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Jul 1, 2016 - Université Grenoble Alpes, F-38000 Grenoble, France. §. CEA, LETI ..... This work was partially funded by the French Région Rhône-. Alpe...
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Impact of surface chemistry on copper deposition in mesoporous silicon Walid Darwich, anthony garron, Piotr Bockowski, Catherine C Santini, Frédéric Gaillard, and Paul-Henri Haumesser Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b00650 • Publication Date (Web): 01 Jul 2016 Downloaded from http://pubs.acs.org on July 5, 2016

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Impact of surface chemistry on copper deposition in mesoporous silicon

Walid Darwich,



Anthony Garron,

Gaillard,





Piotr Bockowski,



Catherine Santini,

and Paul-Henri Haumesser



Frédéric

∗, ‡

CNRS-UMR 5265, 43 Bd du 11 Novembre 1918, 69616, Villeurbanne Cedex, France., and Univ. Grenoble Alpes, F-38000 Grenoble, France ; CEA, LETI, MINATEC Campus, F-38054 Grenoble France. E-mail: [email protected]

Phone: +33 4 38 78 57 59. Fax: +33 4 38 78 30 34

Abstract An easy, ecient and safe process is developed to metallize mesoporous silicon (PSi) with Cu from the decomposition of a solution of Mesitylcopper (CuMes) in an imidazolium-based ionic liquid (IL), [C1 C4 Im][NTf2 ]. The impregnation of a solution of CuMes in IL aords the deposition of metallic islands not only on the surface, but also deep within the pores of a mesoporous Si layer with small pores below 10 nm. Therefore, this process is well suited to eciently and completely metallize PSi layers. An in-depth mechanistic study shows that metal deposition is due to the reduction of CuMes by surface silane groups rather than by Si oxidation as observed in aqueous or water-containing media. This could open a new route to the chemical metallization of PSi by less noble metals dicult to attain by conventional displacement reaction. ∗

To whom correspondence should be addressed



CNRS



CEA LETI MINATEC

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Introduction Porous silicon has exceptional characteristics for microelectronics, integrated optoelectronics, 1 microelectromechanical systems (MEMS), 2 layer transfer technology, solar and fuel cells, biomedicine, etc. 3,4 Its properties can be modied by introducing dierent materials into its pores 5 to create specic composite structures with new electrical, optical or magnetic properties. 6 In particular, the introduction of metals within the porous structure has been proposed. This metallization has been carried out by electrochemical or electroless methods, by evaporation or sputtering deposition of Cu atoms, 6 and by supercritical uid processing. Generally, dry processes lead to an early blocking of the pores by accumulation of the metal near the surface of the porous layer. By contrast, uid carriers capable of wetting the pores allow for a deeper penetration of metal atoms into the porous layer and more uniform metallization. 7 In general, impregnation by aqueous solutions is only ecient at metallizing the surface, but not the internal structure of this hydrophobic material. 811 However, aqueous solutions with suitable wetting agents have been successfully used to thoroughly metallize mesoporous Si layers either by electrolytic or chemical deposition. 1214 In some instances, this reaction needs to be assisted by sonication. 15 This technique is attractive for practical purposes, as it proceeds by simple immersion of the sample in the metal salt solution. However, in the presence of water, metal deposition is accompanied by Si oxidation (displacement reaction). 9,10,16,17 For this reason, this technique is limited to noble enough metals such as Pt, Au or Ag. 1214,18 The chemical deposition of Cu is possible as well, provided that its Nernst potential is not shifted too much by complexation equilibria. 16 It can also be photoassisted. 17 To extend this technique to less noble metals, it is thus necessary to perform the reaction in non-aqueous media, in the strict absence of water traces. For this purpose, organic solvents such as methanol or acetonitrile have been used with glove box or similar techniques. 16 Such techniques are also needed to prevent oxidation of the deposited metal after the reaction. 2

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Ionic liquids (ILs) are an interesting alternative to conventional solvents. They are inert, non-volatile, thermally and chemically stable, thus safer to handle. They also have good wetting properties. Furthermore, they exhibit quite large electrochemical windows compatible with the deposition of non-noble metals. Indeed, they have been used to synthesize various metals through chemical processes involving the reduction/decomposition of organometallic precursors under H 2 . Interestingly enough, the homogeneous nucleation of the metal generally aords mono-disperse metallic nanoparticles (NPs) below 5 nm with dened morphology in the absence of any stabilizer. 19,20 These suspensions are very stable, even if the NPs are not coordinated by ligands (their surface is accessible for, e.g. catalytic reactions). 21 These suspensions can be used in several technological applications. 22 Moreover, the relatively high thermal stability and low volatility of ILs are compatible with annealing conditions needed to achieve for instance sintering of NPs and lm formation. The aim of this work is to take advantage of these unique properties of ILs to metallize mesoporous Si (PSi) with Cu. Indeed, we have demonstrated in previous studies that the decomposition of a solution of Mesitylcopper (CuMes) in 1-butyl-3-methylimidazolium bis(triuoromethylsulphonyl)imide, [C 1 C4 Im][NTf2 ], yields Cu-NPs with a diameter of 5 nm. 23 In the present study, the metallization of PSi by Cu is conducted by impregnation with a solution of CuMes in IL. The precursor readily reacts with PSi to aord metal deposition. The ability of this approach to provide uniform nucleation of the metal within the pores throughout the porous layer is demonstrated. The nature and reactivity of surface groups present on both bulk and porous Si is investigated and discussed to account for the experimental observations.

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Experimental Preparation of IL, solutions and suspensions

All reactions were conducted in the strict absence of oxygen and water under a puried argon atmosphere using glove box (MBraun) or vacuum-line techniques. 1-methylimidazole (>99 %) and 1 chlorobutane were purchased from Sigma Aldrich and distilled prior to use. Distilled water, freshly distilled toluene (> 99 %, Sigma Aldrich) and freshly distilled dichloromethane (> 99 %, Sigma Aldrich) were used for the purication of ILs. Bis(triuoromethanesulphonyl)imide lithium salt (> 99 %, Solvionic) and Mesitylcopper (CuMes, Nanomeps) were kept in a glove box and used as received. The synthesis of 1-butyl-3-methylimidazolium bis(triuoro-methylsulphonyl)imide, [C 1 C4 Im][NTf2 ], was performed as reported in the literature. 24 Its purity was checked by nuclear magnetic resonance (NMR) spectra recorded on a Bruker Advance spectrometer at 300 MHz for 1H and at 75.43 MHz for

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C. After puri-

cation, the halide content was found to be below 100 ppm (HR-SM), and water ∼12 ppm, (limit of Karl Fischer titration). A solution of silver nitrate in distilled water was prepared in order to test chloride traces in IL. The solutions were freshly prepared by dissolving CuMes in [C 1 C4 Im][NTf2 ] to the desired concentration (5 × 10−2 mol · L−1 if not stated otherwise) in a Schlenk tube under stirring at room temperature. Suspensions of Cu-NPs were prepared as needed by decomposing this solution under H2 . For this purpose, 2 mL of this organometallic solution was canuled into an autoclave under argon. At this step, the reaction medium was warmed to 100 ◦C without stirring. After gas purge, the autoclave was pressurized at 0.9 MPa of H2 during 10 min and kept at 100 ◦C during 4 h. At the end of the reaction, the resulting dark suspension was placed under vacuum to eliminate the volatile by-products (mesitylene) and stored under argon in a glove box. The Cu-NPs were observed by transmission electron microscopy (TEM) using a Philips CM120 at 120 kV. For this purpose, in a glove box, the suspension was deposited on a TEM 4

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grid and transferred into the microscope without further preparation. The size of 200 NPs at least was measured. Their size distribution was tted by a lognormal law to 5 ± 1 nm (Figure 1).

Figure 1: TEM image of Cu-NPs formed in [C 1 C4 Im][NTf2 ] at 100 ◦C during 4 h under 0.9 MPa H2 .

Preparation of substrates

A mesoporous layer was electrochemically generated on a boron-doped (p-doped) Si wafer (200 mm in diameter, (100) oriented, with a resistivity of 1500 Ω · cm). An automated industrial equipment composed of a cleaning and an anodizing chamber was used for that purpose. Anodic dissolution of silicon was performed in a double tank cell where the wafer was frontand back-side immersed in the same electrolyte solution. This electrolyte was a mixture of

2.5 volumes of HF (50 wt%) and 1 volume of ispropanol. The anodization was performed galvanostatically at 3 A (10 mA · cm−2 ) under back-side illumination. This protocol aorded a 2 µm-thick mesoporous layer (PSi) with a pore diameter of about 5 nm (40 % porosity) as measured by ellipsometry porosimetry. A p-doped bulk Si wafer with the same resistivity

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served as a reference surface throughout this study. Both wafers were cleaved into centimetric size coupons for the metallization experiments. Native oxide layers appear when Si is exposed to air. The oxygen and humidity react with Si to form SiO 2 and SiOH groups. This supercial oxidation was (probably partially, considering the infra-red spectrum in Figure 6) removed by immersion of the samples into a 1 % solution of hydrouoric acid during 30 s and rinsing with deionized water. Water traces on the surface were dried with a nitrogen-air gun. Finally, all deoxidized samples were put under primary vacuum for at least one hour for complete drying. Various techniques were used to characterize the supercial chemical composition of the samples. For diuse reectance infrared Fourier transform spectroscopy (DRIFT) analysis, the sample was dried under high vacuum ( 10−5 bar) for at least 12 h and transferred in a Harrick high temperature cell under argon into a Thermo Scientic Nicolet 6700 FT-IR equipment. The sample was initially purged for 20 min under 20 mL · min−1 of argon. An IR spectrum was recorded every minute between 25 and 350 ◦C (heating rate: 5 ◦C · min−1 ) under 5 mL · min−1 ow of argon. Temperature-programmed desorption/adsorption experiments were performed in a Belcat B equipment from BEL Japan to characterize the modication of silicon surface upon annealing and to quantify desorbed H 2 . To avoid any external contamination, sample loading into the measurement quartz cell was carried out in a glove box.

Metallization of PSi

All metallization operations were performed in the strict absence of oxygen and water under a puried argon atmosphere using a glove box or a vacuum-line set-up. The PSi samples were metallized by in-situ decomposition of a solution of CuMes in IL. A solution of CuMes in [C1 C4 Im][NTf2 ] was poured on the substrate in an autoclave. The metallization was conducted in up to two steps: 1. Impregnation: the reactor was heated to 50 ◦C for 2 h under argon to allow for the 6

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solution to fully impregnate and react with the porous structure 2. Extended reaction under H 2 : in some experiments, the argon was then replaced by

0.9 MPa H2 and the temperature was raised to 100 ◦C for 4 h to fully decompose CuMes. After cooling down and depressurizing the autoclave, still in the glove box, the sample was extracted from the reactional medium, washed twice with 2 mL of dry distilled dichloromethane and dried under argon ow. An S-5000 high resolution scanning electron microscope (SEM) from Hitachi was used to observe the morphology of the Si samples and perform energy-dispersive X-ray spectroscopy (EDX) analysis. For this analysis, 6×4 mm samples were cut and linked to an aluminium carrier with carbon colloids glue.

Results Metallization of PSi

Spontaneous Cu deposition has been reported upon immersion of PSi in aqueous solutions containing cupric ions, due to the reductive character of this material. 811,17 Cu deposition has also been reported from a solution in methanol. 16 Here, the metallization of the PSi samples was carried out by impregnation with a solution of CuMes in [C 1 C4 Im][NTf2 ]. In this process, it is very important that the solvent penetrates down to the bottom of the pores to allow for metal deposition throughout the porous layer. A short study was undertaken to investigate the interaction of our IL with a mesoporous Si layer similar to the material used in this work (see Supplementary Information). In a rst experiment, it was veried by contact angle measurements that [C 1 C4 Im][NTf2 ] eciently wets the porous material (Figure S2). The penetration of the liquid was tentatively monitored using electrochemical impedance spectroscopy (EIS). A signicant evolution with time of the EIS response of a porous electrode was observed (Figure S4). By comparison with the response of a planar Si 7

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electrode (Figure S3), it was possible to separate the contribution of the double electrical layer (Figure S4b). After a few minutes, a contribution attributed to the porous layer lled with the IL was observed (the initial contribution of the dry porous layer was beyond the tested frequency range). Even though this is an indirect evidence, it could be concluded that the IL-based solution can probably penetrate mesoporous Si layers without the need for further sample preparation in a few minutes only. For the metallization experiments, the samples were thus simply immersed in the solution under argon for 2 h at 50 ◦C to ensure complete wetting of the pores. This step also allowed for complete dissolution of CuMes in the IL. As expected, no Cu deposition was observed on a surface of bulk Si (Figure 2a), whereas an almost closed Cu lm was present on the porous surface (Figure 2b). This demonstrates that CuMes is readily decomposed by PSi under these conditions. More importantly, Cu islands, whose size (about 10 nm in diameter) is comparable to the pore diameter, were present within the pores (Figure 2c). A similar result was obtained with a solution of CuMes diluted 10 times (Figure 3a). Even in this case, the metal was well distributed throughout the porous layer, as shown by EDX measurements (Figure 3b and c). This conrms that the IL-based solution eciently penetrates down to the bottom of the pores and shows that the decomposition reaction is active even deep below the sample surface. Thus, approach is well suited to eciently metallize PSi materials.

(a)

(b)

(c)

Figure 2: (a) Top-view SEM image of bulk Si surface, (b) top-view and (c) cross section SEM images (using backscattering diusion detector) of a PSi sample after impregnation by a solution of CuMes in [C 1 C4 Im][NTf2 ].

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Figure 3: (a) cross section SEM image (using backscattering diusion detector) of a PSi sample after impregnation by a solution of CuMes ( 5 × 10−3 M) in [C1 C4 Im][NTf2 ]. (b) and (c) EDX mapping of Cu and Si, respectively.

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(b)

(c)

Figure 4: (a) Top-view SEM image of bulk Si surface, (b) top-view and (c) cross section SEM images (using backscattering diusion detector) of a PSi sample after impregnation by a solution of CuMes in [C 1 C4 Im][NTf2 ] followed by reaction under H 2 .

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In an attempt to increase metal content in the pores, a second experiment was undertaken, in which an extended reduction under H 2 was conducted. For that purpose, the same procedure was repeated, with an additional reaction step of 4 h under 0.9 MPa H2 at 100 ◦C. This step alone is sucient to decompose all the CuMes present in the solution. Interestingly enough, this additional reaction fostered Cu deposition on bulk Si (Figure 4a, to be compared with Figure 2a). Cu islands were densely distributed over the surface and were characterized by a quite narrow size distribution ( 35 ± 6 nm). This indicates that they were formed by instantaneous nucleation. On the surface of PSi, two populations of Cu islands were observed (Figure 4b). Small islands ( 35 ± 5 nm) similar to those observed after impregnation only (Figure 2b) coexisted with larger Cu particles of about 100 nm in diameter. Finally, the size and number of Cu islands within the pores were not modied upon reaction under H2 (Figure 4c to be compared with Figure 2c). The bimodal size distribution of the Cu clusters at the surface of PSi can be explained by the concomitant growth of the Cu islands formed during the impregnation step and further nucleation of Cu under the reaction with H 2 . However, considering that this reaction may also lead to the formation of Cu-NPs within the liquid phase, it cannot be excluded that the large aggregates were formed by coalescence then deposition of the Cu-NPs from the liquid phase. To verify this hypothesis, two samples (porous and non-porous) were exposed to a suspension of Cu-NPs (the suspension was added dropwise on the samples). The reactor was then kept at 50 ◦C for 2 h under argon (i.e. conditions of the impregnation step). The pictures in Figure 5 show that in this case, no deposit remained on bulk Si after rinsing with CH2 Cl2 . For the PSi sample, Cu was only deposited on the surface of the porous material and not within the pores. This is expected, since the diameter of the Cu-NPs (about 5 nm) is comparable to the pore diameter. The morphology of the supercial deposit was quite dierent from the sample in Figure 4b, though. It was formed of much larger structures (several µm) which seemingly resulted from the aggregation of smaller particles. As expected in this case, the deposition thus seems to proceed from the coalescence (and

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partial sintering at this low temperature) of the Cu-NPs from the suspension onto the sample surface.

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Because such large structures are not likely to transform into the 100 nm islands

observed in Figure 4b, this mechanism is probably not responsible for their formation.

(a)

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Figure 5: (a) Top-view SEM image of bulk Si surface, (b) top-view and (c) cross section SEM images of a PSi sample after treatment with a suspension of Cu-NPs in [C 1 C4 Im][NTf2 ] under Ar, at 50 ◦C for 2 h. To summarize, our results demonstrate that PSi is able to reduce CuMes in IL, leading to the precipitation of Cu islands at the surface and in the pores of the material. Further reaction under H2 does not increase Cu content within the pores, and only promotes further nucleation and growth at the surface.

Surface chemistry of PSi

The nature of the surface reactions responsible for Cu deposition on PSi is still debated in the literature. A few authors consider that Cu ions preferentially react with surface hydrides. 8,14,18 Indeed, these surface functions are commonly present at the surface of PSi. 25 Most published work suggests that the reduction of Cu ions is fostered by the oxidation of Si. 9,10,17 In fact, when PSi is treated in aqueous solutions of metallic ions, there is a gradual oxidation of the material. An oxide layer forms that progressively passivates the surface until the reaction is stopped. In this case, no consumption of (Si) x SiHy surface groups is generally observed by IR spectroscopy. 10 1 Note that the suspension of Cu-NPs is stable for weeks under Ar at room temperature.

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To verify the presence of such (and other) surface groups on our samples, a DRIFT analysis of both the bulk and the PSi surfaces was undertaken. In Figure 6a, the infrared spectra of both materials are compared. Several peaks are common to both types of samples: an intense peak at 1100 cm−1 corresponding to the vibration of siloxane bridges −1 −1 (− −Si−O−Si− −), and broad patterns centered at 3500 cm and at 880 cm associated to the

vibration of silanol groups ( ν (−Si−OH) and δ (− −Si−OH), respectively). Note that in some samples, traces of carbohydrate groups were observed ( 2933 cm−1 ν (CH) and 1455 cm−1 for

δ (− −Si−CH) groups); they are attributed to a pollution by grease during the pre-drying treatment with a nitrogen-air gun. Other features are specic to the PSi sample, such as intense peaks around 2100 cm−1 assigned to Si−H stretching vibrations (Figure 6b). More precisely, the peaks at 2134 cm−1 , 2111 cm−1 and 2086 cm−1 in the IR spectra of PSi are ascribed to ν (SiSiH3 ), ν (Si2 SiH2 ) and ν (Si3 SiH), respectively. 26 An additional peak centered around

2254 cm−1 could be assigned to the vibration ν (O3 SiH) 25 and the vibration at 912 cm−1 corresponds to δ (Si2 SiH2 ). The intensity of all vibrations assigned to the surface silane and siloxy groups decreased with temperature (Figure 6b). This indicates that these groups were removed from the surface and pores upon annealing. To quantify these surface functions, successive temperature programmed desorption and adsorption experiments were thus conducted with a sample of PSi ( 640 mg). The quartz cell was rst ushed with argon, then heated to 900 ◦C (heating rate 5 ◦C · min−1 ) under argon. During this treatment, a rst H 2 desorption peak was detected at 145 ◦C (0.3 mmol · g−1 of H2 ), followed by two other desorptions at 720 ◦C and 860 ◦C (for a total quantity of

0.328 mmol · g−1 of H2 ). Then, the sample was ushed under argon for 1 h and cooled down to room temperature. After this treatment, an adsorption heating cycle was undertaken on the same sample under H 2 (5 vol% in argon) from 25 to 900 ◦C (heating rate of

5 ◦C · min−1 ). During this cycle, two H 2 adsorptions were observed, the rst one at 145 ◦C ( 0.158 mmol · g−1 ) and the second one at 750 ◦C ( 0.153 mmol · g−1 ). It is noticeable that the quantity of adsorbed H 2 corresponds to about 50 % of the desorbed quantity during 12

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(a)

(b)

Figure 6: (a) DRIFT analysis of bulk and porous Si under Ar at 25 ◦C and (b) evolution of the peaks of MiPSi near 2100 cm−1 upon thermal treatment. the rst cycle, indicating that the process is only partially reversible. The sample was again ushed under argon for 1 h and cooled down to room temperature. Finally, a last temperature programmed desorption cycle was applied to the same sample under the same conditions as for the rst cycle. Again, two successive desorptions were observed corresponding to 0.218 mmol · g−1 and 0.044 mmol · g−1 , respectively. This corresponds to an almost full desorption of the H 2 adsorbed during the previous step. All these results conrm the presence of hydrides at the surface (and probably within the pores) of the PSi material. These hydrides can be desorbed upon thermal treatment, then (at least partially) re-adsorbed. However, even if two distinct contributions were measured during thermal desorption, the infrared spectra in Figure 6b show a drastic and simultaneous diminution of all bands in the 20002200 cm−1 range associated to silane groups. Thus, this does not indicate that one kind of surface hydrides desorbs rst. This is in contradiction with the literature, as the H 2 -desorption has been related by others to the hydrogen elimination from Si2 SiH2 and SiSiH3 , 27 aording the reconstruction of Si (100) through the formation of a bridge between two neighboring Si atoms according to: 28

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2 (Si)2 SiH2 −−→ (Si)2 HSi−SiH(Si)2 + H2

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(1)

Discussion In this work, p-doped bulk Si and PSi materials show quite dierent reactivities towards metal deposition. In the previous section, they have been shown to bear dierent chemical functions on their surface. Now, the role of these functions during the metallization process is discussed.

Bulk Si

For bulk Si, no signicant deposition was observed from a suspension of Cu-NPs (Figure 5a). Similarly, plain exposure of this surface to a solution of CuMes did not lead to a metallic deposit (Figure 2a). Conversely, when the solution was further decomposed under H 2 , quite dense Cu islands were formed on the Si surface (Figure 4a). The rst observation indicates that the surface of Si does not oer anchoring sites to metallic Cu. The second result suggests that this surface is not reactive towards CuMes either. Nevertheless, adherent Cu islands were formed on this surface upon reduction under H 2 (they were not washed away during rinsing). This unexpected result could be explained by the presence of − −Si−OH groups on the surface, as evidenced by the DRIFT analysis (Figure 6a). Indeed, the formation of copper siloxy from the reactivity of CuMes with silanol is documented in the literature. 2931 Also, favorable interaction of hydroxyl groups with metallic ions has been found by rst-principle quantum chemical calculations. 32 So, it is reasonable to consider that the − −Si−OH groups may react with CuMes and generate a surface copper siloxide [ −SiO−Cu] intermediate:

− −Si−OH + CuMes −−→ [− −SiO−Cu(I)] + MesH

(2)

This adsorbate is not likely to modify the morphology of the surface. Accordingly, no 14

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change was observed by SEM after impregnation (Figure 2a). According to the literature, its post-treatment under hydrogen allows for the preparation Cu nanoparticles. 31 This would not only explain the formation of the Cu deposit, but also the good adherence of the metallic islands attached to silanol groups. Obviously, such anchoring sites are not generated when the surface is exposed to suspensions of well-formed Cu-NPs.

Mesoporous Si

By contrast with bulk Si, the PSi material can x Cu-NPs from the liquid phase, as shown in Figure 5b. This could be related to the presence on this material of surface silane groups (Si3 SiH and Si2 SiH2 ), even if other eects such as the trapping of Cu-NPs on the rough surface may also explain this behavior. The role of these groups is certainly clearer and more decisive in the case of the in situ reduction of CuMes in the pores observed after impregnation of the solution under Ar (Figure 2). Indeed, these groups possess a reductive character, as already highlighted. The reduction of CuMes by these surface groups rather than by oxidation of PSi itself (which is not likely to occur in IL, by contrast with aqueous solutions) would explain the formation of Cu islands at the surface and deep in the pores, under conditions where CuMes is not decomposed if the PSi material is absent (i.e. in the solution or in the presence of bulk Si). To verify this argument, an additional temperatureprogrammed desorption/adsorption experiment was performed on the Cu-modied MiPSi sample shown in Figure 2. As expected, no H 2 desorption or adsorption could be observed. This indicates that the surface silane groups Si −Hx are no longer accessible after  or have been consumed during  metal deposition. Even if the exact mechanism of the reduction step is still to be elucidated, the rst step of this reaction can be speculated to be the formation of Si−Cu bonds from the reaction of CuMes with Si −H. This would allow for the nucleation of Cu nanoparticle by the decomposition of CuMes. 32

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Conclusion The metallization of a mesoporous Si (PSi) layer was successfully achieved by simple immersion in a solution of CuMes in an IL, [C 1 C4 Im][NTf2 ]. Interestingly enough, this procedure also allows for the metallization of bulk Si samples that do not exhibit silane surface groups. In this case, H2 is required to reduce CuMes. Silanol surface groups are suspected to play an important role in this process, as they act as grafting (anchoring) and nucleation sites for the Cu islands. Even more interestingly, this procedure readily aords deposition of Cu islands not only on the surface, but also deep within the pores of a PSi layer with small pores below 10 nm. This eect is attributed to the reduction of CuMes by surface silane groups detected by IR and thermal desorption analyses. This reaction is quite dierent from the Si oxidation reported for similar processes in aqueous media. Because of this, and owing to the wide electrochemical window of ILs, this reaction can in principle be extended to metals that are less noble than Cu. Further studies are under progress in this direction. Also, the reaction mechanisms with CuMes and other organometallic precursors are currently investigated using soluble silane analogue compounds.

Acknowledgement

This work was partially funded by the French Région Rhône-Alpes through the ARC6 Programme.

Supporting Information Available

The following les are available free of charge.

• Filename: DarwichSI This material is available free of charge via the Internet at http://pubs.acs.org/ .

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References

(1) Thomas, J. C.; Pacholski, C.; Sailor, M. J. Delivery of nanogram payloads using magnetic porous silicon microcarriers. Lab Chip 2006, 6, 782787. (2) Vazquez, M.; Pirota, K.; Hernandez-Valez, M.; Prida, V. M.; Navas, D.; Sanz, R.; Batallan, F.; Velazquez, J. Magnetic properties of densely packed arrays of Ni nanowires as a function of their diameter and lattice parameter. Journal of Applied Physics 2004,

95, 66426644. (3) Buriak, J. M. Organometallic chemistry on silicon surfaces: formation of functional monolayers bound through Si-C bonds. Chem. Commun. 1999, 10511060. (4) Granitzer, P.; Rumpf, K. Porous SiliconA Versatile Host Material. Materials 2010,

3, 943998. (5) Brongersma, M. L.; Kik, P. G.; Polman, A.; Min, K. S.; Atwater, H. A. Size-dependent electron-hole exchange interaction in Si nanocrystals. Applied Physics Letters 2000,

76, 351353. (6) Hérino, R. Nanocomposite materials from porous silicon. Materials Science and Engi-

neering: B 2000, 6970, 7076. (7) Jin, L.; Oya, T.; Tamekuni, S.; Watanabe, M.; Kondoh, E.; Gelloz, B. Copper deposition in microporous silicon using supercritical uid. Thin Solid Films 2014, 567, 82  86. (8) Sham, T. K.; Coulthard, I.; Lorimer, J. W.; Hiraya, A.; Watanabe, M. Reductive Deposition of Cu on Porous Silicon from Aqueous Solutions: An X-ray Absorption Study at the Cu L3,2 Edge. Chemistry of Materials 1994, 6, 20852091. (9) Coulthard, I.; Sham, T. Morphology of porous silicon layers: image of active sites from reductive deposition of copper onto the surface. Applied Surface Science 1998, 126, 287  291. 17

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Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(10) Kumar, P.; Huber, P. Nucleation and growth of copper on mesoporous silicon by immersion plating. Journal of Physics D: Applied Physics 2007, 40, 2864. (11) Bandarenka, H.; Prischepa, S.; Fittipaldi, R.; Vecchione, A.; Nenzi, P.; Balucani, M.; Bondarenko, V. Comparative study of initial stages of copper immersion deposition on bulk and porous silicon. Nanoscale Research Letters 2013, 8, 85. (12) Ogata, Y. H.; Fukami, K. In Handbook of Porous Silicon ; Canham, L., Ed.; Springer International Publishing, 2014; Chapter Porous Silicon and Electrochemical Deposition, pp 629637. (13) Fukami, K.; Koda, R.; Sakka, T.; Urata, T.; Amano, K.; Takaya, H.; Nakamura, M.; Ogata, Y.; Kinoshita, M. Platinum electrodeposition in porous silicon: The inuence of surface solvation eects on a chemical reaction in a nanospace. Chemical Physics

Letters 2012, 542, 99105. (14) Polisski, S.; Goller, B.; Heck, S. C.; Maier, S. A.; Fujii, M.; Kovalev, D. Formation of metal nanoparticles in silicon nanopores: Plasmon resonance studies. Applied Physics

Letters 2011, 98 . (15) Nativ-Roth, E.; Rechav, K.; Porat, Z. Deposition of gold and silver on porous silicon and inside the pores. Thin Solid Films 2016, 603, 8896. (16) Harraz, F. A.; Tsuboi, T.; Sasano, J.; Sakka, T.; Ogata, Y. H. Metal Deposition onto a Porous Silicon Layer by Immersion Plating from Aqueous and Nonaqueous Solutions.

Journal of The Electrochemical Society 2002, 149, C456C463. (17) Recio, G.; Gallach, D.; Silvan, M. M.; Fukami, K.; Palma, R. J. M.; Castro, G. R.; Munoz-Noval, A. Photoassisted Immersion Deposition of Cu Clusters onto Porous Silicon: A Langmuir-Hill Ligand-Locus Model Applied to the Growth Kinetics. The Jour-

nal of Physical Chemistry C 2014, 118, 1490514912.

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Page 18 of 21

Page 19 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

(18) Maley, J. M.; Sham, T. K.; Hirose, A.; Yang, Q.; Bradley, M. P.; Sammynaiken, R. Chemical Reactions and Applications of the Reductive Surface of Porous Silicon. Jour-

nal of Nanoscience and Nanotechnology 2010, 10, 63326339. (19) Dupont, J.; Scholten, J. D. On the structural and surface properties of transition-metal nanoparticles in ionic liquids. Chemical Society Reviews 2010, 39, 17801804. (20) Campbell, P.; HG Prechtl, M.; C Santini, C.; Haumesser, P.-H. Ruthenium Nanoparticles in Ionic Liquids A Saga. Current Organic Chemistry 2013, 17, 414429. (21) Helgadottir, I.; Freychet, G.; Arquillière, P.; Maret, M.; Gergaud, P.; Haumesser, P.; Santini, C. Ru-core/Cu-shell bimetallic nanoparticles with controlled size formed in one-pot synthesis. Nanoscale 2014, 6, 1485614862. (22) Haumesser, P.-H.; Basset, J.-M.; Campbell, P.; Deleonibus, S.; Gutel, T.; Marchand, G.; Santini, C. Method for making electrical interconnections with carbon nanotubes. 2013; US Patent 8,518,816. (23) Arquillière, P. P.; Santini, C.; Haumesser, P.-H.; Aouine, M. Synthesis of Copper and Copper-Ruthenium Nanoparticles in Ionic Liquids for the Metallization of Advanced Interconnect Structures. ECS Transactions 2011, 35, 1116. (24) Magna, L.; Chauvin, Y.; Niccolai, G. P.; Basset, J.-M. The Importance of Imidazolium Substituents in the Use of Imidazolium-Based Room-Temperature Ionic Liquids as Solvents for Palladium-Catalyzed Telomerization of Butadiene with Methanol.

Organometallics 2003, 22, 44184425. (25) Grosman, A.; Ortega, C. In Chemical composition of 'fresh' porous silicon ; Canham, L., Ed.; Institution of Electrical Engineers, 1997; Chapter 5.1, pp 145153. (26) Stryahilev, D.; Diehl, F.; Schroeder, B.; Scheib, M.; Belogorokhov, A. I. On the splitting

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of SiH absorption bands in infrared spectra of hydrogenated microcrystalline silicon.

Philosophical Magazine Part B 2000, 80, 17991810. (27) Tsuboi, T.; Sakka, T.; Ogata, Y.; Naito, S. Structural Changes of Porous Silicon Surface with Thermal Annealing Studied by 29Si Nuclear Magnetic Resonance Spectroscopy and Fourier Transform Infrared Spectroscopy. Journal of Porous Materials 2000, 7, 327330. (28) Chabal, Y. J.; Raghavachari, K. Surface Infrared Study of Si(100)-(2x1)H. Phys. Rev.

Lett. 1984, 53, 282285. (29) Tan, G.; Yang, Y.; Chu, C.; Zhu, H.; Roesky, H. W. Cu24O24Si8R8: Organic Soluble 56-Membered Copper(I) Siloxane Cage and Its Use in Homogeneous Catalysis. Journal

of the American Chemical Society 2010, 132, 1223112233. (30) Schax, F.; Limberg, C.; Muegge, C. Copper(I) Siloxides  Aggregated Solid-State Structures, CuCu Interactions and Dynamic Solution Behavior. European Journal of

Inorganic Chemistry 2012, 2012, 46614668. (31) Roussey, A.; Gentile, P.; Lafond, D.; Martinez, E.; Jousseaume, V.; Thieuleux, C.; Copéret, C. Cu nanoparticles on 2D and 3D silica substrates: controlled size and density, and critical size in catalytic silicon nanowire growth. Journal of Materials Chemistry

C 2013, 1, 15831587. (32) Tatsumura, K.; Watanabe, T.; Hara, K.; Hoshino, T.; Ohdomari, I. Nucleation site of Cu on the H-terminated Si(111) surface. Phys. Rev. B 2001, 64, 115406.

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