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Functionalization of silicon nanowires by conductive and non-conductive polymers
Accepted Manuscript Title: Functionalization of silicon nanowires by conductive and non-conductive polymers Author: S. Belhousse F.-Z. Tighilt S. Sam K. Lasmi K. Hamdani L. Tahanout F. Megherbi N. Gabouze PII: DOI: Reference:
S0169-4332(17)30030-2 http://dx.doi.org/doi:10.1016/j.apsusc.2017.01.030 APSUSC 34825
To appear in:
APSUSC
Received date: Revised date: Accepted date:
13-10-2016 2-1-2017 4-1-2017
Please cite this article as: S.Belhousse, F.-Z.Tighilt, S.Sam, K.Lasmi, K.Hamdani, L.Tahanout, F.Megherbi, N.Gabouze, Functionalization of silicon nanowires by conductive and non-conductive polymers, Applied Surface Science
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Functionalization of silicon nanowires by conductive and non-conductive polymers S. Belhousse1, F-Z. Tighilt1, S. Sam1, K. Lasmi1, K. Hamdani1, L. Tahanout2, F. Megherbi2 and N. Gabouze1 1 Division Couches Minces Surfaces Interfaces (CMSI), Centre de Recherche en Technologie des Semi-conducteurs pour l’Energétique (CRTSE), 2 Bd. Frantz Fanon, B.P. 140, Alger-7 merveilles, Algiers, Algéria. 2 The University M'hamed Bougara, Faculty Engineering Sciences, Route de la Gare Ferroviaire Boumerdes, Algérie * Corresponding authors:[email protected], Tel/Fax: 00 213 21 43 35 11 [email protected] [email protected] [email protected] [email protected] [email protected] [email protected] [email protected]
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Graphical abstract
S +
S
-2H2
S
SiNW
H
S H
S +
S
H
+
.
H
S
S
Si
SiNW Conductive Polymers Polythiophene, polypyrrole
Non-conductive Polymers Polyvinylpyrrolidone SiNW
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HIGHLIGHTS - Silicon nanowires were prepared using metal assisted chemical etching. - Conductive and non-conductive polymers were grafted on SiNW surface using electrochemical polymerization. - The good conductive properties of polythiophene and polypyrrole polymers ensure efficient charge transfer along the SiNW leading to the polymer covering over the entire length of the wire. Unlike, the polyvinylpyrrolidone, non-conductive polymer, polymerizes on SiNW top. - The hybrids structures SiNW/PTh and SiNW/PPy present active surfaces which can increase and improve the transfer and the conductivity when their applications.
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The work reports on the development of hybrid devices based on silicon nanowires (SiNW) with polymers and the difference obtained when using conductive and non-conductive polymers. SiNW have attracted much attention due to their importance in understanding the fundamental properties at low dimensionality as well as their potential application in nanoscale devices as in field effect transistors, chemical or biological sensors, battery electrodes and photovoltaics. SiNW arrays were formed using metal assisted chemical etching method. This process is simple, fast and allows obtaining a wide range of silicon nanostructures. Hydrogen-passivated SiNW surfaces show relatively poor stability. Surface modification with organic species confers the desired stability and enhances the surface properties. For this reason, this work proposes a covalent grafting of organic material onto SiNW surface. We have chosen a non-conductive polymer polyvinylpyrrolidone (PVP) and conductive polymers polythiophene (PTh) and polypyrrole (PPy), in order to evaluate the electric effect of the polymers on the obtained materials. The hybrid structures were elaborated by the polymerization of the corresponding conjugated monomers by electrochemical route; this electropolymerization offers several advantages such as simplicity and rapidity. SiNW functionalization by conductive polymers has shown to have a huge effect on the electrical mobility. Hybrid surface morphologies were characterized by scanning electron microscopy (SEM), infrared spectroscopy (FTIR-ATR) and contact angle measurements. Keywords:
silicon
nanowires;
polypyrrole;
polythiophene;
polyvinylpyrrolidone;
electropolymerization.
1. Introduction: Due to the high surface to volume silicon ratio and unique quasi one dimensional electronic structure, silicon nanowires (SiNW) can be exploited in many ways in electronic devices as in field effect transistors [1–3], battery electrodes [4], photovoltaics [5, 6] and chemical or 4
biological sensors [7–10]. SiNW have gained considerable interest because they have very narrow diameters and provide a link between molecular and solid state physics [11, 12]. Much effort has been made to prepare semiconductor nanowires by different methods, such as Vapor–Liquid–Solid technique (VLS) [13], laser ablation [14, 15], thermal evaporation decomposition [16], supercritical-fluid–liquid–solid (SFLS) synthesis [17], and other methods [18]. However, these growth mechanisms have some limitations as they generally need high synthesis temperatures or high vacuum templates, complex equipment and long synthesis times. Recently, large-area SiNW arrays were formed on silicon substrates when immersing silicon substrates into aqueous HF solution containing silver nitrate [19]. This method called metal assisted chemical etching is simple, fast and allows obtaining a wide range of silicon nanostructures on a large surface. Hydrogen-passivated SiNW surfaces show good stability in air (under ambient conditions) but relatively poor stability in water [20]. Functionalization is an important aspect of the chemistry of 1-D nanostructured materials as chemical modification is often necessary for their functionality and biocompatibility. The functionalization in this work is based upon the protection of silicon surface by polymer layers such as PTh, PPy and PVP. These layers are used as a protective, patterning barrier against surface contamination. Furthermore, the integration of two materials into one composite could synergistically combine the advantages of both materials. The conductive polymers (PTh and PPy) were chosen for their stability in different environments. The choice of conductive polymers allows an improvement of interfacial charge transfer ensuring continuity in the conduction mechanism. We are interested in poly (N-vinyl-2-pyrrolidone) (PVP) as non-conductive polymer because endowed with highly advantageous properties; its introduction into new materials is likely to give them a wide application appearance. Indeed, PVP is capable of forming hydrogen bonds, it exhibits excellent biocompatibility and very low toxicity [21]. It is a very interesting stabilizer and has adhesive features [22-23]. The polymerization can be obtained by electrochemical route which offers several advantages such as simplicity and rapidity. Like in planar materials, functionalization in 1-D nanostructures can generally be divided into covalent and non-covalent functionalization. However, sidewall covalent functionalization of silicon nanowires by conductive and non-conductive polymers gives a different behavior. The basic idea in this work is to assemble two materials to get better properties. The resulted structures can be used in various fields as energy storage [24], optoelectronic devices [25-26]
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and sensors [27-28]. All of these applications are translated into variations leading to a change of the SiNW equivalent conductance.
2. Experimental 2.1. Materials All cleaning and etching reagents were clean-room grade. Milli-Q water (18 MΩ) was used for all experiments. Sulphuric acid 96%, hydrogen peroxide 30%, acid hydrofluoric 48%, nitric acid, trichloroethylene, acetone, acetonitrile, thiophene, silver nitrate and tetrabutyl ammonium tetrafluoroborate were all available from Aldrich. Pyrrole and N-vinyl-2pyrrolidone were provided by Fluka.
2.2. Preparation of SiNW Silicon wafers used in these experiments are (100) oriented, boron doped (p-type) with a resistivity of 1-5 ohm cm. The surface was first degreased and cleaned in trichloroethylene, acetone and rinsed with deionized water then cleaned in 1/3 (by volume) H2O2/H2SO4 piranha solution and rinsed with DI water. SiNW arrays were prepared by chemical etching of the clean substrate in HF (5.25 M)/AgNO3 (0.02 M) solution at 50°C.
2.3. Fabrication of SiNW/ PTh, SiNW/PPy and SiNW/PVP structures Galvanostatic electrochemical polymerization of thiophene, pyrrole and vinyl-2-pyrrolidone on SiNW was performed in CH3CN/Bu4NBF4 (0.1M) solution containing the monomer at 0.05 mM concentration, using an electrochemical cell with three electrodes system. SiNW substrate was used as working electrode. The counter electrode was comprised of a gold wire and all potentials were relative to 0.01 M Ag+/Ag reference.
2.4. Materials characterization The obtained structures were analyzed by FTIR spectroscopy with a ThermoNicolet Nexus 670 equipped with a deuterated triglycine sulfate (DTGS KBr) detector. All FT-IR spectra were collected with 64 scans in mid infrared region at 4 cm−1 resolution. Surface morphology of hybrid structures was investigated by scanning electron microscope (SEM) HITACHI S-4800 equipped with an energy dispersive X-ray analysis device (EDX) used to determine the elements present on SiNW surface. Electrochemical experiments were performed using a potentiostat galvanostat VMP3. The sample was mounted in a PTFE holder
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with copper rear contact. A gold wire was used as counter electrode and all potentials were relative to the 0.01 M Ag+/Ag system. We used a remote-computer controlled goniometer system (DIGIDROP) for measuring contact angles. Water contact angles were measured using deionized water. The accuracy is ± 2°. All measurements were made in ambient atmosphere at room temperature.
3. Results and discussion 3.1. Metal assisted chemical etching of Si Figure 1 displays SEM images of the resulting surfaces. Figure 1a, shows branched silver dendrites formed on the surface of Si sample after etching in aqueous HF/AgNO3 solution at 50°C. Chemical removal of silver deposit using HNO3 acid at room temperature leds to the formation of well-aligned SiNW arrays (Figure 1b). The obtained nanowires are perpendicular to the surface and can be sometimes interconnected to form walls. They have a uniform length of 8 microns and a diameter between 10 and 100 nm. The top view of cleaned surface (figure 1c) shows a homogeneous surface. Silver dendrites deposition and silicon etching in fluoride solution containing Ag+ occurs according to the following equations: Ag+ +
1e+
Ag
Si + 4h + 4HF
SiF4 + 4H+
SiF4 + 2HF
H2SiF6
Figure 2.a shows ATR-FTIR spectrum in 700-4000 cm-1 range recorded from as-prepared SiNW. The strong absorption in 1000-1200 cm-1 range is due to the stretching vibrations of Si-O and Si-O-Si bonds. Upon rinsing with a dilute (10%) aqueous HF solution, new absorption bands attributable to Si-Hx (2000-2200 cm-1) were clearly observed, while the intensity of Si-O absorption bands decrease (figure 2b). Figure 2c shows the vibrational spectrum of HF-etched SiNW in the frequency range of Si-H stretching vibrations. Three broad overlapping bands can be observed. In accordance with the FTIR results of Si (111) and Si (100) wafers [29-31], these bands were assigned to the monohydride (SiH), the dihydride (SiH2), and the trihydride (SiH3), respectively.
3.2. Electrochemical polymerization Figures 3a and 3b show the cyclic voltammograms of thiophene electropolymerisation on SiNW surface with potential sweep rates of 20mV/s and 100mV/s, respectively. Figure 3a 7
shows two oxidation peaks of thiophene. The first one at 0.62V corresponds to the creation of the first active centers on the electrode surface. The second one, appears at 1.37V corresponding to the bipolaron structure of PTh. However, when sweeping at a rate of 100mV/s, one can observe only one peak at 1.37V for the two first cycles that shifts to 1.50V for the other scans. The peak onset of oxidation depends on the scanning speed. The shift of the oxidation peak potential from 1.37V to 1.50V can be explained by the modification of surface state due to polymer chain increase. Figures 4a and 4b show the cyclic voltammograms of pyrrole electropolymerisation on SiNW surfaces of 5 and 11µm length with a potential sweep rate of 50mV/s. The first scan in figure 4a shows an anodic peak at 0.72V which corresponds to the oxidation of the monomer into a radical cation and thus the formation of the first active center. Whilst, a cathodic peak is observed at 0,32V corresponding to the reduction of the polymer. One can observe a shift of the oxidation potential to 0.86V in the second cyclic voltammogram which is likely due to the structural change of the surface by increasing the polymeric chain or by a conformational change in the polymer chain. The start of polymerization on SiNW surface of 11µm length is shown on the cyclic voltammogram (Figure 4b) by a shoulder at about 1.5V which decreases with further scans due to the considerable decrease of active sites, indicating the evolution of the polymerization. However, no reduction peak is observed. From these results, it is clear that SiNW length significantly affects the appearance of monomer redox peaks (oligomer-polymer), and the value of the oxidation potential depends heavily on the surface state. The electropolymerization of vinylpyrrolidone on SiNW surfaces, at 50mV/s, of 5 and 11 µm length show, respectively, a shoulder at 1.83V (Figure 5a) and at 1.80V (Figure 5b). The potential value of the beginning of polymerization remains the same for the two nanowire lengths. However, in figure 5a we can observe the evolution of the polymerization indicated by the polymer oxidation potential decrease while we note the absence of a polymerization peak on SiNW surface of 11 µm length (Figure 5b) which is probably caused by a surface resistance of long nanowires.
3.3. ATR-FTIR spectroscopy analysis ATR-FTIR spectroscopy analysis was used to characterize the changes associated with the surface composition upon SiNW functionalization. FTIR spectrum of SiNW/PTh surface (Figure 6) exhibits bands between 3200 and 2840 cm−1 corresponding to C-H stretching vibrations of thiophene, and its bending vibrations appear between 1450 and 1300 cm−1. The 8
band at 1630 cm−1 is attributed to C═C stretching vibrations. A wide band between 1250 and 970 cm-1 comprising multiple bands is corresponding to the bending vibrations of C-H bonds of thiophene cycle and to the stretching vibrations of Si-C, Si-O-C and Si-O. C-S stretching band of thiophene is appearing at about 862 cm−1 and the corresponding bending vibration appears at 733 cm-1. A small band located around 777 cm-1 is assigned to the bending vibrations of the double bond C═C-H of thiophene cycle. Figure 7 shows FTIR spectrum of SiNW/PPy surface. We can observe a large vibration band between 3020 and 2824 cm−1 ascribed to C-H stretching vibrations of CH2 groups, and its bending vibrations appear between 1478 and 1381 cm−1. The sharp band located at 1727 cm−1 can be attributed to carbonyl C═O groups fixed in ß position of some pyrrole rings and which result from a light overoxidation of the polymer occurring during its electrosynthesis. Likewise, we observe a band situated at 1655 cm−1 which is attributed to C═C stretching vibrations. The band set between 1646 and 1461 cm-1 is attributed to pyrrole ring stretching vibrations. The sharp band at 1032 cm−1 corresponds to N-H in-plane deformation. The band located at 1280 cm−1 is related to Si-C stretching vibrations mode. The deconvolution of a large band between 1250 and 950 cm-1 shows the presence of numerous contributions namely Si-O-Si symmetric (1100 cm-1) and asymmetric (1180 cm-1) stretching vibrations, C-N stretching vibrations at 1132 cm-1, C-C stretching vibrations at 1054 cm-1 and finally N-H binding vibrations at 1032 cm-1. Two bands located at 741 and 678 cm−1 are attributed to the binding out-of-plane vibrations of N-H of pyrrole ring and ═C-H. ATR-FTIR spectrum of SiNW/PVP surface represented in figure 8, shows the presence of a large vibrational band between 3018 and 2807 cm−1 corresponding to CH2 and CH3 stretching vibrations. Bending vibrations of CH2, CH3 and methylene on α position of carbonyl appear between 1483 and 1342 cm−1. We observe a band situated at 1665 cm−1 which is attributed to C═O stretching vibrations. The band around 1280 cm-1 is attributed to C-N stretching vibrations. The large band located between 1180 and 1100 cm−1 is related to Si-O-Si stretching vibrations.
3.4. SEM analysis Figures 9 and 10 show electrochemically deposited PTh films on SiNW surface by applying five and two voltammetric cycles, respectively. The picture in figure 9a shows that after five voltammetric cycles the polymer covers the whole nanowires from top to bottom. However, the covering is done by nanowire bunches and not each wire individually. The first stages of
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the polymer formation are displayed in figure 10 after applying just two voltammetry cycles. It can be seen that the polymerization beginning occurs at the top of the nanowires bunches. EDX analysis (Figure 11) shows the presence of sulfur, silicon, oxygen and carbon which confirm the presence of polythiophene. Electropolymerized PPy on SiNW is represented in figure 12. One can note that the coating of the nanowires by PPy is made similarly as for the PTh. As shown figure 12b, after applying five voltammetric cycles, the thickness of deposited PPy on SiNW is about 0.38µ. The observation is the same by applying two voltammetric cycles (figure 13). In contrast, the elctropolymerization of the non-conductive polymer PVP shows a different morphology on SiNW surface. The polymerization occurs mainly on the nanowires top and we observe the absence of the polymer along the nanowires (figure 14). So, the good conductive properties of Pth and PPy ensure efficient charge transfer along the SiNW leading to the polymer covering over the entire length of the wire. Unlike, the non-conductive polymer PVP polymerizes on SiNW top.
3.5. Contact angle measurements Water contact angle measurements were used to follow the changes in the wetting properties of the freshly- prepared SiNW surface before and after functionalization (Figure 15). Asprepared hydrogen-terminated SiNW surface displays a hydrophobic character with a water contact angle of 124 °. The surface became more hydrophobic after the functionalization with conductive polymers. We observe a contact angle of 140° for SiNW/PTh surface and 143° for SiNW/PPy. These results coincide with those reported in the literature [32]. Whereas, we record a significant decrease of the contact angle of SiNW/PVP surface (29°), these results were due to the chemical structure of polymer.
Contact angle (°)
SiNW
SiNW/PTh
SiNW/PPy
SiNW/PVP
120.5 (a)
140(b)
143 (d)
29 (c)
4. Conclusion Hybrid structures were fabricated assembling SiNW and polymer using electrochemical way. The functionalization of SiNW surface by conductive and non-conductive polymers occurs in different way. Conductive polymer covers all the length of the nanowires while the non10
conductive polymer is deposited primarily on the surface top. The functionalization of surface with conductive polymers provides active surface which can increase and improve the transfer and the conductivity of these hybrid structures. This propriety can be used in electrochemical sensors, which is our current work and subject of our future publications.
Acknowledgement We acknowledge the National Research Fund (DGRSDT) and the Centre de Recherche en Technologie des Semi-conducteurs pour l’Energétique (CRTSE). Funding This work was supported by the National Research Fund (DGRSDT) and the Centre de Recherche en Technologie des Semi-conducteurs pour l’Energétique (CRTSE).
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Figure captions Figure 1: SEM images of: (a) branched silver dendrites formed on Si sample after etching in HF/AgNO3 aqueous solution at 50°C for 6 min, (b) Cross-sectional SEM image of SiNW array after rinsing in HNO3 Figure 2: ATR-FTIR spectrum of SiNW (a) as-prepared, (b) after rinsing in10% HF and (c) in the range of 2020-2150 cm-1..
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Fig.3: Cyclic voltammograms of anodic electropolymerization of thiophene monomer on SiNW surface (a) potential sweep rate of 20mV/s, (b) potential sweep rate of 100mV/s. Figure 4: Cyclic voltammograms of anodic electropolymerization of pyrrole monomer on SiNW surface (a) of 5µm length and (b) of 11μm length. Figure 5: Cyclic voltammograms of anodic electropolymerization of PVP on SiNW surface of (a) 5 µm length and (b) 11μm length. Figure 6: ATR-FTIR spectrum of SiNW/PTh surface. Figure 7: ATR-FTIR spectrum of SiNW/PPy surface. Figure 8: ATR-FTIR spectrum of SiNW/PVP surface. Figure 9: SEM micrographs of electropolymerized PTh on SiNW by applying five voltammetric cycles, (a) Cross-sectional view, (b) Plan view. Figure 10: SEM micrograph of electropolymerized PTh on SiNW by applying two voltammetric cycles Figure 11: EDX analysis of SiNW/ PTh surface. Figure 12: SEM micrographs of electropolymerized PPy on SiNW by applying five voltammetric cycles (a) 10000x magnification, (b) 20000x magnification. Figure 13: SEM micrographs of electropolymerized PPy on SiNW by applying two voltammetric cycles (a) 10000x magnification, (b) 20000x magnification. Figure 14: SEM micrographs of electropolymerized PVP on SiNW by applying five voltammetric cycles (a) Plan view, (b) Cross-sectional view. Figure 15: Water contact angle: (a) SiNW, (b) SiNW/PTh,(c) SiNW/PPy and (c) SiNW/PVP.
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(a)
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Figure 1
1.0 SiNW SiNW after rinsing in HF
Absorbance (a.u)
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Figure 2
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0.016
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1 scan nd 2 scan rd 3 scan th 4 scan th 5 scan
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Figure 3
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Figure 14
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S0169-4332(17)30030-2 http://dx.doi.org/doi:10.1016/j.apsusc.2017.01.030 APSUSC 34825
To appear in:
APSUSC
Received date: Revised date: Accepted date:
13-10-2016 2-1-2017 4-1-2017
Please cite this article as: S.Belhousse, F.-Z.Tighilt, S.Sam, K.Lasmi, K.Hamdani, L.Tahanout, F.Megherbi, N.Gabouze, Functionalization of silicon nanowires by conductive and non-conductive polymers, Applied Surface Science
http://dx.doi.org/10.1016/j.apsusc.2017.01.030 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Functionalization of silicon nanowires by conductive and non-conductive polymers S. Belhousse1, F-Z. Tighilt1, S. Sam1, K. Lasmi1, K. Hamdani1, L. Tahanout2, F. Megherbi2 and N. Gabouze1 1 Division Couches Minces Surfaces Interfaces (CMSI), Centre de Recherche en Technologie des Semi-conducteurs pour l’Energétique (CRTSE), 2 Bd. Frantz Fanon, B.P. 140, Alger-7 merveilles, Algiers, Algéria. 2 The University M'hamed Bougara, Faculty Engineering Sciences, Route de la Gare Ferroviaire Boumerdes, Algérie * Corresponding authors:[email protected], Tel/Fax: 00 213 21 43 35 11 [email protected] [email protected] [email protected] [email protected] [email protected] [email protected] [email protected]
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Graphical abstract
S +
S
-2H2
S
SiNW
H
S H
S +
S
H
+
.
H
S
S
Si
SiNW Conductive Polymers Polythiophene, polypyrrole
Non-conductive Polymers Polyvinylpyrrolidone SiNW
2
HIGHLIGHTS - Silicon nanowires were prepared using metal assisted chemical etching. - Conductive and non-conductive polymers were grafted on SiNW surface using electrochemical polymerization. - The good conductive properties of polythiophene and polypyrrole polymers ensure efficient charge transfer along the SiNW leading to the polymer covering over the entire length of the wire. Unlike, the polyvinylpyrrolidone, non-conductive polymer, polymerizes on SiNW top. - The hybrids structures SiNW/PTh and SiNW/PPy present active surfaces which can increase and improve the transfer and the conductivity when their applications.
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The work reports on the development of hybrid devices based on silicon nanowires (SiNW) with polymers and the difference obtained when using conductive and non-conductive polymers. SiNW have attracted much attention due to their importance in understanding the fundamental properties at low dimensionality as well as their potential application in nanoscale devices as in field effect transistors, chemical or biological sensors, battery electrodes and photovoltaics. SiNW arrays were formed using metal assisted chemical etching method. This process is simple, fast and allows obtaining a wide range of silicon nanostructures. Hydrogen-passivated SiNW surfaces show relatively poor stability. Surface modification with organic species confers the desired stability and enhances the surface properties. For this reason, this work proposes a covalent grafting of organic material onto SiNW surface. We have chosen a non-conductive polymer polyvinylpyrrolidone (PVP) and conductive polymers polythiophene (PTh) and polypyrrole (PPy), in order to evaluate the electric effect of the polymers on the obtained materials. The hybrid structures were elaborated by the polymerization of the corresponding conjugated monomers by electrochemical route; this electropolymerization offers several advantages such as simplicity and rapidity. SiNW functionalization by conductive polymers has shown to have a huge effect on the electrical mobility. Hybrid surface morphologies were characterized by scanning electron microscopy (SEM), infrared spectroscopy (FTIR-ATR) and contact angle measurements. Keywords:
silicon
nanowires;
polypyrrole;
polythiophene;
polyvinylpyrrolidone;
electropolymerization.
1. Introduction: Due to the high surface to volume silicon ratio and unique quasi one dimensional electronic structure, silicon nanowires (SiNW) can be exploited in many ways in electronic devices as in field effect transistors [1–3], battery electrodes [4], photovoltaics [5, 6] and chemical or 4
biological sensors [7–10]. SiNW have gained considerable interest because they have very narrow diameters and provide a link between molecular and solid state physics [11, 12]. Much effort has been made to prepare semiconductor nanowires by different methods, such as Vapor–Liquid–Solid technique (VLS) [13], laser ablation [14, 15], thermal evaporation decomposition [16], supercritical-fluid–liquid–solid (SFLS) synthesis [17], and other methods [18]. However, these growth mechanisms have some limitations as they generally need high synthesis temperatures or high vacuum templates, complex equipment and long synthesis times. Recently, large-area SiNW arrays were formed on silicon substrates when immersing silicon substrates into aqueous HF solution containing silver nitrate [19]. This method called metal assisted chemical etching is simple, fast and allows obtaining a wide range of silicon nanostructures on a large surface. Hydrogen-passivated SiNW surfaces show good stability in air (under ambient conditions) but relatively poor stability in water [20]. Functionalization is an important aspect of the chemistry of 1-D nanostructured materials as chemical modification is often necessary for their functionality and biocompatibility. The functionalization in this work is based upon the protection of silicon surface by polymer layers such as PTh, PPy and PVP. These layers are used as a protective, patterning barrier against surface contamination. Furthermore, the integration of two materials into one composite could synergistically combine the advantages of both materials. The conductive polymers (PTh and PPy) were chosen for their stability in different environments. The choice of conductive polymers allows an improvement of interfacial charge transfer ensuring continuity in the conduction mechanism. We are interested in poly (N-vinyl-2-pyrrolidone) (PVP) as non-conductive polymer because endowed with highly advantageous properties; its introduction into new materials is likely to give them a wide application appearance. Indeed, PVP is capable of forming hydrogen bonds, it exhibits excellent biocompatibility and very low toxicity [21]. It is a very interesting stabilizer and has adhesive features [22-23]. The polymerization can be obtained by electrochemical route which offers several advantages such as simplicity and rapidity. Like in planar materials, functionalization in 1-D nanostructures can generally be divided into covalent and non-covalent functionalization. However, sidewall covalent functionalization of silicon nanowires by conductive and non-conductive polymers gives a different behavior. The basic idea in this work is to assemble two materials to get better properties. The resulted structures can be used in various fields as energy storage [24], optoelectronic devices [25-26]
5
and sensors [27-28]. All of these applications are translated into variations leading to a change of the SiNW equivalent conductance.
2. Experimental 2.1. Materials All cleaning and etching reagents were clean-room grade. Milli-Q water (18 MΩ) was used for all experiments. Sulphuric acid 96%, hydrogen peroxide 30%, acid hydrofluoric 48%, nitric acid, trichloroethylene, acetone, acetonitrile, thiophene, silver nitrate and tetrabutyl ammonium tetrafluoroborate were all available from Aldrich. Pyrrole and N-vinyl-2pyrrolidone were provided by Fluka.
2.2. Preparation of SiNW Silicon wafers used in these experiments are (100) oriented, boron doped (p-type) with a resistivity of 1-5 ohm cm. The surface was first degreased and cleaned in trichloroethylene, acetone and rinsed with deionized water then cleaned in 1/3 (by volume) H2O2/H2SO4 piranha solution and rinsed with DI water. SiNW arrays were prepared by chemical etching of the clean substrate in HF (5.25 M)/AgNO3 (0.02 M) solution at 50°C.
2.3. Fabrication of SiNW/ PTh, SiNW/PPy and SiNW/PVP structures Galvanostatic electrochemical polymerization of thiophene, pyrrole and vinyl-2-pyrrolidone on SiNW was performed in CH3CN/Bu4NBF4 (0.1M) solution containing the monomer at 0.05 mM concentration, using an electrochemical cell with three electrodes system. SiNW substrate was used as working electrode. The counter electrode was comprised of a gold wire and all potentials were relative to 0.01 M Ag+/Ag reference.
2.4. Materials characterization The obtained structures were analyzed by FTIR spectroscopy with a ThermoNicolet Nexus 670 equipped with a deuterated triglycine sulfate (DTGS KBr) detector. All FT-IR spectra were collected with 64 scans in mid infrared region at 4 cm−1 resolution. Surface morphology of hybrid structures was investigated by scanning electron microscope (SEM) HITACHI S-4800 equipped with an energy dispersive X-ray analysis device (EDX) used to determine the elements present on SiNW surface. Electrochemical experiments were performed using a potentiostat galvanostat VMP3. The sample was mounted in a PTFE holder
6
with copper rear contact. A gold wire was used as counter electrode and all potentials were relative to the 0.01 M Ag+/Ag system. We used a remote-computer controlled goniometer system (DIGIDROP) for measuring contact angles. Water contact angles were measured using deionized water. The accuracy is ± 2°. All measurements were made in ambient atmosphere at room temperature.
3. Results and discussion 3.1. Metal assisted chemical etching of Si Figure 1 displays SEM images of the resulting surfaces. Figure 1a, shows branched silver dendrites formed on the surface of Si sample after etching in aqueous HF/AgNO3 solution at 50°C. Chemical removal of silver deposit using HNO3 acid at room temperature leds to the formation of well-aligned SiNW arrays (Figure 1b). The obtained nanowires are perpendicular to the surface and can be sometimes interconnected to form walls. They have a uniform length of 8 microns and a diameter between 10 and 100 nm. The top view of cleaned surface (figure 1c) shows a homogeneous surface. Silver dendrites deposition and silicon etching in fluoride solution containing Ag+ occurs according to the following equations: Ag+ +
1e+
Ag
Si + 4h + 4HF
SiF4 + 4H+
SiF4 + 2HF
H2SiF6
Figure 2.a shows ATR-FTIR spectrum in 700-4000 cm-1 range recorded from as-prepared SiNW. The strong absorption in 1000-1200 cm-1 range is due to the stretching vibrations of Si-O and Si-O-Si bonds. Upon rinsing with a dilute (10%) aqueous HF solution, new absorption bands attributable to Si-Hx (2000-2200 cm-1) were clearly observed, while the intensity of Si-O absorption bands decrease (figure 2b). Figure 2c shows the vibrational spectrum of HF-etched SiNW in the frequency range of Si-H stretching vibrations. Three broad overlapping bands can be observed. In accordance with the FTIR results of Si (111) and Si (100) wafers [29-31], these bands were assigned to the monohydride (SiH), the dihydride (SiH2), and the trihydride (SiH3), respectively.
3.2. Electrochemical polymerization Figures 3a and 3b show the cyclic voltammograms of thiophene electropolymerisation on SiNW surface with potential sweep rates of 20mV/s and 100mV/s, respectively. Figure 3a 7
shows two oxidation peaks of thiophene. The first one at 0.62V corresponds to the creation of the first active centers on the electrode surface. The second one, appears at 1.37V corresponding to the bipolaron structure of PTh. However, when sweeping at a rate of 100mV/s, one can observe only one peak at 1.37V for the two first cycles that shifts to 1.50V for the other scans. The peak onset of oxidation depends on the scanning speed. The shift of the oxidation peak potential from 1.37V to 1.50V can be explained by the modification of surface state due to polymer chain increase. Figures 4a and 4b show the cyclic voltammograms of pyrrole electropolymerisation on SiNW surfaces of 5 and 11µm length with a potential sweep rate of 50mV/s. The first scan in figure 4a shows an anodic peak at 0.72V which corresponds to the oxidation of the monomer into a radical cation and thus the formation of the first active center. Whilst, a cathodic peak is observed at 0,32V corresponding to the reduction of the polymer. One can observe a shift of the oxidation potential to 0.86V in the second cyclic voltammogram which is likely due to the structural change of the surface by increasing the polymeric chain or by a conformational change in the polymer chain. The start of polymerization on SiNW surface of 11µm length is shown on the cyclic voltammogram (Figure 4b) by a shoulder at about 1.5V which decreases with further scans due to the considerable decrease of active sites, indicating the evolution of the polymerization. However, no reduction peak is observed. From these results, it is clear that SiNW length significantly affects the appearance of monomer redox peaks (oligomer-polymer), and the value of the oxidation potential depends heavily on the surface state. The electropolymerization of vinylpyrrolidone on SiNW surfaces, at 50mV/s, of 5 and 11 µm length show, respectively, a shoulder at 1.83V (Figure 5a) and at 1.80V (Figure 5b). The potential value of the beginning of polymerization remains the same for the two nanowire lengths. However, in figure 5a we can observe the evolution of the polymerization indicated by the polymer oxidation potential decrease while we note the absence of a polymerization peak on SiNW surface of 11 µm length (Figure 5b) which is probably caused by a surface resistance of long nanowires.
3.3. ATR-FTIR spectroscopy analysis ATR-FTIR spectroscopy analysis was used to characterize the changes associated with the surface composition upon SiNW functionalization. FTIR spectrum of SiNW/PTh surface (Figure 6) exhibits bands between 3200 and 2840 cm−1 corresponding to C-H stretching vibrations of thiophene, and its bending vibrations appear between 1450 and 1300 cm−1. The 8
band at 1630 cm−1 is attributed to C═C stretching vibrations. A wide band between 1250 and 970 cm-1 comprising multiple bands is corresponding to the bending vibrations of C-H bonds of thiophene cycle and to the stretching vibrations of Si-C, Si-O-C and Si-O. C-S stretching band of thiophene is appearing at about 862 cm−1 and the corresponding bending vibration appears at 733 cm-1. A small band located around 777 cm-1 is assigned to the bending vibrations of the double bond C═C-H of thiophene cycle. Figure 7 shows FTIR spectrum of SiNW/PPy surface. We can observe a large vibration band between 3020 and 2824 cm−1 ascribed to C-H stretching vibrations of CH2 groups, and its bending vibrations appear between 1478 and 1381 cm−1. The sharp band located at 1727 cm−1 can be attributed to carbonyl C═O groups fixed in ß position of some pyrrole rings and which result from a light overoxidation of the polymer occurring during its electrosynthesis. Likewise, we observe a band situated at 1655 cm−1 which is attributed to C═C stretching vibrations. The band set between 1646 and 1461 cm-1 is attributed to pyrrole ring stretching vibrations. The sharp band at 1032 cm−1 corresponds to N-H in-plane deformation. The band located at 1280 cm−1 is related to Si-C stretching vibrations mode. The deconvolution of a large band between 1250 and 950 cm-1 shows the presence of numerous contributions namely Si-O-Si symmetric (1100 cm-1) and asymmetric (1180 cm-1) stretching vibrations, C-N stretching vibrations at 1132 cm-1, C-C stretching vibrations at 1054 cm-1 and finally N-H binding vibrations at 1032 cm-1. Two bands located at 741 and 678 cm−1 are attributed to the binding out-of-plane vibrations of N-H of pyrrole ring and ═C-H. ATR-FTIR spectrum of SiNW/PVP surface represented in figure 8, shows the presence of a large vibrational band between 3018 and 2807 cm−1 corresponding to CH2 and CH3 stretching vibrations. Bending vibrations of CH2, CH3 and methylene on α position of carbonyl appear between 1483 and 1342 cm−1. We observe a band situated at 1665 cm−1 which is attributed to C═O stretching vibrations. The band around 1280 cm-1 is attributed to C-N stretching vibrations. The large band located between 1180 and 1100 cm−1 is related to Si-O-Si stretching vibrations.
3.4. SEM analysis Figures 9 and 10 show electrochemically deposited PTh films on SiNW surface by applying five and two voltammetric cycles, respectively. The picture in figure 9a shows that after five voltammetric cycles the polymer covers the whole nanowires from top to bottom. However, the covering is done by nanowire bunches and not each wire individually. The first stages of
9
the polymer formation are displayed in figure 10 after applying just two voltammetry cycles. It can be seen that the polymerization beginning occurs at the top of the nanowires bunches. EDX analysis (Figure 11) shows the presence of sulfur, silicon, oxygen and carbon which confirm the presence of polythiophene. Electropolymerized PPy on SiNW is represented in figure 12. One can note that the coating of the nanowires by PPy is made similarly as for the PTh. As shown figure 12b, after applying five voltammetric cycles, the thickness of deposited PPy on SiNW is about 0.38µ. The observation is the same by applying two voltammetric cycles (figure 13). In contrast, the elctropolymerization of the non-conductive polymer PVP shows a different morphology on SiNW surface. The polymerization occurs mainly on the nanowires top and we observe the absence of the polymer along the nanowires (figure 14). So, the good conductive properties of Pth and PPy ensure efficient charge transfer along the SiNW leading to the polymer covering over the entire length of the wire. Unlike, the non-conductive polymer PVP polymerizes on SiNW top.
3.5. Contact angle measurements Water contact angle measurements were used to follow the changes in the wetting properties of the freshly- prepared SiNW surface before and after functionalization (Figure 15). Asprepared hydrogen-terminated SiNW surface displays a hydrophobic character with a water contact angle of 124 °. The surface became more hydrophobic after the functionalization with conductive polymers. We observe a contact angle of 140° for SiNW/PTh surface and 143° for SiNW/PPy. These results coincide with those reported in the literature [32]. Whereas, we record a significant decrease of the contact angle of SiNW/PVP surface (29°), these results were due to the chemical structure of polymer.
Contact angle (°)
SiNW
SiNW/PTh
SiNW/PPy
SiNW/PVP
120.5 (a)
140(b)
143 (d)
29 (c)
4. Conclusion Hybrid structures were fabricated assembling SiNW and polymer using electrochemical way. The functionalization of SiNW surface by conductive and non-conductive polymers occurs in different way. Conductive polymer covers all the length of the nanowires while the non10
conductive polymer is deposited primarily on the surface top. The functionalization of surface with conductive polymers provides active surface which can increase and improve the transfer and the conductivity of these hybrid structures. This propriety can be used in electrochemical sensors, which is our current work and subject of our future publications.
Acknowledgement We acknowledge the National Research Fund (DGRSDT) and the Centre de Recherche en Technologie des Semi-conducteurs pour l’Energétique (CRTSE). Funding This work was supported by the National Research Fund (DGRSDT) and the Centre de Recherche en Technologie des Semi-conducteurs pour l’Energétique (CRTSE).
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Figure captions Figure 1: SEM images of: (a) branched silver dendrites formed on Si sample after etching in HF/AgNO3 aqueous solution at 50°C for 6 min, (b) Cross-sectional SEM image of SiNW array after rinsing in HNO3 Figure 2: ATR-FTIR spectrum of SiNW (a) as-prepared, (b) after rinsing in10% HF and (c) in the range of 2020-2150 cm-1..
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Fig.3: Cyclic voltammograms of anodic electropolymerization of thiophene monomer on SiNW surface (a) potential sweep rate of 20mV/s, (b) potential sweep rate of 100mV/s. Figure 4: Cyclic voltammograms of anodic electropolymerization of pyrrole monomer on SiNW surface (a) of 5µm length and (b) of 11μm length. Figure 5: Cyclic voltammograms of anodic electropolymerization of PVP on SiNW surface of (a) 5 µm length and (b) 11μm length. Figure 6: ATR-FTIR spectrum of SiNW/PTh surface. Figure 7: ATR-FTIR spectrum of SiNW/PPy surface. Figure 8: ATR-FTIR spectrum of SiNW/PVP surface. Figure 9: SEM micrographs of electropolymerized PTh on SiNW by applying five voltammetric cycles, (a) Cross-sectional view, (b) Plan view. Figure 10: SEM micrograph of electropolymerized PTh on SiNW by applying two voltammetric cycles Figure 11: EDX analysis of SiNW/ PTh surface. Figure 12: SEM micrographs of electropolymerized PPy on SiNW by applying five voltammetric cycles (a) 10000x magnification, (b) 20000x magnification. Figure 13: SEM micrographs of electropolymerized PPy on SiNW by applying two voltammetric cycles (a) 10000x magnification, (b) 20000x magnification. Figure 14: SEM micrographs of electropolymerized PVP on SiNW by applying five voltammetric cycles (a) Plan view, (b) Cross-sectional view. Figure 15: Water contact angle: (a) SiNW, (b) SiNW/PTh,(c) SiNW/PPy and (c) SiNW/PVP.
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(a)
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Figure 1
1.0 SiNW SiNW after rinsing in HF
Absorbance (a.u)
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Figure 2
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Figure 3
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Figure 14
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