WO2008071504A2

WO2008071504A2 - Porous silicon

Info

Publication number: WO2008071504A2
Application number: PCT/EP2007/062032
Authority: WO
Inventors: Frank-Martin Petrat; Martin Stutzmann; Joachim Diener; Dmitri Kovalev; Martin S. Brandt
Original assignee: Evonik Degussa Gmbh; Technische Universität München
Priority date: 2006-12-15
Filing date: 2007-11-08
Publication date: 2008-06-19
Also published as: TW200842109A; DE102006059318A1; WO2008071504A3; TWI380951B

Abstract

Porous silicon composed of particles having an average diameter of 10 to 100 nm, the particles having pores throughout, and the particles in turn containing partially fused crystallites having an average diameter of less than 5 nm.

Description

Porous silicon

The invention relates to porous crystalline silicon and to a process for its preparation.

US 2004/0229447 discloses a two-stage process for preparing photoluminescent silicon, the process involving first preparing nanoparticulate silicon from a silicon starting material, SiH₄ for example, in the presence of a sheath gas and a photosensitizer, by means of the heat generated by a radiation source, preferably a laser, and in a second step etching the nanoparticulate silicon. The nanoparticulate silicon obtained in the first step has an average particle size of 5 to 20 nm. These are spherical unaggregated nano- particles. The subsequent etching operation is carried out preferably using a solution which comprises hydrofluoric acid and nitric acid. This operation produces a reduction in the particle size of the silicon used, so that in order to achieve a photoluminescent effect, which occurs at particles sizes of less than 5 nm, a major portion of the material used is fully dissolved in the etching solution.

US 2004/0166319 discloses silicon particles having a porous shell and a non-porous core. These particles are obtained by grinding a silicon body, a wafer for example, into silicon particles of approximately 100 μm, and etching these particles by means of hydro- fluoric acid in the presence of, for example, iron nitrate, to form porous silicon particles. In the dispersed state, these silicon particles are treated with ultrasound and subsequently separated from coarse particles. With this process, too, the yield of photo- luminescent silicon is low.

An object of the present invention was to provide a process for preparing porous silicon that avoids the disadvantages of the known processes. A particular aim of the process is to minimize the fraction of material used which is lost during an etching operation.

A further object of the invention was the provision of porous, easily handled silicon having photoluminescent properties .

In contrast to US 2004/0166319, the silicon particles produced ought preferably to be fully porosified silicon particles, without a crystalline core, and with as largely as possible a number of nanoparticles or aggregates and/or agglomerates of silicon particles.

The invention provides a porous silicon which is composed of particles having an average diameter of 10 to 100 nm, preferably 25 to 70 nm, the particles having pores throughout, and the particles in turn contain partially fused crystallites having an average diameter of less than 5 nm.

The fraction of the crystallites gives the silicon of the invention photoluminescent properties. The particles may in turn be composed of partially fused crystallites having an average diameter of less than 5 nm.

Pores and crystallites can be detected by means of techniques of electron microscopy (an example being high-resolution transmission electron microscopy (TEM) ) .

In one preferred form the porous silicon particles of the invention, together with non-porous silicon particles, form aggregates and/or agglomerates of silicon particles. Figure 1 shows a high-resolution TEM image which shows a section of an aggregate with the porous silicon particles of the invention (regions A) and non- porous silicon particles (regions B) .

The average aggregate diameter may preferably be less than 1 μm. Particular preference is given to a range from 100 to 500 nm. The average aggregate diameter can be determined by means, for example, of image analysis. In that case approximately 100 to 2000 aggregates are evaluated from TEM images. The evaluation can take place in accordance with ASTM 3849-89.

An aggregate is a three-dimensional structure composed of a plurality of firmly fused particles. These aggregates are difficult if not impossible to break up again using dispersing equipment. A number of aggregates and/or unaggregated particles may join loosely together to form agglomerates. That process can be reversed by appropriate dispersing.

The invention further provides a process for preparing porous silicon, in which first of all non-porous silicon is added with stirring to a solution comprising hydrofluoric acid, so removing any silicon oxide formed, after which dilute nitric acid is added in portions, the nitric acid being left to react in each case before any further addition, and the amount of nitric acid added in total is such that HNθ3/Si, in mol/gram atom, is less than 1, and thereafter the solid is separated from the liquid.

The gases formed during the reaction are allowed to escape from the reaction vessel, preferably during the reaction .

In one preferred embodiment 3 ≤ HF/Si ≤ 50, in mol/gram atom. Particular preference may be given to the range 5 < HF/ S i < 15 .

Further preferred is an embodiment in which 0.1 < HNOs/Si ≤ 0.8, in mol/gram atom. Particular preference may be given to the range 0.2 < HNOs/Si < 0.5.

Further preference is given to an embodiment in which the nitric acid is added in portions such that the ratio of added nitric acid to silicon used originally is 0.01 < HNO₃/Si < 0.1, in mol/gram atom.

In addition it is possible preferably to use a non- porous silicon having a BET surface area of 5 to 300 m²/g.

Particularly suitable is a non-porous silicon obtainable by a process in which a continuous stream 1 of an SiH₄/argon mixture and a continuous stream 2 of argon or an H₂/argon mixture are reacted in a hot- walled reactor at temperatures of up to 1000⁰C, the reaction mixture is cooled or left to cool, and the reaction product in the form of a powder is separated off from gaseous substances, the SiH₄/argon ratio in stream 1, in mol/mol, being 0.2 to 5, and the H₂/argon ratio in stream 2, in mol/mol, being 0 to 2.

In the process for preparing the non-porous silicon the SiH₄/ (sum of argon from stream 1+2) ratio, in mol/mol, can be chosen preferably from 0.1 to 0.5.

Furthermore, the H₂/ (sum of argon from stream 1+2) ratio, in mol/mol, can be chosen preferably from 0.5 to 0.8.

Additionally the SiH₄/H₂ ratio, in mol/mol, can be chosen from 0.1 to 0.3. Also suitable, additionally, is a non-porous silicon obtainable by a process in which at least one silane in gas or vapour form, an inert gas, hydrogen and oxygen or an oxygen-containing gas are transferred to a reactor and mixed therein, and a plasma is generated by energy input by means of electromagnetic radiation in the microwave range, under a pressure of 10 to 1100 mbar, preferably 100 to 300 bar, the reaction mixture is cooled or left to cool, and the reaction product in the form of a powder is separated off from gaseous substances.

In the process for preparing the non-porous silicon a fraction of the silane can be preferably 0.1% to 90% by weight and with particular preference 1% to 10% by weight, based on the sum of silane, inert gas, hydrogen and oxygen.

Furthermore, the fraction of oxygen can be preferably 0.01 to 25 mol%, based on the silane.

The power input of the microwave radiation is not limited. It is preferentially chosen such that the back-radiated, unabsorbed microwave power is minimal, and a stabile plasma is developed. Generally speaking, the energy input is situated between 100 W and 100 KW, and with particular preference between 500 W to 6 KW. The skilled worker will match the energy input to the reactor volume.

The particle size distribution can be varied by means of the irradiated microwave power. Accordingly, for given gas compositions and volume flows, higher microwave powers may lead to a smaller particle size and to a narrower particle size distribution.

The microwave range for the purposes of the invention is a range from 900 MHz to 2.5 GHz, particular preference being given to a frequency of 915 MHz.

The process for preparing the non-porous silicon can be carried out, furthermore, in such a way that the reaction mixture obtained after the microwave treatment is thermally aftertreated. Particularly advantageous for this purpose is a wall-heated hot-walled reactor.

Its dimensions should be such that the residence time in the hot-walled reactor is between 0.1 s and 2 s. The maximum temperature in the hot-walled reactor ought not to exceed 1000⁰C.

Besides the thermal aftertreatment of the reaction mixture, it is likewise possible to carry out thermal aftertreatment of the silicon itself.

The present invention incorporates in full the content of German Patent Application DE-A-10353996.

Examples

A. Preparation of non-porous silicon Example A-I:

Apparatus: The hot-walled reactor used is a tube having a length of 200 cm and a diameter of 6 cm. It is made of quartz glass or Si/SiC with a quartz glass inliner. The tube is heated externally by means of resistance heating, over a zone of 100 cm, to 1000⁰C.

Via a two-fluid nozzle, an SiH₄/argon mixture of 2000 seem silane (1 seem = 1 cm³/min at 0⁰C and 1013 mbar pressure) and 1000 seem argon, and also 5000 seem argon, are supplied from above to the hot-walled reactor. The pressure in the reactor is 1080 mbar. In a downstream filter unit the powderous product is separated from gaseous substances.

The powder obtained has a BET surface area of 10.5 m²/g.

B. Preparation of porous silicon Example B-I :

In an open propylene or Teflon container, 5 grams of the silicon particles from Example A-I are introduced into 50 ml of a solution of hydrofluoric acid in water (49% by weight HF) with stirring at a temperature of 20⁰C.

This removes the shell of silicon oxide from the silicon particles. As a result of the hydrogen termination of the silicon surface, the silicon particles are hydrophobic and remain in the form of a foam on the surface of the solution.

Subsequently 5 ml of 18% by weight of nitric acid in water are added to this mixture, the reaction vessel is sealed, and the vessel and contents are shaken for 10 s. As result of this operation the foam expands to approximately ten times its original volume, and the temperature rises. The reaction mixture is shaken further until the nitric acid has been consumed and the volume of the foam subsides again. The gas mixture produced during the reaction, which is composed of species including hydrogen and oxides of nitrogen, is taken off continuously via the lid of the reaction vessel .

Subsequently 5 x 2 ml portions of the 18% by weight nitric acid are added, the consumption of the nitric acid used being awaited before the next addition is made in each case. Thereafter the pressure in the reaction vessel remains constant, since no further gases are produced.

Towards the end of this process, under irradiation with UV light, it is possible to observe photoluminescence .

At this point the 18% by weight nitric acid is added in portions of 5 X 1 ml to the closed reaction vessel.

Following each consumption by reaction of the nitric acid added in portions, an increase is found in the quantum yield of the photoluminescence.

The reaction is at an end after the consumption by reaction of the nitric acid at the desired photoluminescence quantum yield.

The foam comprising the porous silicon particles is then removed mechanically from the solution and dried at 50°C under 10 mbar for 1 hour.

Figure 1 shows a high-resolution TEM image of the porous silicon of the invention from Example B-I. The image shows an inventive aggregate/agglomerate with the inventive porous silicon particles (regions A, without a macroscopic silicon core) and non-porous silicon particles (regions B) .

Figure 2 shows an IR spectrum in transmission of the non-porous silicon from Example B-I. The transmission (Y axis) is plotted against the wavenumber in cm^"1 (X axis) . In contrast to the "Original Particles" from US 2004/0229447, Fig. 7, the non-porous silicon particles of B-I exhibit hydrogen termination, which can be inferred from Figure 2 on the basis of characteristic absorption modes, around 2100 cm^"1 and 640 cm^"1, for example.

Figure 3 shows the photoluminescence of the inventive aggregates/agglomerates with the inventive porous silicon particles under resonant excitation (at 1.54 eV and a temperature of 5 K) . The plot is of the intensity of the photoluminescence in arbitrary units as a function of the detection energy in eV. The photoluminescence exhibits a fine structure which is typical of porous silicon.

Plot A in Figure 4 shows the photoluminescence of the inventive porous silicon under non-resonant excitation

(at 2.54 eV and a temperature of 50 K). The plot is of the intensity of the photoluminescence in arbitrary units as a function of the detection energy in eV. This photoluminescence of porous silicon is described in the literature: for example, in "Optical properties of silicon nanocrystals", D. Kovalev, H. Heckler,

G. Polisski, F. Koch, Phys . Stat. Solidi 215, 871-932

(1999) .

If gaseous oxygen is added, there is a reduction in the intensity of the photoluminescence. The spectrum observed accordingly is reproduced as plot B in Fig. 4 (the intensity is being multiplied by a factor of 10) . The inventive porous silicon generates singlet oxygen.

Claims

Claims :

1. Porous silicon characterized in that it is composed of particles having an average diameter of 10 to 100 nm, the particles having pores throughout, and the particles in turn contain partially fused crystallites having an average diameter of less than 5 nm.

2. Porous silicon according to Claim 1, characterized in that the average particle diameter is 25 to 70 nm.

3. Porous silicon according to Claim 1 or 2, charac- terized in that together with non-porous silicon particles it forms aggregates and/or agglomerates of silicon particles.

4. Porous silicon according to Claim 3, characterized in that the average aggregate diameter is less than 1 μm.

5. Process for preparing porous silicon, characterized in that first of all non-porous silicon particles are added with stirring to a solution comprising hydrofluoric acid, then any silicon oxide formed is removed, after which dilute nitric acid is added in portions, the nitric acid being left to react in each case before any further addition, and the amount of nitric acid added in total is such that HNθ3/Si, in mol/gram atom, is less than 1, and thereafter the solid is separated from the liquid.

6. Process according to Claim 5, characterized in that 3 < HF/Si < 50, in mol/gram atom.

7. Process according to Claim 5 or 6, characterized in that 0.1 < HNO₃/Si < 0.8, in mol/gram atom.

8. Process according to Claims 5 to 7, characterized in that the nitric acid is added in portions such that the ratio of added nitric acid to silicon used originally is 0.01 < HNO₃/Si < 0.1, in mol/gram atom.

9. Process according to Claims 5 to 8, characterized in that the non-porous silicon particles have a BET surface area of 5 to 300 m²/g.

10. Process according to Claims 5 to 9, characterized in that the non-porous silicon particles are obtainable by reacting a continuous stream 1 of an SiH₄/argon mixture and a continuous stream 2 of argon or an H₂/argon mixture in a hot-walled reactor at temperatures of up to 1000⁰C, cooling the reaction mixture or leaving it to cool, and separating off the reaction product in the form of a powder from gaseous substances, the SiH₄/argon ratio in stream 1, in mol/mol, being 0.2 to 5, and the H₂/argon ratio in stream 2, in mol/mol, being 0 to 2.

11. Process according to Claim 10, characterized in that the SiH₄/ (sum of argon from stream 1+2) ratio, in mol/mol, is 0.1 to 0.5.

12. Process according to Claim 10 or 11, characterized in that the H₂/ (sum of argon from stream 1+2) ratio, in mol/mol, is 0.5 to 0.8.

13. Process according to Claims 10 to 12, characterized in that the SiH₄/H2 ratio, in mol/mol, is 0.1 to 0.3.

14. Process according to Claims 5 to 9, characterized in that the non-porous silicon particles are obtainable by continuously transferring at least one silane in gas or vapour form, an inert gas, hydrogen and oxygen or an oxygen-containing gas to a reactor and mixing them therein, and generating a plasma by energy input by means of electromagnetic radiation in the microwave range, under a pressure of 10 to 1100 mbar, cooling the reaction mixture or leaving it to cool, and separating off the reaction product in the form of a powder from gaseous substances.

15. Process according to Claim 15, characterized in that the fraction of the silane is 0.1% to 90% by weight, based on the sum of silane, inert gas, hydrogen and oxygen.

16. Process according to Claim 14 or 15, characterized in that the fraction of oxygen is 0.01 to 25 mol%, based on the silane.