silicon nanopafticle material forms the basis, for example, for emitters, sensors, or filters that are efficient and compatible with the existing silicon based integrated circuit technology.

A number of methods have been developed for the production of silicon nanoparticles that fluoresce. Prior methods for the production of silicon nanoparticles include a variety of physical, electrochemical and chemical techniques. The methods generally produce distributions of nanoparticles.

Many techniques have practical limitations, such as the quantities of silicon nanoparticle material that may be produced in a reasonable amount of time.

Many techniques are difficult to implement, and would scale poorly if used for a manufacturing scale synthesis of silicon nanoparticles. Some techniques produce silicon nanoparticles in a form that is difficult to disperse or collect.

Example prior methods include the following. Silicon nanoparticle clusters (Si-nc) have been formed, for example, in the matrices of glass and

SiO ₂ by implanting high energy Si ions into quartz, followed by annealing at elevated temperatures. Silicon wafers have also been dispersed by ablation using a variety of agents, such as lasers, to produce isolated Si particles. Collection or dispersion in the latter example requires that nanoparticles be transported downstream from the spot of ablation by an inert gas jet, to be collected by filters.

There are also gas-phase formation techniques. One type of formation method obtains fluorescent silicon material from silanes via slow combustion, thermal decomposition, microwave plasma, gas evaporation or chemical vapor deposition (CVD). This class of methods may involve particle formation in a discharge of gas mixtures that include the highly toxic silane (SiH ₄ ), followed by collection in filters, and recovery from filters.

There are also liquid phase formation techniques. An example is chemical synthesis via a reduction of anhydrous silicon halogen ionic salts (SiCl ₄ or SiBr ₄ ) dispersed in water- free reverse-micelles solutions, with LiAlH ₄ . Silicon nanoparticle clusters may also be produced by reduction of SiCl ₄ and RSiCl ₃ (R = H, C ₈ Hi ₇ ) in the presence of sodium (Na) metal according to: SiCl ₄ + RSiCl ₃ + Na → Si-nc (diamond lattice) + NaCl. Transformation of the alkali silicon salts ASi (A = Na, K), via interaction with SiCl ₄ results in formation of Cl-capped Si nanoparticle clusters, which may be followed by replacing the Cl by methyl groups. Tetrahedral shaped silicon nanoparticle clusters may also be obtained by a reduction of SiCl ₄ with Na naphtalenide followed by termination with butyl lithium. In another liquid phase formation technique octyloxi- terminated Si nanocrystals were produced in supercritical fluid. By thermally degrading the Si precursor, diphenyl silane (SiH ₂ Ph ₂ ) in the presence of octanol, relatively size-monodisperse sterically stabilized Si nanocrystals ranging from 1.5 to 4.0 nm in diameter were obtained in significant quantities. [See, J. D. Holmes, K. J. Ziegler, R. C. Doty, L. E. Pell, K. P. Johnston, B. A. Korgel, J. Am. Chem. Soc. 123, 3743 (2001)]. Work by one of the present inventors, Dr. Munir Nayfeh, and others has resulted in silicon nanoparticle formation methods that are capable of

producing quantities of silicon nanoparticles, including quantities of highly uniform lnm silicon nanoparticles and also particles from a family of discrete sizes. See, Nayfeh et al U.S. Patent No. 6,585,947 entitled Method for Producing Silicon Nanoparticles (July 1, 2003), and Nayfeh et al U.S. Patent No. 6,743,406 entitled Family of Discretely Sized Silicon Nanoparticles and Method for Producing the Same. (June 1 , 2004). Those techniques involve the electrochemical etch of a silicon anode as it is gradually advanced into an etchant solution.

DISCLOSURE OF INVENTION The invention provides a method for the formation of silicon nanoparticles, in sizes that fluoresce, by electrodeposition of silicon material onto a non-reactive (with HF) metal (e.g., platinum) surface from a solution of silicate and HF or HF/H ₂ O ₂ . In an embodiment of the invention, a positively biased substrate with a platinum surface is immersed in a solution of sodium metasilicate in HF/H ₂ O ₂ , a current is drawn and a coating of silicon nanoparticles is formed on the platinum surface.

BEST MODE OF CARRYING OUT THE INVENTION In the invention, silicon nanoparticles are formed on a non-reactive (with HF) metal (e.g., platinum) surface from a solution of silicate and HF or HFfH ₂ O ₂ by electrodeposition. A positive bias is applied to the platinum surface while a counter electrode is negatively biased to draw a current and the nanoparticles deposit from the silicate onto the platinum surface. The deposited nanoparticle material is fluorescent, and includes a distribution of nanoparticle sizes. A standard electrodeposition cell may be used to conduct the method of the invention.

Metals that are non-reactive with HF may be used to provide a metal surface for the deposition. A platinum surface in embodiments of the invention is preferably a thin platinum layer formed on a substrate, such as a semiconductor or insulator substrate. A platinum substrate may also be used,

but silicon nanoparticle formation proceeds more efficiently when a thin platinum layer is used. Various silicate sources may be used. A preferred example is sodium metasilicate, known also as water glass or soluble glass.

Electrodeposition solutions of either HF or HF/H ₂ O ₂ may be used, while the HF/H ₂ O ₂ solution is preferred as silicon nanoparticles deposit with more efficiency, as evidenced by stronger fluorescence responses from example silicon nanoparticle depositions produced by electrodeposition from HF/H ₂ O ₂ as compared to weaker responses from example silicon nanoparticle depositions in HF solutions. Silicon nanoparticle formation has been verified experimentally. The example experimental results will now be discussed, and artisans will appreciate various additional inventive features from the discussion while appreciating broader aspects of the invention as well. In example experiments, a lmg/liter commercial metasilicate water solution (Na ₂ SiO ₃ .5H ₂ O ₂ ) in an HF:H ₂ O ₂ mixture was used as an electrodeposition solution. The silicate had 0.02 percent of pentachlorophenol (C ₆ Cl ₅ OH) as a preservative. The substrate in experiments was either a platinum coated material or a platinum plate.

A standard electrodeposition cell configuration was used to conduct the experiment. The cell itself must be a material that is resistant to HF/H ₂ 0 ₂ The electrodeposition cell in the experiments was Teflon beaker. The substrate including a non-reactive (with HF) metal (e.g., platinum) surface on which deposition is to be conducted is immersed vertically in the etchant, to contact the etchant with the non reactive metal surface. The substrate can be left stationary, or can be moved downward into the bath as the process proceeds. Countering this substrate is an electrode, e.g., a pure platinum wire, mesh, or foil. The platinum electrode is negatively biased, while the substrate is positively biased.

Many other silicate solutions may be used. Sodium silicate, also called water glass or soluble glass, is any one of several compounds containing sodium oxide, Na ₂ O, and silica, Si ₂ O, or a mixture of sodium silicates with varying ratios of SiO ₂ to Na ₂ O, solid contents, and viscosity. These include

Na ₄ SiO ₄ ; Na ₂ SiO ₃ ; Na ₂ Si ₂ Os; Na ₂ Si ₄ Og. All these compounds are colorless, transparent, glasslike substance available commercially as a powder or as a transparent, viscous solution in water. They are produced chiefly by fusing sand and sodium carbonate in various proportions. Sodium metasilicate is widely available as it is used in many applications. For example, it is used as a raw material for making silica gel, as a basic material for the detergent industry and as cement for glass, pottery, and stoneware. Granular sand ingredients may also be used to form silicate solutions.

Preferred embodiments use a semiconductor or insulator substrate with a thin platinum layer. A silicon substrate may be used, for example. In experiments, a platinum layer was formed on a silicon substrate, particularly a Si wafer of 10 ω-cm resistivity. Any technique that permits formation of a platinum layer on a substrate may be used. The example experimental technique used a seed layer formation technique known in the art. The silicon substrate was first coated with a thin platinum layer using a two-step process. The first coating is an electrode less seed process and the second is using an electrode configuration. In this first treatment, the silicon substrate was sonicated in methanol, dipped in diluted HF and rinsed in deionized water. A section of it was then dipped in chloroplatinic acid/hydrogen fluoride (H ₂ PtCl ₆ /HF) solution for fifteen minutes to form a thin platinum seed layer according to PtCl ₆ ^2" (aq)+ Si°(s) + 6F ^" (aq) → Pt°(s) + SiF ₆ ^2" (aq) + 6Cl. ^" (aq). This process may also lead to direct chemical reaction between Pt and Si to form platinum suicide. FTIR measurement of the substrate at this stage (after the seed step) shows Si-H with some residual Si-OH passivation, typical of an HF treatment. The sample is not fluorescent to the naked eye under UV irradiation. However, under a fluorescence microscope, the sample shows extremely weak spotty fluorescence. Once the first seed layer/coating is formed, a second platinum coating is placed over the first platinum coating by electrodeposition. In the example experimental technique, a five minute platinum electrodepositing process in chloroplatinic acid with the substrate as the cathode was used. This produces a thicker platinum film that covers all

sides of the treated section of the wafer. The electroplated sample is then rinsed with deionized water and flushed with an inert gas. This completed the formation of a silicon wafer substrate with a platinum surface layer. The substrate is then rinsed in acetone. Testing was conducted to see if the platinum film would maintain its integrity in an etching system. A sample was immersed into an HFZH ₂ O ₂ solution to nearly the level of the platinum film, and biased positively with respect to an immersed counter platinum wire electrode. A current flow of ~ 10 mA was established. After processing for a period of one hour, the substrate was removed from the bath. The substrate was found to not to be fluorescent to the naked eye under UV irradiation. Also, under a fluorescence microscope, the substrate shows no fluorescence. This demonstrated that the platinum film has protected the underlying silicon wafer from HF attack and etching. If a substrate is immersed in the etching solution to a level above the platinum coating, fluorescence is established in the unprotected silicon wafer part, as nanoparticles are created on the unprotected part by the electrochemical etching process when the substrate is gradually advanced into the etching solution. See, i.e., Nayfeh et al U.S. Patent No. 6,585,947.

Electrodeposition of silicon nanoparticles onto the platinum coated silicon substrate was then conducted. The substrate was dipped into the silicate/acid solution (to nearly the level of the platinum coating — this is unnecessary — remove it) The wafer substrate was not moved during the electrodeposition process. The substrate was positively biased with respect to an immersed counter platinum wire electrode. An electrodepositing current flow in the current range 1-100 mA works. It is established by applying a positive bias (relative to the counter electrode) to the substrate. The process is not sensitive to the biasing voltage. Once established, deposition of silicon nanoparticles occurs on the platinum surface. The process is self-limiting. The current decreases with time as more and more nanomaterial forms on the immersed part of the platinum coated substrate.

Under irradiation from a 365 nm incoherent mercury lamp, red luminescence was observed from an electroplated wafer. Under similar irradiation from a 365 nm incoherent mercury lamp, no red luminescence was observed from substrate wafer that had been treated without the inclusion of the silicate in the process.

The luminescence spectrum of the silicon nanoparticle electroplated wafer consists of a red band rising at 550 nm and extending to 850 nm. In the measurement we used a fiber optic spectrometer that utilizes a UV-VIS holographic grating with groove density of 600/mm and a blaze wavelength of 0.4 μm for dispersion. The spectrometer uses optical fibers to transport the excitation and to extract the luminescence.

We next deposited silicon nanoparticles on a platinum substrate using the same electrodepositing technique described above. This deposition showed that the material concentrates at the sharp edges due to concentration of current. Moderately conducting substrates, such as a semiconductor or an insulator coated with a metal film (platinum) produce more uniform films. Other metals that are non-reactive with HF may also be used.

Patterning was demonstrated with the platinum substrate. Essentially, by masking the substrate, the electrodepositing of silicon nanoparticles from the silicon solution may be limited to non masked areas of the substrate. In the experiment, a platinum substrate was masked with a paraffin wax layer of 300 nm. Patterns were scraped to provide current paths that define the area of silicon nanoparticle formation. Imaging with a fluorescent microscope showed that the material selectively deposits in the pattern area. In addition to enabling deposition in a pattern, the definition of current paths also eases the sharp edges of particle deposition areas.

FTIR transmission spectroscopy in the range 500 cm ^"1 - 4000 cm ^"1 was conducted on the silicon nanoparticle coated Si wafer. It showed strong Si-H signals at 615 cm ^"1 to 670 cm ^"1 , at 903 cm- ^"1 to 910 cm ^"1 and at 2070 cm ^"1 to 2090 cm ^"1 , and a strong Si-OH peak at 3500 cm ^"1 . A Si-O contribution is observed at 1 100 cm ^"1 . The spectrum showed C-H vibration near ~ 2950.

Bands in the region 1250-1175 cm ^"1 are due to SiCH _n or Si-C vibration. Sharp peaks at 1416, and 1378 cm ^"1 may be associated with an Si-C bond, which may be due to residual carbonates or pentachlorophenol (C ₆ Cl ₅ OH). Sodium carbonate is fused with sand to produce sodium silicates. FTIR spectra of the Si control wafer sample showed weaker Si-H and

Si-OH peaks. Residual Si-H signals are due to the HF treatment during the electroless platinum coating process used in preparing the substrate which stains etch the surface. It also shows the absence of vibrations near 1416, 1378 cm ^'1 , and 1250-1175 cm ^"1 . XPS spectra taken of a processed luminescent sample show a Si state, confirming the presence of silicon material.

We believe the electrodeposition process of the invention involves deposition of Si atoms from silicates followed by nucleation into nanostructures. In the process, positive 2Na ⁺ ions proceed to the negatively biased platinum wire, whereas the negative selicic ions [(H ₂ SiO ₄ )^H ₂ O ₂ ] ^2" proceed to the platinum coated substrate surface. On the platinum surface, the negative ion neutralizes resulting in the deposition of Si atoms. With Si on the substrate, nucleation produces clusters. The wire counter electrode, on the other hand, showed little fluorescent material. Also, reversing the polarity of the substrate inhibited formation of fluorescent material. When we used HF/silicate solutions, namely without adding H ₂ O ₂ , the deposition proceeds but the resulting coating is not as bright. Finally the process does not proceed without HF or HF/H ₂ O ₂ , i.e., when a pure silicate solution is used.

We performed material depth profiling of atomic percentages to test the formation and deposition of silicon nanomaterial on the platinum coated silicon wafer substrate. In these measurements we used Auger electron spectroscopy. In some cases we find some platinum from the platinum coating appearing to begin at the top surface of the nanomaterial coating (zero depth), indicating that the Si coating is inhomogeneous and has gaps, and effectively creating nanomaterial mixed with platinum. With depth from the top surface of the nanomaterial, the platinum signal stays nearly flat at 35% but then suffers a sharp drop to 17% level at a depth of 130 run. The thickness of the Si deposit

is ~ 220 nm. Si increases from 30 % at top surface of the nanomaterial coating front to a steady level of ~ 48% at a depth of 150 nm, before starting to drop at a depth of 220 nm from the top surface of the nanomaterial. The oxygen percentage stays nearly flat at a level of 22%. The carbon contribution is larger on the surface and deep in the first platinum coating (~ 15%) than in the second platinum coating (—7%).

In quantum confinement-induced radiative recombination of photoexcited electron-hole pairs in nanostructures, the luminescence wavelength correlates with the size of the structure. We used a fiber optic sensor, which provides 1-2 mm spatial resolution. We examined the platinum coated substrate, which was kept stationery in the bath. We find variation in the luminescence wavelength across the film, pointing to a non-homogeneous cluster size. Optical spectra from a platinum coated substrate after electrodeposition from a region near the meniscus shows a band near 610 nm. This band has been correlated to the luminescence of dispersions of 2.85 nm silicon nanoparticles. Photoluminescence from near the bottom of the sample, i.e., the deepest point in the liquid shows a band near 750 nm. The region looks dark as 750 nm is outside the sensitivity range of the naked eye. The likely source for this band is clusters of ~ 3.6 nm across. We can also identify regions near the mid section of the sample, i.e., halfway from the meniscus, which indicate that both kinds of silicon nanoparticle clusters are present

While specific embodiments of the present invention have been shown and described, it should be understood that other modifications, substitutions and alternatives are apparent to one of ordinary skill in the art. Such modifications, substitutions and alternatives can be made without departing from the spirit and scope of the invention.