Chemical Functionalization of Silicon Nanowires by an Electroactive

Feb 12, 2010 - by vapor-liquid-solid (VLS) process were modified directly on their growth substrate and then break away by ultrasonication and were an...
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Chemical Functionalization of Silicon Nanowires by an Electroactive Group: A Direct Spectroscopic Characterization of the Hybrid Nanomaterial Cle´ment Suspe`ne,† Re´gis Barattin,‡ Caroline Celle,† Alexandre Carella,† and Jean-Pierre Simonato*,† CEA-Grenoble, LITEN/DTNM/LCRE, and CEA-Grenoble, LETI/DTBS/LFCM, 17, rue des Martyrs, 38054 Grenoble Cedex 9, France ReceiVed: December 23, 2009; ReVised Manuscript ReceiVed: January 26, 2010

We report on the optical and electrochemical characterization of hybrid nanowires prepared by chemical functionalization of silicon nanowires (SiNWs) with triphenylamine (TPA) derivatives. The SiNWs synthesized by vapor-liquid-solid (VLS) process were modified directly on their growth substrate and then break away by ultrasonication and were analyzed directly in solution by UV-vis and fluorescence spectroscopies. These two techniques allowed a direct analysis of molecules grafted on the SiNWs whereas indirect methods are commonly used for this purpose. Coupled with X-ray photoelectron spectroscopy analysis, we demonstrate the effective anchoring of the molecules to the inorganic semiconductor and we suggest the formation of a cation radical TPA+•, which is attributed to an electronic transfer from the TPA to the silicon nanowire. 1. Introduction Silicon-based one-dimensional nanomaterials have recently attracted a great deal of attention because of the technological importance of silicon in diverse areas ranging from electronics, optoelectronics, energy, healthcare, and others.1-5 They can be prepared with reproducible electronic properties in high yield by various methods,6-8 like the vapor-liquid-solid (VLS) process,9 which implies that materials with distinct chemical compositions, structures, or sizes can be integrated into different systems within the framework of the bottom-up approach for nanotechnologies.2,10-12 The morphology and other properties of SiNWs such as photoluminescence, electron field emission, or thermal and electronic conductivities have been studied.1 Surface chemical reactivity of SiNWs has become of great interest to produce original nano-objects. In fact, chemical functionalization of SiNWs is emerging as an important area in the development of new semiconductor-based devices.13-18 Combining organic materials with conventional semiconductor technology provides indeed the design of hybrid devices with unique properties used, for example, for chemical or biological sensing.19,20 Characterization of the nano-objects formed after the grafting of molecules on nanowires is therefore crucial. Most techniques employed to determine properties of modified SiNWs are electrical and electrochemical methods in a field-effect transistor configuration or by observing a change of (trans-)conductance or resistance.21-23 However, it implies that SiNWs are already integrated into devices. Other mentioned techniques including X-ray photoelectron spectroscopy (XPS) or fluorescence spectroscopy are used at the solid state requiring SiNWs to be attached to their growth support.24 There is clearly a lack of direct characterization of these nano-objects when they are in solution. More generally, direct studies on interactions between grafted molecules and silicon nanowires are scarce so far to our knowledge.25 * To whom correspondence [email protected]. † LITEN/DTNM/LCRE. ‡ LETI/DTBS/LFCM.

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The highly electron-rich triphenylamine (TPA) is a unique molecule possessing useful properties such as redox activity, fluorescence, and ferromagnetism because of the high oxidizability of the nitrogen center and the transportability of positive charge centers via the radical cation species (TPA+•).26 Today, the TPA derivatives have been chosen as important candidates for electrical and optical applications such as organic photoconductors, electroluminescence, and electrochromic devices.27-29 Because of their optical properties and their electroactive site, TPA derivatives appear to be good candidates for studies focused on functionalization of SiNWs and on interactions between the inorganic semiconductor and the grafted molecules. Herein, we report a direct spectroscopic study of SiNWs dispersed in ethanol before and after their functionalization by TPA derivatives. We took care to characterize the organic moieties previously by optical and electrochemical based techniques. Spectroscopic analyses are expected to reveal the attachment of this derivative to the nanowires and to evidence an electronic transfer between the organic moiety and the inorganic semiconductor. Contrary to most published results in this field, we show that a direct characterization of chemically functionalized nanowires is possible. 2. Experimental Section 2.1. Materials. All chemical reagents were purchased from Aldrich with analytical or chemical purity and were used as received. Organic solvents were degassed by purging with argon and were dried before use. Water was purified by a Milli-Q system (Millipore). Silicon samples were single-side-polished wafers (100-oriented, p-type, 0.20-0.30 Ω · cm resistivity) purchased from Siltronix. For electrochemical experiments, propylene carbonate was distilled under vacuum on sodium, and tetrabutylammonium hexafluorophosphate (n-Bu4NPF6) was dried under vacuum. 2.2. Synthesis. The molecule 1-(4-(diphenylamino)phenyl)3-(3-(triethoxysilyl)propyl)urea (TPASi) was synthesized according to a procedure described in a previous work.30 The alkyne derivative of triphenyamine named TPAA (4-ethynyl-

10.1021/jp912118m  2010 American Chemical Society Published on Web 02/12/2010

Chemical Functionalization of Silicon Nanowires N,N-diphenylaniline) was prepared according to a method from literature31 with modifications presented below. 2.2.1. Synthesis of p-(Trimethylsilyl)ethynyl)triphenylamine. A 100 mL three-necked flask was equipped with a three-way stopcock and a magnetic stirring bar and was flushed with dry nitrogen. The following were added to the flask: 0.5 g (1.5 mmol) of p-bromotriphenylamine, 160 µL (1.15 mmol) of ethynyltrimethylsilane, 24.3 mg (0.035 mmol) of bis(triphenylphosphine)palladium(II) dichloride, 270 µL (1.5 mmol) of diisopropylethylamine, and 5 mL of dimethylformamide. The reaction mixture was heated with stirring at 60 °C for 15 h and then was cooled to room temperature. Thirty milliliters of dichloromethane were then added, and an insoluble salt was filtered. The filtrate was washed with 1 N hydrochloric acid and then with water. The organic solution was dried over anhydrous Na2SO4 and was concentrated with a rotary evaporator. Purification of the crude product by flash silica gel column chromatography eluted with n-hexane provided a yellow oil (0.4 g, yield: 89%). 1 H NMR (200 MHz, CDCl3, 300 K): δ (ppm) ) 0.24 (s, 9H, SiMe3), 6.92 (m, 2H, HAr), 7.09 (m, 6H, HAr), 7.28 (m, 6H, HAr). 13 C[1H] NMR (200 MHz, CDCl3, 300 K): δ (ppm) ) 2.5 (SiMe3), 98.1 (C≡CSiMe3), 106.3 (C≡CSiMe3), 119.2 (CAr), 123.2 (CAr), 124.5 (CAr), 126.5 (CAr), 129.8 (CAr), 135.6 (CAr), 142.0 (CAr), 142.8 (CAr). MALDI-TOF: m/z ) 341.15 [M]+•. IR (ATR, cm-1): 3075 (ν C-HAr), 2154 (ν C≡C), 1591 and 1490 (ν CdCAr), 1281 (ν C-N), 841 (δ Si-CH3 (rocking)). 2.2.2. Synthesis of 4-Ethynyl-N,N-diphenylaniline (TPAA). A mixture of 0.2 g (0.6 mmol) of the protected alkyne dissolved in 20 mL of dichloromethane/methanol 1:1 and 0.65 g (4.7 mmol) of potassium carbonate was stirred at room temperature in a 100 mL round-bottom flask for 15 h. Residual salt was filtered, and the organic layer was washed with a saturated solution of sodium chloride, was dried over anhydrous Na2SO4, and was concentrated to afford a yellow solid. Purification of the crude product by silica gel column chromatography eluted with n-hexane/ethyl acetate 90:10 provided a pale yellow solid (0.15 g, yield: 95%). 1 H NMR (200 MHz, CDCl3, 300 K): δ (ppm) ) 3.06 (s, 1H, C≡CH), 7.03 (m, 2H, HAr), 7.12 (m, 6H, HAr), 7.32 (m, 6H, HAr). 13 C[1H] NMR (200 MHz, CDCl3, 300 K): δ (ppm) ) 77.2 (C≡CH), 83.9 (C≡CH), 114.8 (CAr), 122.1 (CAr), 123.7 (CAr), 125.0 (CAr), 129.4 (CAr), 133.1 (CAr), 146.4 (CAr), 147.1 (CAr). MALDI-TOF: m/z ) 270.13 [M + H]+. IR (ATR, cm-1): 3290 (ν C-H), 3039 (ν C-HAr), 2105 (ν C≡C), 1590 and 1490 (ν CdCAr), 1266 (ν C-N). UV/vis (EtOH): λmax/nm (ε/L mol-1 cm-1) ) 229 (19 800), 301 (24 300), 327 (25 200). mp ) 107-108 °C (lit.31 104-105 °C). 2.3. Growth of SiNWs. SiNWs were prepared by chemical vapor deposition at 500 °C via the gold-catalyzed VLS method as previously described using SiH4 as silicon precursor.32 After synthesis, gold catalyst and gold traces remaining along the sides were etched by a KI/I2 treatment. 2.4. Functionalization of SiNWs with TPASi. A piece of silicon wafer supporting SiNWs was first washed with acetone and ethanol and was blown dry with nitrogen. Then, it was immersed in a piranha solution (1 vol 30% wt aqueous hydrogen peroxide to 3 vol sulfuric acid) for 15 min, was rinsed with an excess amount of deionized water, and was blown dry with nitrogen. It was exposed to UV light from a low-pressure

J. Phys. Chem. C, Vol. 114, No. 9, 2010 3925 mercury lamp for approximately 45 min in air. The UV lamp generated ozone which oxidized organic contaminants and left a carbon-free surface.24 After that, the SiNWs were reacted in a flask in a solution of toluene containing the precursor TPASi (10-3 M) at 80 °C for 4 h under argon. Finally, the piece was washed with ethanol and acetone and was blown dry with nitrogen. 2.5. Grafting of TPAA on Silicon Substrates. TPAA was grafted on Si substrates according to two different routes, thermal hydrosilylation and “click chemistry”.33 2.5.1. Grafting by Hydrosilylation. A piece of p-doped silicon wafer (100) (approximately 5 × 10 mm2) was immersed in a piranha solution (1 vol 30% wt aqueous hydrogen peroxide to 3 vol sulfuric acid) for 15 min, was rinsed with excess amount of deionized water, and was blown dry with nitrogen. It was then dipped in a 1% HF solution for 2 min, was quickly rinsed with excess amounts of deionized water, and was blown dry with nitrogen. It was reacted in a flask in a solution of mesitylene containing TPAA (10-3 M) at 180 °C for 2 h under argon, was rinsed with dichloromethane and ethanol, and finally was blown dry with nitrogen. 2.5.2. Grafting by Click Chemistry. A piece of p-doped silicon wafer (100) (approximately 5 × 10 mm2) was immersed in a piranha solution (1 vol 30% wt aqueous hydrogen peroxide to 3 vol sulfuric acid) for 15 min, was rinsed with excess amount of deionized water, and was blown dry with nitrogen. It was then dipped in a 1% HF solution for 2 min, was quickly rinsed with excess amounts of deionized water, and was blown dry with nitrogen and was further dried under vacuum. It was reacted in a flask in a solution of mesitylene containing 6-chlorohex1-ene (10-3 M) at 180 °C for 2 h under argon, then was rinsed successively with acetone, ethanol, and dichloromethane, and was blown dry with nitrogen. Chloride was then exchanged by azide by reaction of the silicon substrate with a saturated solution of sodium azide in dimethylformamide (DMF) at 80 °C overnight.34 The surface was rinsed successively with excess amounts of deionized water, acetone, ethanol, and dichloromethane and then was blown dry with nitrogen. The azidoterminated surface was then reacted in a solution of dichloromethane containing TPAA (10-3 M), copper iodide (2% mol/ TPAA), and N,N-diisopropylethylamine (DIEA, 2% mol/TPAA) at room temperature for 10 h. The silicon substrate was finally rinsed with dichloromethane and ethanol and was blown dry with nitrogen. 2.6. Instruments. 1H and 13C NMR spectra were recorded on a Bru¨ker Avance 200 MHz spectrometer with TMS as the internal reference using CDCl3 as solvent in all cases. IR spectra were measured in ATR Pike Germanium mode on a Bru¨ker Vertex 70. UV-vis spectra were recorded as a dilute solution in absolute EtOH on a Cary 5000 UV-vis-NIR spectrophotometer from Varian. Mass spectra were obtained with a Bru¨ker Daltonics Ultraflex MALDI TOF/TOF mass spectrometer with dithranol as the reference. Fluorescence spectra, including the excitation spectrum and the emission spectrum, were recorded on an Edinburgh FS920 fluorescence spectrophotometer. Scanning electron microscopy (SEM) was performed on an SEMFEG (Leo 1530). All X-ray photoelectron spectroscopy (XPS) data were recorded using an ultrahigh vacuum XPS system SSISProbe equipped with a monochromatized Al KR X-ray source (1486.6 eV) at a 35° incident angle measured from the sample surface. All binding energies were referenced to the C1s hydrocarbon peak at 284.6 eV. Electrochemical properties were measured at different scan rates by cyclic voltammetry (CV)

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SCHEME 1: Synthesis of TPAA

on an Autolab potentiostat PGSTAT 12 with a three-electrode configuration. 3. Results and Discussion Before their grafting on SiNWs, TPA derivatives were characterized optically and electrochemically in solution and on Si substrates to prove their ability to be used as models for our spectroscopic study on functionalized SiNWs. 3.1. Synthesis. The TPA derivative used for optical and electrochemical studies is TPAA whose synthesis is outlined in Scheme 1. Preparation of this compound was achieved by a two-step synthesis. The intermediate p-((trimethylsilyl)ethynyl)triphenylamine was obtained starting from p-bromotriphenylamine by a Sonogashira coupling with ethynyltrimethylsilane in the presence of Pd(PPh3)2Cl2 and DIEA under conditions adapted from literature.31 Our procedure does not include the use of copper iodide and additional triphenylphosphine. Deprotection of the alkyne group was carried out with K2CO3 in a mixture of dichloromethane/methanol 1:1 to give TPAA with a global yield of 85%. The triple bond is introduced on the TPA moiety to further enable the grafting of the molecule on Si surface. The structures of the compounds were confirmed by IR, 1H and 13C NMR, and mass spectrometry. 3.2. Optical Properties of TPAA. The optical properties of TPAA were investigated using UV-vis spectroscopy. The UV-vis absorption spectrum of TPAA in EtOH is presented in Figure 1. It is composed of absorption bands below 260 nm related to benzenoid transitions of phenyl groups and of strong absorption bands between 270 and 400 nm (ε ≈ 25 000 L mol-1 cm-1) corresponding to π-π* and n-π* transitions of a diphenylamine unit and of the entire conjugated backbone of phenylacetylene. These bands are better separated in a polar solvent as EtOH than in the case of CHCl3.31 The onset absorption of the UV-vis spectrum of TPAA is about 400 nm, which corresponds to an energy band gap of 3.1 eV by using the formula Egap ) 1240/λonset. 3.3. Electrochemical Properties of TPAA. The electrochemical properties of TPAA were measured by cyclic voltammetry (CV) at room temperature at different scan rates ranging from 0.1 V s-1 to 2 V s-1. CV was carried out in a three-electrode cell setup with 0.1 M n-Bu4NPF6 as a supporting electrolyte in propylene carbonate, Pt wires as the working and

Figure 1. UV-vis spectrum of TPAA in EtOH (absorbance, AU). Concentration: 6 × 10-5 M.

Figure 2. Cyclic voltammogram of TPAA in propylene carbonate with 0.1 M of n-Bu4NPF6 at different scan rates. Redox current is shown as a function of the working electrode voltage.

the counter electrodes, and a saturated calomel electrode (SCE) as the reference electrode. As shown in Figure 2, TPAA exhibits a reversible oxidation process with an onset oxidation potential (Eonset) estimated from the anodic oxidation wave at a scan rate of 0.1 V s-1 to be 0.85 V. This value is very close to the data reported for TPA (0.9 V).35 The oxidation process at half-wave potential E1/2 ) 0.95 V for an average scan rate of 0.75 V s-1 is attributed to the oxidation of the TPA segment which results in a radical cation. The energy level of the highest occupied molecular orbital (HOMO) was calculated according to an empirical formula EHOMO ) -(Eonset + 4.74) eV36 to be -5.59 eV. The energy level of the lowest occupied molecular orbital (LUMO) was determined from the values of the energy band gap and HOMO energy level by the relation ELUMO ) EHOMO + Egap ) -2.49 eV. A high-lying HOMO energy level and a reversible electrochemical oxidation are in agreement with the fact that TPAA is suitable for hole injection and transport properties for OLED applications as previously observed for other TPA derivatives.37 3.4. Electrochemical Properties of TPAA Grafted on Si Substrates. TPAA was grafted on pieces of silicon wafers (100) by thermal hydrosilylation or click chemistry to characterize the electrochemical behavior of the TPA moiety linked to silicon substrates. On one hand, as described in Scheme 2, the silicon surface was first etched with a dilute solution of HF, and then hydrosilylation proceeded by immersion of Si-H surface in a solution of TPAA (10-3 M) in mesitylene at 180 °C. This reaction resulted in TPA directly grafted on Si surface through a vinylic bond. On the other hand, as described in Scheme 3, Si-H surface was dipped in a dilute solution of 6-chlorohex1-ene (10-3 M) in mesitylene at 180 °C to allow the formation of a chloro-terminated silicon surface, which was reacted a second time with a solution of sodium azide in DMF at 100 °C to form the azide precursor for the Huisgen 1,3-dipolar cycloaddition reaction. This reaction proceeded with TPAA, copper iodide, and DIEA in dichloromethane to create the triazole derivative.33,34 The electrochemical properties of TPAA grafted on Si substrates were measured by cyclic voltammetry (CV) at room temperature at different scan rates ranging from 0.1 V s-1 to 1.5 V s-1. CV was carried out in a three-electrode cell setup with 1 M n-Bu4NPF6 as a supporting electrolyte in propylene carbonate, the Si substrates as working electrode, a Pt wire as the counter electrode, and SCE as the reference electrode. Figure 3 displays the two voltammograms recorded for the two types

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SCHEME 2: Direct Grafting of TPAA on Si Substrate by Hydrosilylation

SCHEME 3: Grafting of TPAA on Si Substrate by Click Chemistry with a C6-Spacer

of modified Si surface. They exhibit in each case a reversible oxidation wave as it was previously observed for free TPAA. As shown in Figure 3a and b (insets), the cathodic peak current exhibits an almost linear dependence on the scan rate indicating that the triphenylamine entities are surface-confined electroactive molecules and that the electrochemical processes observed on the surface are only due to the surface-bound species. The halfwave potentials are located at E1/2 ) 0.65 and 0.90 V for an average scan rate of 0.75 V s-1 for the direct grafting by hydrosilylation (Figure 3a) and the grafting by click chemistry via a six-carbon-long linker (Figure 3b), respectively. As a consequence, the oxidation occurs at a higher voltage for TPAA in solution and for the TPA derivative linked to the Si surface by the triazole group (click chemistry) than for the TPA derivative linked to the Si surface by a Si-C bond (direct grafting). This can be mainly ascribed to the electron-withdrawing effect of the triazole group, and of the triple bond, which is higher than that of the double bond. These oxidation redox potentials agree with the Hammet relationship in the electrondonating/withdrawing properties of the substituents at the paraphenyl positions of TPA.38 Comparison of the three voltammograms (Figure 2 and Figure 3) allows us to conclude that the oxidation of the TPA moiety remains reversible in solution and after grafting on Si substrates even when electron-withdrawing groups are present at the paraphenyl positions. Such a stabilization of the cation radical for a TPA-functionalized Si substrate was not obvious since electron-withdrawing groups like nitro or halide tend to deactivate this cation radical.39 In the case of the click chemistry based grafting method, the system tends to be more quasireversible than fully reversible. Indeed, as it can be seen in Figure 3b, the anodic and cathodic peak potentials shift with the scan rate. This scan-rate dependence of the peak separation can be explained by a slower electron transfer from the TPA moiety to the silicon surface through the six-carbon-long linker.34 A better resolution of the oxidation wave is also remarked in this case compared to the direct grafting of TPAA by hydrosi-

lylation, which could be related to a higher density of organic groups grafted on the surface. Surface coverage for both surfaces was estimated by cyclic voltammetry: for the substrate functionalized by hydrosilylation, surface coverage is approximately 6.2 × 10-12 mol cm-2, whereas for the substrate functionalized by click chemistry, it is ca. 1.6 × 10-11 mol cm-2. In fact, when bulky groups are directly grafted by hydrosilylation, the surface coverage is lower because of steric hindrance.40,41 This implies an oxidation of the silicon surface leading to the measurement of an important capacitive current, which is indeed observed in Figure 3a. Finally, TPA derivatives exhibit interesting optical properties and an ability to easily give an electron by formation of a stable cation radical in solution when grafted on Si substrates. Hence, it makes this molecule a good model for spectroscopic studies on the grafting of electroactive organic moieties onto SiNWs. 3.5. Functionalization of SiNWs by TPA Derivatives and Spectroscopic Characterization. SiNWs were grown via the vapor-liquid-solid (VLS) mechanism on silicon substrates according to a previously reported procedure.32,42-44 SEM images were obtained to characterize the growth of SiNWs. Figure 4 shows SEM images of the SiNWs; their average diameter was estimated to be ca. 100 nm. The higher-resolution image shows catalyst droplets remaining at the tip of SiNWs which is characteristic of the VLS mechanism. These droplets as well as tiny amounts of gold present along the walls of SiNWs (not shown) were etched by a KI/I2 treatment before chemical modification of the surface of SiNWs. The TPA derivative chosen for the functionalization of SiNWs is TPASi, a molecule previously synthesized for preparation of self-assembled monolayers (SAMs) on silicon dioxide.30 Grafting of TPASi on SiNWs was achieved by silanization as depicted in Scheme 4. A piece of Si substrate supporting SiNWs (approximately 10 × 10 mm2) was first cleaned by piranha and UV/ozone treatments and was immersed in a solution of toluene containing TPASi (10-3 M) at 80 °C for 3 h under argon. Silanization was chosen to be the method of

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Figure 4. Cross-sectional SEM images of as-prepared SiNWs on Si substrate. Scale bars are 1 and 10 µm.

Figure 3. Cyclic voltammograms of TPAA grafted on Si substrate (a) by direct hydrosilylation and (b) by click chemistry via a C6-linker in propylene carbonate with 1 M of n-Bu4NPF6 at different scan rates. Redox currents are shown as a function of the working electrode voltages. Insets display peak current (µA) as a function of the potential scan rate (V s-1).

grafting preferentially to hydrosilylation in order to create a higher density of organic compounds around SiNWs and thus to maximize the intensity of spectra recorded by spectroscopic techniques. After a thorough rinse of the substrate with ethanol and acetone and blowing dry with nitrogen, SiNWs were cleaved from their growth substrate by sonication at 160 W in ethanol for 1 min. The solution was centrifuged, and the supernatant was pipeted off (carefully avoiding removal of solid material) and analyzed by UV-vis spectroscopy to check that no free TPASi was remaining. SiNWs were finally dispersed in 1 mL of absolute ethanol. The ethanol solution containing functionalized SiNWs was qualitatively analyzed by fluorescence and UV-vis spectroscopies. Excitation and emission spectra of solutions of TPASi in ethanol (1 × 10-5 M) and modified SiNWs are presented in Figure 5. The Stoke’s shift for the free TPASi in ethanol (red curves) determined by the difference between the wavelengths corresponding to the emission and excitation maxima was found to be 50 nm. By examining the excitation and emission spectra of the solution of functionalized SiNWs (blue curves), signals related to the TPA moiety could be recovered meaning that organic compounds were effectively attached to the nanomaterials. The Stoke’s shift is in this case equal to 87 nm, which is higher than for TPASi in solution. This could be due to a modification of the environment of the

TPA moiety once grafted on SiNWs and to the fact that nonradiative recombinations take place more frequently than for the free molecule in solution as the Stoke’s shift is caused by the nonradiative recombinations, such as Auger recombination or surface recombination.45 Furthermore, it can be noticed that the emission spectra show a characteristic partially resolved vibronic structure that is probably due to the coupling of the exciting transitions to the stretching vibrations. Results obtained by UV-vis spectroscopy are presented in Figure 6. The blue curve represents the absorption of the precursor TPASi in a dilute solution of EtOH (9 × 10-5 M), the green curve is related to the absorption of a solution of pristine SiNWs in EtOH, the pink curve corresponds to the absorption of TPASi functionalized SiNWs dispersed in EtOH, and the orange curve is the difference between the pink and the green curves. Unfunctionalized SiNWs were degrafted from their substrate directly after the VLS growth by sonication in EtOH. The absorbances of pink and green curves are multiplied by a factor of 4 and the orange one by a factor of 8 to allow an easier comparison between spectra. The spectrum of TPASi in solution exhibits absorption bands below 260 nm related to benzenoid transitions of phenyl groups and exhibits the characteristic band of TPA at 300 nm corresponding to π-π* and n-π* transitions.46 The onset absorption of this spectrum is about 340 nm, which is lower than in the case of TPAA meaning that the urea group does not show an electron-withdrawing behavior and that conjugation is located on the TPA core. The spectrum of unfunctionalized SiNWs dispersed in EtOH presents a main absorption band located around 400 nm, which is associated with the Γ25-Γ15 direct transition observed for silicon. The latter is normally positioned at 364 nm (3.4 eV), and its bathochromic shift could be explained by quantum confinement effects resulting from small nanocrystals which are preferentially formed at the SiOx/Si interface in SiNWs because of nucleation kinetics.47 Nevertheless, the red-shift of this transition does not seem to be a problem because, on the contrary, it decreases the absorbance of the X4-X1 direct transition around 300 nm which could hide the characteristic peak of TPA. The spectrum

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SCHEME 4: Grafting of TPASi on SiNWs by Silanization

obtained by substraction between the pink and the green curves represents the absorption of the TPA grafted on SiNWs and confirms the fluorescence results: the organic moiety is effectively attached to the inorganic semiconductor. By comparing this spectrum and the one of TPASi in EtOH (blue), it can be observed that benzenoid transitions are located at the same positions below 260 nm but that many differences appear in the case of the band corresponding to π-π* and n-π* transitions. For the molecule grafted on SiNWs, this band is indeed larger, red-shifted by around 10 nm and less intense. This could be due to the modification of the environment of TPA moiety previously mentioned and to a partial or slow electronic transfer between the organic group and the inorganic semiconductor. This would suggest that an oxidation of TPA core could occur even though it is difficult to clearly observe characteristic bands of the cation radical around 370 nm and 600-650 nm. XPS spectra of TPASi functionalized SiNWs were recorded to clear this issue. As it is not possible to carry out XPS experiments on SiNWs dispersed on a substrate because of their

Figure 5. Non-normalized excitation and emission spectra for (a) TPASi in EtOH (1 × 10-5 M) (red curves) and (b) TPASi functionalized SiNWs dispersed in EtOH (blue curves). Excitation wavelengths in both cases are equal to 300 nm.

Figure 6. UV-vis spectra in EtOH of (a) TPASi (blue curve, concentration 9 × 10-5 M), (b) unfunctionalized SiNWs (green curve), (c) TPASi functionalized SiNWs (pink curve), and (d) TPASi functionalized SiNWs - unfunctionalized SiNWs (orange curve).

size and shape, XPS measurements were realized on SiNWs still attached to their growth substrate, before the sonication step, to analyze a dense pack of nano-objects. Figure 7a displays the Si(2p) spectrum with two peaks at 99.6 and 103.2 eV corresponding to Si-Si and Si-O bonds, respectively. Each peak holds two components visible after deconvolution resulting from the spin-orbit splitting of the Si 2p energy levels. The C(1s) spectrum depicted in Figure 7b consists after deconvolution of a peak at 285.2 eV related to the C-C bond, a component at 286.7 eV linked to C-N and C-O bonds of the molecule and the remaining EtOH groups, and a peak at 289.1 eV attributed to the CdO bond of the urea group. The N(1s) spectrum in Figure 7c shows a single peak at 400.6 eV corresponding to N-C and N-H bonds of the TPA and urea group. However, this peak is not symmetrical and after deconvolution it appears that a component is located at 402.1 eV. As the area over 401 eV refers to charged or electron-poor nitrogen atoms,48 and according to the facts that no N-O bond is present and that no protonation can occur,49 this peak is assigned to the cation radical TPA+•. These measurements do not rule out the possibility that TPASi might link to both SiNWs and the underlying SiO2 substrate, but the signal visible on XPS N(1s) spectrum is given by a majority of nitrogen atoms from the molecular layer around SiNWs because of geometric factors.24 Finally, all the spectroscopic methods used to characterize TPASi functionalized SiNWs tend to prove the presence of the cation radical TPA+• and thus that an electronic transfer occurs between the organic part and the SiNWs. Such a transfer could result from a mechanism of electron donation out of the highest occupied molecular orbital (HOMO) of the TPA derivative into SiNWs as it was recently described in the case of a terpyridine molecule grafted on SiNWs.50 4. Conclusion TPA derivatives were optically and electrochemically characterized in solution and after grafting onto Si substrates. They showed optical properties and a reversible oxidizability that enable them to serve for investigation of modified SiNWs. XPS, UV-vis, and fluorescence spectroscopies have validated the grafting of organic molecules on SiNWs and have evidenced the formation of the cation radical TPA+• by the presence of a peak at 402.1 eV on the N(1s) XPS spectrum and by a modification of the band related to the π-π* and n-π* transitions comparing to the free precursor in solution suggesting that an electronic transfer occurs between the organic part and the SiNWs. All these results could be useful for further integration of chemically modified SiNWs into devices for

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Figure 7. XPS spectra (a) silicon(2p), (b) carbon(1s), and (c) nitrogen(1s) for TPASi functionalized SiNWs on SiO2/Si.

electronic or photovoltaic applications and for other fields requiring design of organic-inorganic interactions in hybrid nanomaterials. Acknowledgment. The authors would like to thank E. De Vito for XPS spectra, G. Delapierre for grafting protocol through click chemistry, and E. Rouviere and C. Mouchet for their help in the synthesis of SiNWs. References and Notes (1) Cui, Y.; Lieber, C. M. Functional Nanoscale Electronic Devices Assembled Using Silicon Nanowire Building Blocks. Science 2001, 291 (5505), 851–853. (2) Lu, W.; Lieber, C. M. Nanoelectronics from the bottom up. Nat. Mater. 2007, 6 (11), 841–850. (3) Kuchibhatla, S. V. N. T.; Karakoti, A. S.; Bera, D.; Seal, S. One dimensional nanostructured materials. Prog. Mater. Sci. 2007, 52 (5), 699– 913. (4) Baca, A. J.; Ahn, J. H.; Sun, Y.; Meitl, M. A.; Menard, E.; Kim, H. S.; Choi, W. M.; Kim, D. H.; Huang, Y.; Rogers, J. A. Semiconductor

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