WO2018204963A1 - Method for producing noble metal-modified silicon nanowires - Google Patents

Abstract

The invention relates to a method for producing noble metal-modified silicon nanowires, comprising the following steps: creating the silicon nanowires in a silicon wafer by means of silver-catalysed chemical wet etching; and applying one or more noble metals to the surface of the nanowires, characterised in that silicon nanowires coated with gold are produced by a) first creating the silicon nanowires in the silicon wafer by means of silver-catalysed chemical wet etching with an etching solution, wherein the concentration and the dwell time of the etching solution are coordinated with each other such that nanowires with a length < 15 μm are formed; and b) then coating the created nanowires with gold as the noble metal by sputtering, wherein the duration of the sputter coating is selected such that a gold coat with a thickness between 20 and 130 nm is formed.

Description

 Process for producing noble metal-modified silicon nanowires

The invention relates to a method for producing noble metal-modified silicon nanowires as SERS substrates.

STATE OF THE ART

 Nanowires or nanowires (NWs) have for some years been common substrates for surface-enhanced Raman scattering (or "spectroscopy", in short: SERS), for which purpose they are surface-treated with silver or gold. In SERS, the Raman signal of molecules adsorbed to the noble metal surface is several orders of magnitude greater than that of conventional Raman spectroscopy, although the exact mechanism for this phenomenon is still unclear. Largely accepted, however, is the explanation that the laser beam induces an electromagnetic field on the surface of the precious metal. Areas with particularly strong electromagnetic fields, called "hotspots", are located between two neighboring noble metal nanostructures that are close enough to each other. When an analyte molecule enters such a hotspot, its Raman signal is significantly enhanced. Various methods are known for making such silver or gold-plated NWs. For example, Jiwei et al., Nanoscale Res. Lett. 8, 495 (2013), the preparation of nanowire networks of sheet-shaped NW clusters of anodized alumina (AAO) by treating a porous aluminum foil with a mixture of phosphoric acid and chromic acid. Then a 50 nm thick gold layer was deposited.

Chen et al., Nanotechnology 19, 275712 (2008) disclose indium phosphide NWs provided with gold nanoparticles (Au-NPs) prepared by metalorganic vapor phase epitaxy about 2 pm in length and up to 50 nm in diameter and between individual NWs of about 120 nm and deposition of gold particles about 50 nm in diameter from an aerosol phase can be produced. Recently, silicon NWs have become increasingly important as SERS substrates. For example, Convertino et al., Scientific Reports 6, Article 25099 (2016), disclose SiNWs prepared by plasma enhanced chemical vapor deposition (PECVD) to a length of 2 to 3 pm and diameters of 40 to 70 were grown on the then a gold layer of about 150 nm thickness was evaporated. Due to the production by means of vapor deposition, however, the SiNWs are highly disordered.

Yang et al., Dalton Trans. 42, 14324 (2013) disclose, in addition to the general mention of electron beam lithography, photolithography and oxide-assisted growth, in particular a silver-catalyzed chemical etching process ("Ag-assisted chemical etching"). ) for the production of SiNW arrays, followed by the formation of gold nanoparticles on the Si surface by means of galvanic displacement. SiNWs with diameters between 80 and 200 nm and a length of more than 20 m will be observed and gold particles with diameters of up to 80 nm.

Despite sometimes quite good performance of the Si-NWs thus produced, the disadvantages of these and generally all known embodiments of gold-modified SiNWs lie in the uniformity of the structure and in the reproducibility of the respective production process, which is why no Au-modified Si-NWs have been included consistently high sensitivity can be produced.

US 2014/030873 A1 discloses a method for producing structured Si NWs in which the pattern is produced by wet chemical etching. At first, the silicon surface is partially covered with a protective layer, for example an oxide layer, and the uncovered parts are subsequently etched, for example using aqueous KOH as the etching solution. Additionally or alternatively, selected areas of the Si surface may be provided with a thin metal layer of Au, Ag, or Pt as a catalyst for subsequent noble metal-catalyzed wet chemical etching, eg, with an aqueous solution of HF and H2O2 as an etch solution to effect the etch process therein Catalyze areas. Preferably, Ag used as a catalyst, since it is most easily removable with HNO3 again. The noble metal layer may be deposited, for example, by electron beam evaporation, physical vapor deposition, chemical vapor deposition or sputtering, and the metal layer is preferably between 5 and 50 nm, in particular about 20 nm, thick. The structured Si NWs produced in this way are used, inter alia, for solar cells.

CN 105039942 A discloses a process for preparing "cactus" (or "Christmas tree") shaped dendrites from Ag-coated silicon by silver-catalyzed wet chemical etching of a Si substrate with HF in the presence of AgNÜ3 to simultaneously form dendritic branches coat them with Ag. On the Ag surface, a 5 to 10 nm thick Au layer is deposited by sputtering, after which the whole is first annealed in a CVD chamber to produce an alloy of the metals as well as Ag droplets which serve as a catalyst for the following CVD process with SiH ₄ gas (with H2 as a carrier gas), which are formed on the structure Si tips.

Against this background, the object of the invention was the development of an improved, in particular more reproducible, method for producing more uniformly high sensitivity silicon nanowires for use as SERS substrates, in particular as substrates for SERS imaging.

DISCLOSURE OF THE INVENTION

 The invention achieves the above object in a first aspect by providing a method for producing noble metal-modified silicon nanowires comprising a step of producing the silicon nanowires in a silicon wafer by silver-catalyzed wet chemical etching and a step of depositing one or more noble metals on the surface Nanowires, with the characteristic that gold-coated silicon nanowires are manufactured by

a) first the silicon nanowires are produced in the silicon wafer by silver-catalyzed wet chemical etching with an etching solution, wherein the concentration and the duration of exposure of the etching solution are coordinated so that nanowires are formed with a length of <15 pm; and

 b) then the obtained nanowires are coated with gold as precious metal by sputtering, wherein the duration of the sputter coating is selected so that a gold layer is formed with a thickness between 20 and 130 nm.

The inventors have gained in the course of their research several findings. On the one hand that gold is preferable to silver or a mixture of gold and silver, but especially in the known silver-catalyzed wet chemical etching process, preferably using an aqueous solution of HF and H2O2 as the etching solution, the length of the generated Si -NWs can be easily controlled reproducible by either the exposure time and / or the concentration of the etching solution is / are varied. Preferably, the exposure time is kept constant and the concentration of the etching solution varies, as this allows a finer variation of the etching conditions and thereby ensures better reproducibility of the process.

Furthermore, it has surprisingly proven to be advantageous to produce in step a) silicon nanowires with a sometimes significantly shorter length than in the prior art, namely NWs with a length <15 pm, preferably <10 pm, more preferably <5 pm, more preferably between 0.8 and 2 pm, in particular between 1 and 1, 5 pm, as the later embodiments prove. This is so surprising because the skilled artisan would expect that longer NWs, thus having a larger surface area for the noble metal coating, would give better results.

Furthermore, it has surprisingly been found advantageous to apply to the nanowires in step b) a relatively thin gold layer, namely a gold layer with a thickness between 20 and 130 nm, preferably between 25 and 100 nm, more preferably between 30 and 80 nm, even more preferred sputtered between 40 and 60 nm, in particular with a thickness of about 50 nm. Also in this case, the skilled person would have expected that a thicker gold layer would provide better results, as by a thicker gold layer, a more uniform texture and a larger amount of noble metal can be provided for the formation of the electromagnetic field.

Finally, according to the present invention, sputtering, or sputtering, is preferred as a method for depositing the gold layer over alternatives such as electron beam evaporation, physical vapor deposition, or chemical vapor deposition, because in this way the thickness of the gold layer is more controllable. The Si-NWs produced by the process according to the invention with the above-defined lengths and thicknesses of the gold layer are characterized by a high uniformity of the surfaces and by a particularly high sensitivity, i. high activity in the SERS process, as demonstrated by the later examples. In a second aspect, the present invention provides gold-coated silicon nanowires obtained by the method according to the first aspect of the invention, particularly silicon nanowires coated with a gold layer, in particular those having a length <15 pm and a gold layer having a thickness between 20 and 120 nm, were previously unknown.

And in a third aspect, the invention provides the use of such gold-coated silicon nanowires as SERS substrates, as the Si-NWs of the present invention, by virtue of their characterization of the present invention

The present invention will now be further described by way of various non-limiting embodiments and comparative examples, and with reference to the sole drawing, Fig. 1, which shows scanning electron micrographs of silicon nanowires of different lengths. EXAMPLES

 Commercially available silicon wafers were purchased from MEMC (1 -0-0, p-type, B-doped, 10-20 .mu.cm, 500-550 .mu.m thick, double-sided polished) and all reagents purchased from Sigma Aldrich and used without further purification.

The silicon wafers were cut with a diamond tip along crystallographic axes to obtain rectangular pieces each 25 x 18 mm in size. These were cleaned by sonicating in toluene for 10 minutes and then wiped with a cloth soaked in toluene, rinsed with about 1 ml of toluene and dried in an argon stream. Thereafter, the specimens were further cleaned in a UV / ozone chamber on both sides for 10 minutes each.

Examples 1 to 8 and Comparative Examples 1 to 13 Step a): Silver-Catalysed Wet Chemical Etching

 The specimens were subsequently treated in 20 ml of aqueous HF (6.2 M) and AgN3 (5 mM) aqueous solution for exactly 1 minute to deposit silver as a catalyst, rinsed with water, and then treated with water for exactly 10 minutes aqueous solution of HF and H2O2 etched with varying concentrations. The samples were then rinsed again with water and then treated with a 1: 1 mixture of HNO 3 (65% by weight) and water for 5 minutes to remove the silver, rinsed again and finally with a 4: 1 mixture of H2SO4 (96 wt .-%) and H2O2 (36 wt .-%) treated to form an oxide layer. The samples were then placed in water for 5 minutes and then in acetone and finally dried in an argon stream.

Table 1 below shows the concentrations of the etching solution and the resulting (rounded) lengths of the Si-NWs produced in the silicon wafers. Table 1

FIG. 1 shows SEM images of the Si NWs obtained in step a) of the process according to the invention, in each case in side view and in plan view.

Step b): Sputter coating

Coating of the Si NWs was performed on a BAL-TEC MED020 high vacuum sputter coating system. The etched silicon wafer as described above were 10 ^"4 mbar evacuated. Subsequently, argon was metered to an argon plasma atmosphere with a pressure of 2 x ^10" minutes to a final pressure of 7 x 10 to produce ² mbar. Gold or gold and silver were deposited at a plasma current of 60 mA, giving an average deposition rate of 0.6 nm / s. The coating time was subsequently varied to apply gold and gold and silver layers of different thickness, respectively. The obtained layer thicknesses were measured with a quartz crystal.

Table 2 below shows the lengths of the Si NWs, the layer thicknesses of the noble metal coatings and the precious metal used in each case.

Table 2

The precious metal-coated Si-NWs reported in Table 1 and obtained in the above procedure were used as active surfaces in a SERS experiment. For this purpose, the respective Au-coated Si-NWs were placed in a 0.05 M solution of thiophenol in ethanol for 24 h, then rinsed thoroughly with pure ethanol for 2 min and blown dry in an argon stream. For excitation was a laser beam with focussed at a frequency of 633 nm on each 25 measurement points of the surface of each Si NWs in a 5x5 grid at a distance of 30 pm as potential hotspots and recorded the respective Raman spectrum. Each measurement point at which an amplification of the Raman signal was detected by at least two powers of ten was rated as the actual hotspot.

The experiments were repeated three times with all coated Si NWs, and the results were averaged. Nanowires, where on average at least 1 out of 25 measurement points (ie 4% of the measurement points) proved to be a hotspot, were in principle suitable as SERS substrates and were therefore considered to be a positive result. Those in which at least 3 out of 25 measurement points (i.e., 12% of the measurement points) have proven to be hotspots on average are considered to be preferred embodiments. It should be noted, however, that it is not excluded that certain combinations of nanowire length and coating thickness, which had previously not cut off positive, may prove suitable in future experiments, since the correct selection of the measurement points - despite the mentioned multiple repetition of the individual experiments - strongly depends on chance, especially since the irradiated area of 150 x 150 pm represents only a tiny fraction of the total area of the wafers used (25 x 18 mm).

Table 3, overleaf, gives the results to date for all NWs listed in Table 2.

Table 3

The table shows that the best results were achieved with Au-coated Si-NWs with lengths of 0.45 prn and 1.25 prn with layer thicknesses above 20 nm and below 70 nm, whereas all combinations of Ag and Au as have proved ineffective.

From a comparison between Comparative Example 1 and Examples 1 and 2, it can be concluded that with the short NW length of only 0.45 prn and the small layer thickness of only 27 nm in Comparative Example 1, the amount of gold deposited thereon is insufficient was to generate effective hotspots, while in Example 1 also at 0.45 prn NW length, but the double layer thickness of 54 nm with 36% hotspots the best result of all attempts was achieved. In examples 3 and 4, with a NW length of 1.25 μm and only slightly varying Au layer thicknesses of 25 nm and 27 nm, on the one hand good 20% and on the other hand only moderate 12% hotspots were detected, which was the mentioned random dependence of the method - de underlines.

The latter also follows from a comparison of Examples 5 and 6 with an NW length of 1.25 pm and nearly identical layer thicknesses of 49 nm and 50 nm, respectively: In one case, 20%, i. 5 out of 25 hotspots, in the other case, on average, even 30%, i. 7.5 out of 25 hotspots detected.

At the shortest NW length of only 0.45 m tested, at the high layer thickness of 74 nm in Example 2, also 12% hotspots were also detected, while in Comparative Examples 8 and 9 with an NW length of 1.25 pm and Au Layer thicknesses of 75 nm and 76 nm (mean) not a single hot spot occurred.

It is also noticeable that the best results were achieved with layer thicknesses around 50 nm. Even with very large NW lengths of around 5 pm or around 1 1 pm, a hotspot was detected at a layer thickness of 51 nm.

Examples 9 to 19, Comparative Examples 14 to 19

In the above examples, since Si-NWs having lengths between 0.45 and 11, 07 nm and having a gold coating provided positive results, further uncoated Si-NWs obtained in the above step a) were again subjected to a sputter coating according to the above step b ) using gold as the noble metal, but more closely adhering to the respective coating time to reproducibly obtain gold-coated Si-NWs having gold layer thicknesses of about 10, 25, 50, 75, and 100 nm. As a result, the gold-coated Si NWs thus obtained were again used as active surfaces in a SERS experiment, but now with 5 repetitions for each sample, in which case the SERS activity therein was determined for each measurement point Percent and the standard deviation were measured as shown in Table 4 below, with SERS activities in the double-digit percentage range being considered acceptable, since they were average values from 5 measurements and sometimes individual values were quite better.

Table 4

From Table 4 it can be seen that at all tested lengths of Si-NWs from 0.45 to 11, 07 pm for layer thicknesses above 10 nm at least a positive result and at a layer thickness of 50 nm thickness for all lengths positive results were obtained , Given the large differences between 10 nm and 25 nm for the same length, it can be assumed that a gold layer thickness of less than about 20 nm will hardly give any positive results. From a comparison of the values at the apparently optimal Thickness of about 50 nm for the individual NW lengths can be concluded that the lower limit for the length of the Si NWs is likely to be about 0.25 to 0.3 pm. On the other hand, the upper limit of the length seems to be about 15 pm, possibly as low as about 12 pm, and longer lengths will probably no longer produce any positive results. Even lengths over 10 pm are clearly no longer to be preferred. Since the values at 50 nm layer thickness are comparatively good for 0.45 and 5, 13 pm length, and make up about half of the top value for 1.25 pm length, a particularly preferred length range is between 0.4 and 4 μm, in the range between 0.8 and 2 pm even better results are to be expected and the range of 1 to 1.5 pm is the most preferable.

Considering the results for 1, 25 pm length, where good results were obtained even with a layer thickness of 100 nm, it may be assumed that the upper limit for the thickness of the gold layer should be about 130 to 140 nm and a range between 25 and 100 nm is preferable. From a comparison of the values for the different lengths, especially between 1.25 pm and 5.13 pm, in which it is noticeable that the difference between 25 nm and 75 nm is regularly considerable, a preferred range between 30 and 80 nm results and particularly preferred is the range between 40 and 60 nm for the thickness of the gold layer. The optimum should be between about 50 nm and about 55 nm.

In summary, it is expected that other Au-coated Si-NWs made in accordance with the present invention will be able to produce comparably good results - especially when on the one hand NW lengths between about 0.4 and 4 pm, more preferably between 0.8 pm and 2 pm, in particular between about 1 and 1.5 pm, and / or Au layer thicknesses between about 30 nm and about 80 nm, preferably between about 40 nm and about 60 nm and in particular in the range of about 50 nm to about 60 nm, eg about 50 nm or about 55 nm. All such embodiments will be very well suited as SERS substrates, especially for SERS imaging.

Claims

1 . A method of making noble metal-modified silicon nanowires, comprising a step of producing the silicon nanowires in a silicon wafer by silver-catalyzed wet chemical etching, and a step of applying one or more noble metals to the surface of the nanowires, characterized in that gold-coated silicon nanowires are produced by  a) the Siiicium nanowires are initially produced in the Siiiciumwafer by silver-catalyzed wet chemical etching with an etching solution, the concentration and the duration of the etching solution are coordinated so that nanowires are formed with a length <15 pm; and  b) then the obtained nanowires are coated with gold as precious metal by sputtering, wherein the duration of the sputter coating is selected so that a gold layer is formed with a thickness between 20 and 130 nm. 2. The method according to claim 1, characterized in that step a) is carried out using an aqueous solution of HF and H2O2 as an etching solution. 3. The method according to claim 1 or 2, characterized in that in step a), the exposure time is kept constant and the concentration of the etching solution is varied in order to control the length of the nanowires. 4. The method according to any one of claims 1 to 3, characterized in that in step a) Siiicium nanowires are produced with a length of <10 pm or <5 pm. 5. The method according to claim 4, characterized in that Siiicium nanowires are produced with a length between 0.8 and 2 pm. 6. The method according to claim 5, characterized in that Siiicium nanowires are produced with a length between 1 and 1, 5 pm. 7. The method according to any one of claims 1 to 6, characterized in that on the nanowires in step b) a gold layer is sputtered with a thickness between 25 and 100 nm or between 30 and 80 nm. 8. The method according to claim 7, characterized in that a gold layer is sputtered with a thickness between 40 and 60 nm. 9. The method according to claim 8, characterized in that a gold layer is sputtered with a thickness of about 50 nm. 10. Gold-coated silicon nanowires obtained by a method according to one of claims 1 to 9, which have a length <15 pm and a gold layer with a thickness between 20 and 120 nm. 1 1. Gold-coated silicon nanowires according to claim 10, characterized in that they have a length <10 pm or <5 pm. 12. Gold-coated silicon nanowires according to claim 1, characterized in that they have a length of between 0.8 and 2 μm or between 1 and 1.5 μm. 13. Gold-coated silicon nanowires according to one of claims 10 to 12, characterized in that they have a gold layer with a thickness between 25 and 100 nm or between 30 and 80 nm. 14. Gold-coated silicon nanowires according to claim 13, characterized in that they have a gold layer with a thickness of 40 and 60 nm or of about 50 nm. 15. Use of gold-coated silicon nanowires according to one of claims 10 to 14 as SERS substrates.