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Controlled Deposition of Gold Nanoparticles on Well-Defined Organic Monolayer Grafted on Silicon Surfaces D. Aureau,*,† Y. Varin,‡ K. Roodenko,† O. Seitz,† O. Pluchery,‡ and Y. J. Chabal† Laboratory for Surface and Nanostructure Modifications, UniVersity of Texas at Dallas, 800 West Campbell Road, Dallas, Texas 75080, and Institut des NanoSciences de Paris, UniVersite´ Pierre et Marie Curie, UPMC, CNRS, 140 rue de Lourmel, 75015 Paris, France ReceiVed: May 7, 2010; ReVised Manuscript ReceiVed: June 30, 2010
The use of nanoparticles for advanced applications critically depends on the control of the interfaces between the substrate, the intermediate/linking layer, and the nanostructures. While much work has been done to attach nanoparticles and to determine their properties, less effort has been devoted to the quality of starting surfaces and little is known about the impact of the attachment process on the pre-existing interfaces. In this study, the properties of interfaces of two model surfaces obtained by covalently grafting alkyl chains directly to oxide-free silicon surfaces, either via Si-O-C or Si-C bonds are compared with those currently obtained by attaching organic silane molecules (e.g., (aminopropyl)triethoxysilane) on oxidized silicon surfaces. Using FTIR, Raman spectroscopy, atomic force microscopy, and spectroscopic ellipsometry, we show that nanopatterned Si-O-C surfaces suffer some oxidation upon attaching nanoparticles, although they remain stable in ambient environments. In contrast, surfaces with Si-C bonds remain oxide free and remarkably stable during and after gold nanoparticle attachment. The attachment of AuNP to these oxide-free, stable semiconductor surfaces opens the way to applications sensitive to interface state quality. Introduction In the development of technologies based on nanomaterials, most of the focus has been on the properties of the nanomaterials and their attachment to metallic and semiconductor substrates. A number of approaches are based on self-assembly of an intermediate organic layer between an inorganic substrate and the nanoparticles. Molecules for such layers are chosen with headgroups that can chemically bind nanoparticles. In many applications for catalysis,1 biosensing,2 optical,3 and electronic4 devices, the control of the initial interfaces (“electronic substrate/ organic layer” and “nanoparticle/organic layer”) is critical. For instance, very low concentrations of interface defects on the substrate surface negatively affect the properties of tunnel junctions or the efficiency of energy transfer. On silicon substrates, substoichiometric surface oxides or partial surface oxidation are known to generate electronic traps that degrade the performance of devices. Covalent grafting of organic monolayers makes it possible to passivate the surface (e.g., prevent subsequent oxidation) while presenting a headgroup that can attach other structures. However, the quality of the initial interfaces becomes irrelevant if they do not resist to subsequent processing for attaching nanoparticles. Furthermore, the overall long-term stability of the initial interfaces and the nanoparticles is critical for biological applications when the system is in contact with aqueous biological media (e.g., buffer solutions).5 The most common method for the deposition of gold nanoparticles (AuNP) on silicon substrates is based on the reaction of (aminopropyl)triethoxylsilane (APTES) molecules with oxidized silicon surfaces.6 However, APTES-terminated * To whom correspondence should be addressed, damien.aureau@ utdallas.edu. † University of Texas at Dallas. ‡ Universite´ Pierre et Marie Curie, UPMC, CNRS.
oxide surfaces are poorly defined and unstable in aqueous solutions, which greatly reduces potential applications.7 To identify the factors influencing the stability of silicon/organic layer interfaces, we have studied three types of amino-terminated surfaces, as schematically shown in Figure 1: (a) APTES on SiO2, currently used by the community; (b) alkoxyl monolayer (Si-O-C bonds), prepared using a novel method that leads to ordered nanopatterning of atomically flat H/Si(111) surfaces with 1/3 monolayer methoxy groups;8 and (c) organic monolayers anchored by a Si-C bond via hydrosilylation.9 As an outcome of this study, we present a new chemical route for depositing monodisperse AuNP on oxide-free silicon with a 1.6 nm thick homogeneous layer of organic molecules inserted as a spacer and covalently attached via a Si-C bond to the silicon. The attachment of AuNP to this organic monolayer is then established through bonding with amino groups. The chemical nature of the interface between the substrate and the organic layer is characterized by infrared spectroscopy in transmission after each processing step. The homogeneity of the deposited nanoparticles is monitored by atomic force microscopy (AFM), and the surface density determined independently using AFM and spectroscopic ellipsometry. The chemical bonding of the nanoparticles to the organic layer is investigated by surface-enhanced Raman spectroscopy. We find that the layer attached via Si-C bonds remains oxide-free and stable in solution during all processing steps, including AuNP attachment. Experimental Section Chemicals and Samples. The different organic reactants (anhydrous ethanol >99.5%, ethylenediamine 98%, 6-aminohexanol 97%, (aminopropyl)triethoxysilane 98%) were purchased from Sigma Aldrich. The cleaning reagents (H2O2 30%, H2SO4 96%, acetic acid 100%) were purchased from Fisher
10.1021/jp104183m 2010 American Chemical Society Published on Web 08/02/2010
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Figure 1. Schematic description of amino-terminated Si surfaces for gold nanoparticle deposition.
Scientific. The etching reagents (HF 40%, NH4F 40%) were of VLSI grade and supplied by J.T. Baker. N-Hydroxysuccinimide 98% (NHS), N-(3-(dimethylamino)propyl)-N′-ethylcarbodiimide 97% (EDC), hydrogen tetrachloroaurate 99% (HAuCl4 · 3H2O) and 1,2,3-tricarboxy-2-hydroxypropane 99.5% (citric acid) were also purchased from Aldrich. Ultrapure water was provided by a Millipore station, which ensures a resistivity of 18.2 MΩ cm at 25 °C. The silicon samples were cut from [111]-oriented double-side polished Float Zone silicon wafers. Preparation of Silicon Surface. The samples were initially cleaned in “piranha” at 80 °C, a 3:1 mixture of concentrated H2SO4 (98%) and H2O2 (30%) and copiously rinsed with ultrapure water. The surface was directly used at this stage for the preparation of the APTES layer. Atomically flat, hydrogenterminated Si(111) surfaces were prepared by removal of oxide in a concentrated HF solution for 30 s, followed by a 150 s immersion in 40% NH4F solution and through rinsing in deionized water.10 Grafting of Organic Monolayers. Surface (a). The surface denoted (a) is prepared by silanization of the oxidized surface7,11 and represents a reference system in this study. Silanization of silicon oxide surfaces using (aminopropyl)triethoxysilane proceeds via hydrolysis of the ethoxy moiety, followed by siloxanation of the surface OH groups. This reaction is therefore strongly dependent on the local concentration of water and surface silanol, leading to surface inhomogeneity. The reaction was conducted in a N2-purged glovebox where 1 mL of pure APTES was diluted in 10 mL of anhydrous toluene. After 12 h of immersion at 105 °C, the surface was rinsed in hot toluene with sonication. On average, a reasonably dense amineterminated layer is formed after such a treatment, represented in Figure 1a, which can efficiently capture gold nanoparticles. The roughness of the resulting APTES layer remains below 3 nm with typical root mean square values of 0.9 nm, which is much smaller than the diameter of the nanoparticles (15 nm). Surface (b). The total absence of oxygen in the environment during the preparation of organic monolayers covalently grafted to the silicon via a Si-O-C bond is known to be a critical point to avoid any oxidation of the interface. Consequently, the thermal silylation reaction with aminohexanol was done following an anhydrous grafting method performed under nitrogen atmosphere in a glovebox (similar to the method used to prepare methoxy surfaces without oxide).8 Aminohexanol (solid at room temperature) was heated and 2 mL was diluted in 10 mL of toluene. In addition, 200 µL of chlorotrimethylsilane was added to the solution to remove trace amounts of water or fluoride from the surface and the reaction mixture, ensuring an anhydrous environment without affecting the reactions.12 After 4 h of immersion (T ) 105 °C), the surface was rinsed in hot isopropanol with sonication. This surface, shown in Figure 1b, is made of an ordered monolayer terminated with amine groups
separated from silicon by a short C6 alkyl chain linked via Si-O-C bonds, on an otherwise oxide-free surface.8 Surface (c). 10-Carboxydecyl organic monolayers were grafted on H-Si(111) via direct thermal hydrosilylation of undecylenic acid. The neat alkene was outgassed under argon in a Schlenk tube at 90 °C for 30 min and then cooled to room temperature under continuous argon bubbling to insert the freshly prepared H-terminated silicon sample. Grafting was performed for 20 h at 180 °C. The functionalized surface was then rinsed in hot acetic acid (60-70 °C) under argon for 15 min. (Acetic acid is used to remove the hydrogen-bonded undecylenic acid molecules.13). The transformation of the functional headgroup into succinimidyl ester-terminated chains was performed by reaction with an aqueous solution of N-ethylN′-(3-(dimethylamino)propyl)carbodiimide (EDC) and N-hydroxysuccinimide (NHS).14 The EDC and NHS solutions were prepared with cold water. The appropriate concentration mixture of EDC (10-2 M) and NHS (10-2 M) was deoxygenated with argon for 15 min in a Schlenk tube placed in a water bath at 0 °C. The freshly prepared acid-terminated alkyl surface was then placed in the Schlenk tube and allowed to react under continuous argon bubbling for 90 min at room temperature. The resulting succinimidyl ester-terminated surface was copiously rinsed with water and dried under a stream of nitrogen. The surface was then dipped in a Schlenk tube containing ethylenediamine for 12 h at room temperature. Finally the surface was rinsed with ultrapure water. During the last step, one amine group of the ethylene diamine molecule (H2N-CH2-CH2-NH2) reacts with the activated ester leading to the attachment of the molecule via an amide bond (-C(dO)-NH- CH2-CH2-NH2). Therefore, the resulting surface presents amine groups on top of the layer. Preparation and Deposition of Gold Nanoparticles. AuNP are synthesized with the Turkevich-Frens15 method under conditions yielding 15 nm nanoparticles stabilized by a citrate layer in aqueous solution. A 1 mL solution of citrates (8.5 × 10-4 mol · L-1) was added to a 19 mL solution of HAuCl4 (2.5 × 10-4 mol · L-1) at 80 °C under vigorous stirring. The stirring was maintained for 45 min at 80 °C (color gradually changing from slight yellow to gray, purple, and finally dark red). Then, the solution was cooled down to room temperature and finally sonicated. This well-documented method yields reproducible monodispersed AuNP with average diameter of 15 nm (standard deviation less than 1 nm).11 The aggregation of AuNP is prevented by the negatively charged citrate layer surrounding the particles. However, citrates do not form a covalent bond with gold and can be easily replaced by other molecules. AuNPs were subsequently deposited from the solution to the sample by dipping the grafted silicon surface into the gold colloidal solution for 90 min. Under these conditions, the pH of the solution is around 5.5, so that the amino-terminated
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TABLE 1: Bruggeman EMA Model Results for Au Nanoparticlesa
ellipsometric analysis
surface (a)
surface (b)
surface (c)
SiO2 d ) 1.6 nm APTES n ) 1.9, d ) 0.4 nm
-
-
Mixed organic/SiO2/Si n ) n (SiO2), d ) 3.7 nm, 13% SiO2/Organic Au/air, d ) 14 nm, 9.5% Au s ≈ 7 × 1010 cm-2 d ≈ 14.8 ( 1.0 nm s < 4.5 × 1010 cm-2
Organic layer n ) 1.9, d ) 1.5 nm
Au/air, d ) 15 nm, 1.7% Au, s ≈ 1.4 × 1010 cm-2 d ≈ 14.8 ( 1.0 nm s ≈ 4 × 1010 to 6 × 1010 cm-2
AFM analysis
Au/air, d ) 13 nm, 8% Au s ≈ 6.8 × 1010 cm-2 d ≈ 14.8 ( 1.0 nm s ≈ 8.5 × 1010 cm-2
a
The thickness d of SiO2 layer is determined by ellipsometry before the APTES grafting. The thicknesses d of the organic layers are determined by ellipsometry before the deposition of gold nanoparticles. The surface densities s of the gold films are independently determined by ellipsometry and AFM. n is the refractive index of the media.
monolayers (pKa ≈ 10) are protonated (acid form: NH3+) in contact with the gold suspension.11 The quality (size, shape, and dispersion) of the nanoparticles solution is characterized by UV-visible absorption. When the surface plasmon resonance (SPR) is monitored, which is dependent on these properties,16 a systematic control of the particle size and dispersion is possible. For 15 nm particles, for instance, the SPR has a sharp maximum at 520 nm. Atomic Force Microscopy (AFM). The AFM images were collected using the tapping mode of a Dimension 3100 microscope from Nanoscope. The AFM images were processed with WSxM.17 Spectroscopic Ellipsometry Measurement. Samples were measured with a HORIBA Jobin Yvon ellipsometer (iHR320) over a 250-800 nm spectral range. The fits are performed in several successive steps. For surface (a), the thickness of the silicon oxide interface is determined prior to any organic modification. This step is not necessary for surfaces (b) and (c) as they are oxide free (i.e., H-passivated) prior to the organic modification. After organic modification, the thickness and the dielectric constant of each organic layer are determined using a simple three or two layer model: Si/SiO2/organic layer stack for sample (a), and Si/organic layer stack for samples (b) and (c). Next, to describe the attachment of the AuNPs to each anchoring layer, a Bruggeman effective medium approximation (EMA)18 is applied, as summarized by the following equation
0)
ε -ε
∑ fi εi i + 2εeffeff
(1)
i
where fi and εi are the volume fraction and the dielectric constant of the ith inclusion of material and εeff is the effective dielectric function established by the Bruggeman approximation. In performing this analysis, it is possible to test whether the interface between the silicon and the layers is stable or chemically modified during the AuNP deposition process. Several layer models were tested and the convergence of the fits examined. The only layer stack that can be fitted without changing the layers existing prior to AuNP deposition is surface (c). For surfaces (a) and (b), the best model (i.e., giving consistent results with the lowest minimum square error (MSE)19) requires some modification of the substrate-organic interface and a mixed Au/air layer. For instance, the fits with the lowest MSE are obtained when the dielectric properties and the thicknesses of the organic layers are modified during the fitting procedure. In contrast, such modifications are not necessary to achieve the best fit for surface (c) after AuNP deposition.
For a comparison with AFM analysis, correlation between the volume coverage f and the surface density s is calculated based on geometric considerations, summarized in the following equation20
s)
3 f 2 πr2
(2)
where r is the radius of the nanoparticles. The results of the fits are summarized in Table 1 and will be discussed in the Results and Discussion section. FTIR Spectroscopy. IR absorption spectra were recorded in transmission (probing both sides of the sample) using a Nicolet 6700 FTIR spectrometer from Thermo Scientific equipped with a DTGS detector. Reference spectra were obtained using unfunctionalized surfaces, i.e., oxidized Si for the APTES monolayers and H-terminated Si for the other two monolayers. Raman Spectroscopy. The samples were measured using a Nicolet Almega XR Dispersive Raman spectrometer from Thermofisher. The spectra were obtained with 532 nm laser at 0.045 mW power as measured at the sample plane. The Raman spectrometer was equipped with a microscope with a 10× objective (N.A. 0.25). Results and Discussion IR Characterization of Amino-Terminated Silicon Surfaces. Figure 2 shows the infrared spectra of the three aminoterminated layers on silicon before the deposition of the gold nanoparticles. The IR transmission spectrum of surface (a) referenced to the initial oxidized surface features all modes of chemisorbed APTES, including the formation of an additional Si-O-Si interfacial layer as discussed in earlier publications.7 The IR transmission spectrum of the surface (b) referenced to the initial H-Si(111) surface shows that ∼1/3 monolayer Si-H terminations have been reacted (loss of the band intensity at 2082.8 cm-1) to form Si-O-C bonds that exhibit a very clear band at 1093 cm-1, which is distinct from SiO2 characterized by a pair of modes (TO at ∼1050 cm-1 and LO at 1100-1250 cm-1 depending on the thickness). In the same spectrum, the 2860 and 2933 cm-1 symmetric and asymmetric vibrations of the methylene functions from the C6 alkyl chain are clearly observed. Moreover the bands from the amino group at 1387 and 1458 cm-1 are weak but detectable. Importantly, the Si-O-C mode is distinct enough from the contribution from silicon oxide, typically seen in the 900-1250 cm-1 region. In particular, the absence of any spectral feature in the SiO2 TO phonon range (1030-1050 cm-1) unambiguously rules out the
Controlled Deposition of Gold Nanoparticles
Figure 2. Infrared absorption spectra of the three surfaces before deposition of gold nanoparticles: (a) APTES on SiO2 referenced to SiO2; (b) SiOC6H10NH2; (c) SiC10H20CONH-C2H4-NH2 referenced to Si-H surfaces.
assignment of the 1093 cm-1 to a SiO2 LO phonon mode of a very thin oxide film. The IR transmission spectrum of surface (c) referenced to the initial H-Si(111) surface shows that attachment of amino-terminated layers is obtained after reaction with ethylene diamine. The spectrum is similar to the one of surface (b) except that the hydrocarbon absorption (CH2 stretch modes at 2850 and 2920 cm-1) is larger, in accordance to the alkyl chain lengths, i.e., total number of CH2 units: 12 CH2 in the alkyl chains of surface (c) vs 6 for surface (b). The integrated area of these bands for surfaces (b) and (c) gives a ratio of 1/2, showing a similar coverage for both surfaces, which has been demonstrated to be in the range 0.32-0.39 for surface (c) after grafting of the acid-terminated chains.13 Furthermore, there is no Si-O-C band for surface (c) since the layer is grafted to silicon via Si-C bonds. In addition to the band related to amino groups and C-H vibrations already discussed for surface (b), there are bands associated with the amide link created on the chains (1540-1650 cm-1). There are also features at 1718 and 1738 cm-1, assigned to CdO groups that remain unreacted or due to side reactions during the process.14 Even under the best deposition conditions (i.e., the formation of tightly packed APTES layers7), APTES layers are not as welldefined as the organic layers formed on surfaces (b) and (c) that are covalently grafted on oxide-free silicon surfaces. These
J. Phys. Chem. C, Vol. 114, No. 33, 2010 14183 latter surfaces form a single monolayer without any silicon oxides (except for the Si-O-C bonds in surface b). The absence of the LO and TO modes of SiO2 in the 1000-1250 cm-1 spectral region for spectra 2-b and 2-c demonstrates the quality of these surfaces at this stage. Homogeneity and Density of the Gold Nanoparticles Layers. Figure 3 shows AFM images of the three aminoterminated surfaces after AuNP deposition (APTES on SiO2, SiOC6H10NH2, and SiC10H20CONH-CH2-CH2-NH2), corresponding to the surfaces (a), (b), and (c) shown in Figure 1. The homogeneity is checked by probing three different 2 × 2 µm spots on each surface. The gold nanoparticles are successfully attached on all three amino-terminated surfaces. In contrast, a similar treatment to attach AuNP (i.e., 90 min immersion) to clean silicon oxide, carboxyl- or methyl-terminated monolayers leads to no measurable nanoparticle deposition, confirming the importance of amino end groups to attach AuNP. The nature of the bond between the AuNP and the amino group of the SAM is not unambiguously established, although it probably involves the lone pair of the nitrogen atom.22 Importantly, the AuNPs are firmly attached and remain stable in air for months. In contrast to what is observed when using drop casting the AuNP solutions on oxidized silicon surfaces, we do not observe multilayers and cannot remove particles with sonication. On the three amino-terminated surfaces, AFM pictures show that the AuNPs have a diameter of 14.8 ( 1.0 nm based on their height. They confirm that the nanoparticles have not been modified during the deposition process. Due to tip convolution, their apparent lateral size is overestimated (ca. 50 nm for the images of Figure 3). These images may therefore convey the erroneous impression that the surface is saturated. Surface (a) exhibits a rather reproducible surface density with typical values of 5 × 1010 AuNP · cm-2 (20%. Such density is an order of magnitude less than the maximum possible density of 5.3 × 1011 AuNP · cm-2 corresponding to close packed spheres of the same diameter. For surface (b), the density of nanoparticles deposited with the same dipping time is lower with values of 5 × 1010 AuNP · cm-2 (40%, which also indicates poor homogeneity Finally, surface (c) exhibits the highest density of nanoparticles with a typical value of 8.0 × 1010 AuNP · cm-2 (10%, i.e. with a much lower variability between successive samples than the other two surfaces. These images and their corresponding cross sections show essentially no occurrence of aggregated AuNP. The average distance between deposited particles is around 30 nm, i.e., enough space to accommodate another AuNP. This gap is consistent with a strong electrostatic
Figure 3. 500 × 500 nm2 AFM images of the three amino-terminated surfaces: (a) APTES on SiO2, (b) SiOC6H10NH2, and (c) SiC10H20CONHC2H5-NH2.
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Figure 4. Fits for the ellipsometric parameters Ψ (red) and ∆ (blue) for three amino-terminated surfaces after gold nanoparticles deposition: (a) APTES on SiO2, (b) SiOC6H10NH2, and (c) SiC10H20CONHC2H5-NH2; open circles, measured ∆ data; closed circles, measured Ψ data; lines, fitted data. The parameters for the fits are given in Table 1.
repulsion between negatively charged citrate molecules that surround each nanoparticle. Spectroscopic ellipsometric analysis of surfaces (a), (b), and (c), as described in detail in the Experimental Section, provides an independent estimate of the AuNP density on a macroscopic area, i.e., providing an average over a much larger area than what can be done with AFM. Indeed, the diameter of the light spot is ∼0.7 cm in diameter. All fits to the data are shown in Figure 4 and summarized in Table 1 that also reports the AuNP densities obtained by AFM and ellipsometry. The fits are performed using the Bruggeman effective medium approximation (EMA).16 The thickness obtained for the AuNP layer (d ∼ 13-15 nm) is well in the range of the nanoparticle diameter. The best agreement between the gold densities obtained by AFM and ellipsometry analyses is obtained for surface (c), indicating that the distribution of AuNP on the surface is homogeneous. Surface (c) is also the system for which the best model and fit requires no change of the parameters (dielectric function and thickness) for the underlying surface (silicon + organic layer), indicating that there is no chemical modification during AuNp deposition. The somewhat lower density obtained on surface (a) by ellipsometric analyses suggests that the surface is inhomogeneous, as described in the next paragraph. A critical consideration of ellipsometric data is important, however, because the correlation between the nanoparticle coverage obtained from the AFM images and from the ellipsometric analysis is a topic under ongoing research, as summarized for example by Kooij et al.20 When a quantitative analysis of the optical ellipsometric measurements is attempted, several considerations need to be taken into account. The dielectric function of the AuNP may need to be modified from their bulk value,21 even if the void fraction to account for dispersed nanoparticles is considered through the effective medium approximation. For instance, Kooij et al.20 have performed modification of the bulk dielectric function based on considerations of limited electron mean free path due to the small particle size. They also point out that additional effects which might be significant in interpretation of the optical signal, such as the effect of image charges, are not taken into account in the effective medium approach.18,20 Although these considerations are possible reasons for the discrepancy between the particle coverage as determined by AFM and ellipsometry for sample (a) (see Table 1), two important results need to be highlighted. First, the fit and
Aureau et al.
Figure 5. Infrared spectra of SiOC6H10NH2 (i) and SiC10H20CONHC2H4-NH2 (ii) after deposition of gold nanoparticles referenced to Si-H surfaces.
convergence for AuNP on surface (c) are excellent, requiring no change in the Au dielectric function and no introduction of image charges. Second, the fit convergence of our data for surfaces (a) and (b) by simply applying the effective medium approach with the bulk optical properties for AuNPs is much better than that for the data reported by Kooij et al.20 Based on these results, we acknowledge that further investigations and development of optical models is important for understanding of the correlations between the nanoparticle coverages as found from AFM and ellipsometric measurements. Complementary chemical analyses based on IR data that support interface modification derived from optical models is provided in the next section. Stability of the Interface. As described above, only for surface (c) does the refractive index determined for the spacer layer remain constant. That is, the ellipsometric data are easily modeled by simply adding to the modelsconsisting of the initial Si/organic layer system with a ∼1.7 nm thick organic layersa uniform ∼13 nm thick AuNP layer, indicating that the distribution of AuNP on the surface is homogeneous. In contrast, modeling of both surfaces (a) and (b) requires a modification of the organic layer after AuNP deposition. This suggests that the interface between the silicon and the organic layer is chemically modified during AuNP deposition. Infrared spectroscopy provides a more direct access to the information of the chemical structure of the interface than ellipsometry. We investigated the modifications of the interface of the two well-defined surfaces (b and c) due to the nanoparticles deposition using infrared spectroscopy. Figure 5 shows the spectra of those surfaces, referenced to the initial Si-H, after deposition of the nanoparticles. After immersion of surface (b) into the AuNP suspension, the surface appears drastically modified. Figure 5 (spectrum (i) shows the growth of an oxide on the surface (very large TO and LO modes). The area of the TO and LO mode at 1047 and 1200 cm-1 is of 0.27 cm-1, which accounts for 85% of the area of the native oxide removed by the initial HF etching. Although there are still enough organic chains to deposit gold nanoparticles as seen in the AFM picture (Figure 3b), an important amount of silicon oxide has grown. Such modifications of the interface also occur on APTES layers where the solution can reach and modify the silicon oxide interface. In contrast, organic layers grafted via a Si-C bond (surface (c)) remain resistant against oxidation, even in contact with aqueous solutions.22 Indeed, the infrared absorption spectra associated with the amino-terminated layer linked via Si-C bond (referenced to the initial Si-H) do not change in the
Controlled Deposition of Gold Nanoparticles
Figure 6. Raman spectra of amino-terminated silicon surface after deposition of gold nanoparticles.
-1
900-1200 cm region throughout all processing steps, including immersion in the NP-containing solution (Figure 5, spectrum (ii)). This indicates that the silicon-organic layer of surface (c) remains stable without chemical modification of the interface. Consequently, this surface exhibits a more organized network of protonated amino groups. Bonding of the Nanoparticles to the Amine Groups. The mechanism of AuNP attachment to amino-terminated surfaces involves a complex interplay between electrostatics and donor-acceptor interactions.11,23 The negatively charged shell of citrates around the particles plays two roles: it ensures a repulsion that explains the rather regular dispersion of the particles as shown on AFM pictures (figure 3) and it produces an attractive force with the NH3+ end of the protonated amino group. However, the bonding occurs by taking advantage of the labile character of the citrate layer,24 which makes it possible to establish a direct bond between Au and the amino-terminated surface. In the resulting surface, the citrates are displaced by the amino end group. The unambiguous detection of citrates with IR absorption is difficult because there are many contributions in the region of the citrates vibrational modes (e.g., amine, amide, CdO) and because there may be screening by the AuNPs of species in contact with the metal (e.g., for modes parallel to the surface as expected for the citrate CdO mode). Details of the chemical nature of the AuNP layer can be better obtained using enhanced Raman spectroscopy. The presence of AuNP on top of the layer provides the adequate enhancement to sensitively detect the chemical environment around the nanoparticles. Figure 6 shows the Raman spectrum obtained after deposition of gold nanoparticles on surface (c). The spectrum exhibits ν(CH) vibrations and strong Raman peaks at 1605 and 1340 cm-1. The 1605 cm-1 band has been assigned to NH3+ deformation modes.25 The 1340 cm-1 broader band has been attributed to COO- modes (and potentially CH bending modes), consistent with citrates surrounding the AuNP. These data suggest that the AuNP are directly bonded through the NH3+ headgroups of the monolayer, even though it remains covered by citrate molecules over the nonbonded area. Conclusion We have shown that the formation of Si-C bonds is essential to withstand the chemistry associated with nanoparticle deposition. On the basis of this understanding, we have developed a
J. Phys. Chem. C, Vol. 114, No. 33, 2010 14185 robust chemical procedure for organizing monodisperse gold nanoparticles on oxide-free silicon via an amino-terminated thick homogeneous layer of molecules as spacer. The resulting surface, characterized by Si-C and Si-H bonds at the interface, remains stable in aqueous solutions. The absence of Si-O bonds at the interface is particularly important for electronic applications sensitive to interfacial states, such as gas sensors and biosensors and other highperformance electronic devices. In particular, the molecular layer that has been prepared can play the role of a tunnel barrier between the silicon substrate and the AuNP. This system provides an excellent starting substrate to design one electron transistors and is therefore ideal to test single electron conductance properties.26 The homogeneity of AuNP achieved with this organic layer is adequate to detect Coulomb staircase effects not only on one nanoparticle with the help of scanning tunneling spectroscopy27 but over the whole assembly of nanoparticles using other contact schemes. Acknowledgment. The work performed at UT Dallas was fully supported by the National Science Foundation under CHE0911197. The authors are grateful to Jean-Franc¸ois Veyan for technical support. Note Added after ASAP Publication. Figures 2 and 5 and Table 1 have been corrected. This paper was published on the Web on August 2, 2010. The corrected version was reposted on August 19, 2010. References and Notes (1) Haruta, M.; Kobayashi, T.; Sano, H.; Yamada, N. Chem. Lett. 1987, 16, 405–408. (2) Lazarides, A. A.; Schatz, G. C. J. Phys. Chem. B 2000, 104 (3), 460–467. (3) (a) Kreibig, U.; Vollmer, M. Optical Properties of Metal Clusters; Springer Verlag: Berlin, 1995. (b) Sardar, R.; Funston, A. M.; Mulvaney, P.; Murray, R. W. Langmuir 2009, 25 (10), 5840–5846. (4) (a) Ray, V.; Subramanian, R.; Bhadrachalam, P.; Liang-Chieh, Ma; Kim, C.-U.; Koh, S. J. Nat. Nanotechnol. 2008, 3, 603–608. (b) Novembre, C.; Guerin, D.; Lmimouni, K.; Gamrat, C.; Vuillaume, D. Appl. Phys. Lett. 2008, 92, (10). (c) Har-Lavan, R.; Ron, I.; Thieblemont, F.; Cahen, D. Appl. Phys. Lett. 2009, 94 (4), 043308. (5) Norman, A.; Lapin, N. A.; Chabal, Y. J. Phys. Chem. B 2009, 113, 8776. (6) (a) Taue, S.; Nishida, K.; Sakaue, H.; Takahagi, T. J. Surf. Sci. Nanotechnol. 2007, 5, 74–79. (b) Wu, C.-H.; Yeh, N. Jpn. J. Appl. Phys. 2009, 48, 04C152. (7) (a) Pasternack, R. M.; Rivillon Amy, S.; Chabal, Y. J. Langmuir 2008, 24, 12963–12971. (b) Tian, R.; Seitz, O.; Li, M.; Hu, W.; Chabal, Y. J.; Gao, J. Langmuir 2010, 26, 4563–4566. (8) (a) Michalak, D.; Rivillon Amy, S.; Esteve, A.; Chabal, Y. J. Phys. Chem. C 2008, 112 (31), 11907–11919. (b) Michalak, D.; Rivillon, A. S.; Aureau, D.; Dai, M.; Esteve, A.; Chablal, Y. J. Nat. Mater. 2010, 9, 266– 271. (9) Seitz, O.; Dai, M.; Aguirre-Tostado, F. S.; Wallace, R. M.; Chabal, Y. J. J. Am. Chem. Soc. 2009, 131, 18159–18167. (10) (a) Higashi, G. S.; Chabal, Y. J.; Trucks, G. W.; Raghavachari, K. Appl. Phys. Lett. 1990, 56 (7), 656. (b) Jakob, P.; Chabal, Y. J. J. Chem. Phys. 1991, 95 (4), 2897. (11) (a) Ji, X. H.; Song, X. N.; Li, J.; Bai, Y. B.; Yang, W. S.; Peng, X. G. J. Am. Chem. Soc. 2007, 129, 13939–13948. (b) Diegoli, S.; Mendes, P. M.; Baguley, E. R.; Leigh, S. J.; Iqbal, P.; Diaz, Y. R. G.; Begum, S.; Critchley, K.; Hammonds, G. D.; Evans, S. D.; Attwood, D.; Jones, I. P.; Preece, J. A. J. Exp. Nanosci. 2006, 1 (3), 333–353. (12) Boukherroub, R.; Morin, S.; Sharpe, P.; Wayner, D. D. M. Langmuir 2000, 16, 7429–7434. (13) Faucheux, A.; Gouget-Laemmel, A.-C.; Henry de Villeneuve, C.; Boukheroub, R.; Ozanam, F.; Allongue, P.; Chazalviel, J.-N. Langmuir 2006, 22, 153–162. (14) (a) Sam, S.; Touahir, L.; Salvador, A. J.; Allongue, P.; Chazalviel, J.-N.; Gouget-Laemmel, A. C.; Henry de Villeneuve, C.; Moraillon, A.; Ozanam, F.; Gabouze, N.; Djebbar, S. Langmuir 2009, 26 (2), 809–814. (b) Voicu, R.; Boukherroub, R.; Bartzoka, V.; Ward, T.; Wojtyk, J. T. C.; Wayner, D. D. M. Langmuir 2004, 20, 11713. (c) Lahiri, J.; Isaacs, L.;
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