Surface Modification of Silicon Nanowires via Copper-Free Click

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Surface Modification of Silicon Nanowires via Copper-Free Click Chemistry Anders Henriksson,† Gernot Friedbacher,‡ and Helmuth Hoffmann*,† † ‡

Institute of Applied Synthetic Chemistry, Vienna University of Technology, Getreidemarkt 9/163, A-1060 Wien, Austria Institute of Chemical Technology and Analytics, Vienna University of Technology, Getreidemarkt 9/164, A-1060 Wien, Austria ABSTRACT: A two-step process based on copper-free click chemistry is described, by which the surface of silicon nanowires can be functionalized with specific organic substituents. A hydrogen-terminated nanowire surface is first primed with a monolayer of an R,ω-diyne and thereby turned into an alkyneterminated, clickable platform, which is subsequently coupled with an overlayer of an organic azide carrying the desired terminal functionality. The reactive, electron-deficient character of the employed diyne enabled a quantitative coupling reaction at 50 °C without metal catalysis, which opens up a simple and versatile route for surface functionalization under mild conditions without any potentially harmful additives.

’ INTRODUCTION Over the last decades, impurity doping has been the main technique to tune the electronic properties of semiconductors by changing the density of the mobile charge carriers. As device sizes continue to shrink and approach nanoscale dimensions, this concept is no longer applicable because the correspondingly low numbers of dopant atoms become subject to serious fluctuations and device-to-device variations. A promising alternative, which benefits from the exponential increase of the surface-area-tovolume ratio with decreasing device dimensions, is surface modification.1 5 It has been shown recently1 3 that organic molecules adsorbed onto the channel silicon layer of a field-effect transistor change the channel conductance in a similar way to conventional doping, whereby a charge transfer between the device channel and the adsorbed molecules is assumed to be responsible for this effect: Electron-donating groups inject negative charges into the device layer and act as n-type dopants, whereas electron-accepting groups withdraw negative charges and enhance p-type conduction. A reversible transition from p-type to n-type conductivity was recently achieved with silicon nanowires6 (SiNW) by exposing them to an either acidic or basic ambient gas. Acetic acid vapor enhanced p-type conduction by more than 4 orders of magnitude, whereas ammonia yielded a similar increase in n-type conduction. A thin water layer acting as an electron-donating or electron-accepting electrolyte was believed to be responsible for this observation. In order to exploit this effect and fine-tune the electrical device properties via their surface composition, a versatile and reliable protocol for attaching diverse chemical functionalities to low-dimensional silicon would be desirable. A previously described chlorination/alkylation procedure applied to SiNW7 yielded alkyl and alkenyl surface layers with excellent protection against silicon oxidation, r 2011 American Chemical Society

but with limited options for further functionalization. We report here on a process using click chemistry on silicon nanowires for surface modification. The workhorse of click chemistry is the copper-catalyzed azide alkyne cycloaddition (CuAAC),8 which has proven its versatility and efficiency not only for solution phase reactions but also for surface modifications of a broad range of solid substrates9 including flat silicon wafers10 and porous silicon.11 For certain applications, however, the use of a metal catalyst is precluded, for example, in biological environments due to the cytotoxic properties of copper12 or for surface modifications of electronic materials, where traces of copper are retained on the surface and alter the electronic properties dramatically.13 Copper ions can also cause degradation of DNA molecules,14,15 induce protein denaturation,16 and inhibit the luminescence of quantum dots attached via click chemistry to biomolecules for in vivo imaging purposes.17 Omission of the catalyst, however, slows down this reaction by 7 orders of magnitude18,19 and thus requires some alternative activation of the CtC triple bond. Bertozzi’s group has pioneered the ringstrain promoted reaction of cyclooctynes with azides,12 which has been used for bioorthogonal labeling and surface immobilization of proteins20 as well as for tagging living cells and organisms with fluorescent markers.21 Alternatively, electronwithdrawing groups adjacent to the alkyne group enable catalystfree azide alkyne coupling in high yields at moderate temperatures both in solution22 24 and on surfaces.25 28 In addition, electron-deficient alkynes are also considerably more reactive with respect to their covalent bonding to H-terminated silicon Received: March 14, 2011 Revised: May 4, 2011 Published: May 17, 2011 7345

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surfaces.29 31 Thus, the symmetrical, electron-deficient R,ωdiyne 1,2-ethanediol dipropiolate (EDDP) was chosen here to convert H-terminated SiNW surfaces into a “clickable” monolayer platform, whose terminal alkyne groups were subsequently reacted in a Huisgen 1,3-dipolar cycloaddition with an alkyl azide.

’ EXPERIMENTAL SECTION Chemicals and Materials. All chemicals and solvents were purchased from Sigma-Aldrich and were used as received. Silicon wafers ((100) oriented, N(Phosphorus)-doped, single-sided polished, 0.5 mm thickness, 1 5 Ω cm resistivity) were purchased from Si-Mat Silicon Materials (Landsberg, Germany) and were used for the preparation of nanowires. Synthesis. 1,2-Ethanediol-dipropiolate (EDDP) was synthesized according to a general procedure described in ref 32. Ethylene glycol (5.5 mL, 0.1 mol) and propiolic acid (19 mL, 0.3 mol) were dissolved in toluene (100 mL). p-Toluenesulfonic acid (4 g, 0.021 mol) was added and the solution was refluxed for 5 h while continuously removing water via the water toluene azeotrop (bp 83 °C). The remaining solution was diluted with 300 mL of toluene, washed with water, dried with anhydrous MgSO4, concentrated, and distilled (bp 50 °C at 10 2 mbar), yielding 14.1 g (85%) of EDDP as a colorless liquid. 1H NMR (250 MHz, CDCl3): 2.98 (s, 2H), 4.4 (s, 4H). IR (neat): 3282 (ν(H—CtC)), (2966, 2888) (ν(CH2)), 2122 (ν(CtC)), 1717 (ν(CdO)), 1453 (δ(CH2)), 1212 (ν(C O)). 1-Azidooctane was prepared following a literature procedure for the analogous C12 compound.33 NaN3 (12.1 g, 0.18 mol) was stirred in 80 mL of dried dimethyl sulfoxide (DMSO) for 1 h whereupon 1-bromooctane (20 g, 0.1 mol) in tetrahydrofuran (THF, 300 mL) was added. The mixture was stirred for 2 days, diluted with H2O (100 mL), extracted with Et2O (3  150 mL), washed with H2O (3  100 mL), and concentrated, yielding 1-azidooctane (14.2 g, 90%) as a colorless liquid. 1H NMR (250 MHz, CDCl3): 0.86 (t, 3H), 1.2 (m 10 H), 1.58 (q 2 H), 3.25 (t, 2 H). IR (neat): 2958 (ν(CH3)), (2925, 2855) (ν(CH2)), 2098 (ν(NdNdN)), 1464 (δ(CH2)). Silicon Nanowire Preparation. Silicon nanowires were prepared on commercial silicon wafers by silver-catalyzed etching following a general recipe by Zhang et al.34 Si wafers were cut into 20 mm  25 mm pieces and were cleaned by sonication in ethanol for 3 min followed by UV-ozone oxidation for 10 min in a commercial UV-ozone cleaning chamber (Boekel Industries). The remaining native oxide layer was subsequently removed by 2 min etching in 5% HF. The samples were washed with distilled water and transferred into the Ag coating solution containing 0.005 M AgNO3 and 4.5 M HF in water, where they were left for 1 min under slow stirring. Afterward they were washed with water to remove excess Ag and were immersed in an etching solution containing 4.8 M HF and 0.4 M H2O2 in water for 30 min in the dark. Finally, the samples were immersed in 33% HNO3 to dissolve the deposited Ag catalyst, were rinsed with distilled water and ethanol, and were blowdried with high purity nitrogen (99.999%). A homogeneous black surface of densely packed and uniformly oriented nanowires was reproducibly obtained following this procedure. The length of the nanowires and the transmittance in the mid-IR range strongly depend on the immersion time in the HF/H2O2 etching solution. Figure 1 shows typical SEM images and IR transmittance spectra of SiNW samples etched for 30 and 240 min, yielding nanowires with diameters between 50 and 200 nm and lengths of about 4 and 20 μm. The IR transmittance decreases strongly with increasing nanowire length and falls below 2% in the mid-IR range for 20 μm long wires (Figure 1b). An immersion time of 30 min, yielding about 4 μm long NW, was chosen in this study to give a satisfactory signal-to-noise ratio in the IR spectra. Surface Modification of Silicon Nanowires. i. Oxide Etching. Immediately before further use, the silicon wafers covered with silicon

Figure 1. SEM images and IR transmittances of SiNW covered silicon wafers etched for (a) 30 min and (b) 240 min. nanowires were placed in piranha solution (95% H2SO4/30% H2O2 4:1 v/v) for 30 min, rinsed with water, and immersed in 5% HF solution for 2 min to remove the native oxide and create a H-terminated surface of the nanowires. ii. Hydrosilylation. A 0.5 M solution of EDDP in mesitylene was deoxygenated by a flow of argon for 1 h. A freshly etched SiNW covered wafer was immersed in the solution, and the argon flow was maintained for another hour, after which the solution was heated to 120 °C for 12 h under a weak argon flow. The sample was removed from the solution, rinsed with mesitylene, immersed for 1 h in mesitylene at 50 °C to remove physisorbed reactants, and finally rinsed with toluene and blowdried in nitrogen. iii. Click Coupling. Immediately after hydrosilylation, the SiNW sample was placed in a 0.5 M solution of 1-azidooctane in ethanol at 50 °C for about 24 h, was subsequently rinsed with ethanol, and was blow-dried in nitrogen. Sample Characterization. i. Secondary Electron Microscopy (SEM) Measurements. SEM images were recorded on a FEI Quanta 200 MK2 electron microscope equipped with an Everhart-Thornley detector. For the measurements, the SiNW covered silicon wafers were 7346

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Figure 2. Surface functionalization of silicon nanowires via hydrosilylation with an electron-deficient alkyne 1,2-ethanediol dipropiolate (EDDP) followed by alkyne azide click coupling with an azidoalkane R N3 (R = octyl). cleaved, mechanically fixed on a sample holder for cross-sectional specimens, and sputter-coated with gold using an Agar Scientific sputter coater B7340. Sputtering was performed for 50 s at a current setting of 10 mA. SEM images were recorded at working distances between 8 and 13 mm, and an electron beam high voltage of 5 keV with the crosssection of the samples perpendicularly oriented to the beam. In order to investigate potential chemical or mechanical damage to the nanowire structure in the course of the chemical modification procedure, SEM measurements were also carried out with SiNW after click coupling and were compared to images of untreated SiNW samples. No discernible changes caused by the chemical modification procedure applied here were observed in these images. ii. Infrared Measurements. Infrared spectra were measured on a Bruker Vertex 80 FT-IR spectrometer equipped with a narrow-band MCT detector. Spectra of the surface-modified SiNW samples were measured in transmission at normal incidence. A native oxide covered SiNW sample, freshly cleaned in piranha solution, was used as a reference. A total of 1024 scans at 4 cm 1 resolution were measured from sample and reference, and the resulting transmission spectra were baseline corrected using the OPUS interactive baseline correction software supplied with the Bruker spectrometer.

’ RESULTS AND DISCUSSION Several different synthetic routes for the preparation of silicon nanowires, based on either gas-phase or condensed-phase techniques, are known today.35 Herein, they were prepared according to a simple solution phase procedure,36 38 which is based on the silver-catalyzed etching of nanoporous, vertical channels into a standard silicon wafer surface, leaving behind an array of silicon pillars with diameters of 30 200 nm and lengths up to 140 μm.38 SiNW about 4 μm long and 100 nm thick were prepared by this procedure (Figure 1a) and were subjected to a surface modification procedure depicted in Figure 2: After removing the native oxide layer by HF etching, the H-terminated nanowires were reacted with 1,2-ethanediol dipropiolate (EDDP) to form a monolayer of EDDP on the nanowire surface via hydrosilylation of the CtC triple bond.29 The terminal alkyne groups of this monolayer were reacted at 50 °C with 1-azidooctane under formation of the triazole-bridged addition product. The course of this surface modification was followed in the infrared spectra shown in Figure 3. Starting with a H-terminated Si surface (Figure 3a) with its characteristic SiHx (x = 1, 2, 3) stretching vibrations39 at 2142 cm 1 (SiH3), 2110 cm 1 (SiH2), and 2187 cm 1 (SiH),40 these bands disappear completely upon adsorption of EDDP (Figure 3b) and are replaced by absorptions of the terminal alkyne groups of adsorbed EDDP at 3263 cm 1 (ν(H C)) and 2121 cm 1 (ν(CtC)) as well as the ester ν(CdO) peak at 1726 cm 1. The alkyne (ν(H C)) absorption is unusually broad and is shifted by almost 50 cm 1 to lower

Figure 3. Infrared spectra of surface-modified silicon nanowires: (a) hydrogen-terminated SiNW, (b) after hydrosilylation of (a) with 1,2-ethanediol dipropiolate, and (c) after click reaction of (b) with 1-azidooctane.

wavenumbers compared to adsorbed 1,8-nonadiyne.11a The CH stretching absorptions of EDDP appear below 3000 cm 1 and the CH2 deformation mode at 1451 cm 1. Clicking 1-azidooctane onto this EDDP monolayer (Figure 3c) results in the disappearance of the alkyne ν(H C) and ν(CtC) peaks and the growth of the ν(CH) and δ (CH2) absorptions of the octyl group at 2960 cm 1 (νas(CH3)), 2930 cm 1 (νas(CH2)), 2859 cm 1 (νs(CH2)), and 1455 cm 1 (δ(CH2)). A small peak at 3142 cm 1 is presumably the ν(H CdC) stretching absorption of the single CH group in the triazole ring. The ν(CdO) absorption shifts from 1726 to 1734 cm 1 upon click coupling. This high frequency shift, together with the broad, low-frequency absorption of ν(H CtC) of adsorbed EDDP in Figure 3b, indicates a hydrogen bonding interaction between the alkyne hydrogen and the ester carbonyl group in the EDDP layer, which weakens both the alkyne H C bond and carbonyl bond and causes a low frequency shift of their stretching absorptions. The click reaction removes the alkyne hydrogen and ν(CdO) shifts back to higher wavenumbers in the click product (Figure 3c). Under the current reaction conditions (24 h exposure at 50 °C), the yield of the surface click reaction is essentially quantitative as judged from the complete disappearance of the terminal alkyne absorptions. A blind experiment (exposure of the EDDP monolayer to pure solvent under the same conditions as in the click reaction) yielded an unchanged EDDP monolayer spectrum. Likewise, no reaction occurred when the nonactivated 1,8nonadiyne was adsorbed and exposed under the same reaction conditions to the click reagent 1-azidooctane. In the presence of a Cu-catalyst, however, this latter reaction proceeded quantitatively. Ongoing studies in our laboratory aim at the attachment of electron-donating and electron-withdrawing groups using the described modification scheme and the assessment of the corresponding changes of the SiNWs electronic properties. In summary, we have presented here a simple two-step procedure based on click chemistry, by which the surface of oxide-free, H-terminated silicon nanowires can be functionalized with organic substituents. A reactive but air-stable primer layer is first attached to the SiNW, onto which the desired functionalities can subsequently be clicked via alkyne azide coupling. The mild, catalyst-free reaction conditions for this process in combination with the extensive pool of available click transformations offer a promising approach for fine-tuning the electronic 7347

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Langmuir properties of SiNW through surface functionalization and present a biocompatible route for the fabrication of SiNW based bioanalytical devices.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: helmuth.hoff[email protected].

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