LETTER pubs.acs.org/Langmuir
Curing Induced Structural Reorganization and Enhanced Reactivity of Amino-Terminated Organic Thin Films on Solid Substrates: Observations of Two Types of Chemically and Structurally Unique Amino Groups on the Surface Joonyeong Kim,*,† George J. Holinga,‡,§ and Gabor A. Somorjai‡,§ †
Department of Chemistry, Buffalo State, State University of New York, 1300 Elmwood Avenue, Buffalo, New York 14222, United States Department of Chemistry, University of California at Berkeley, Berkeley, California 94720, United States § Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States ‡
bS Supporting Information ABSTRACT: Infrared-visible sum frequency generation vibrational spectroscopy (SFG) was used to characterize the structure of 3-aminopropyltriethoxysilane (APTES) films deposited on solid substrates under controlled experimental conditions for the first time. Our SFG spectra in combination with complementary analytical data showed that APTES films undergo structural changes when cured at an elevated temperature. Before the films are cured, well-ordered hydrophobic ethoxy groups are dominantly present on the surface. A majority of hydrophilic surface amino groups are protonated, and they are either buried or randomly oriented at the interface. After the films are cured, chemically and structurally different neutral amino groups are detected on the surface. Unlike the protonated amino groups, a new class of neutral amino groups is ordered at the interface and shows enhanced reactivity.
he formation of organosilane-based thin films has greatly extended the utility of silicon wafers as solid substrates in many areas of science and technology.13 For example, proteins and oligomeric nucleic acids are covalently immobilized on either amino- or derivatized amino-terminated silicon surfaces during the fabrication of a variety of biosensing devices. 3-Aminopropyltriethoxysilane (APTES) is currently one of the most commonly used organosilane agents for preparing amino-functionalized organic thin films on silicon substrates.49 Previous studies using various analytical techniques have revealed that the formation of APTES films on silicon substrates is a complex multistep process that is very sensitive to reaction conditions. The complexity of the APTES silanization reaction mainly stems from the presence of a reactive amino group in APTES and its inherent propensity to enter into competing reactions. The amino groups in adsorbed APTES interact with silanols present on the silicon surface and/or in the adjacent hydrolyzed APTES via hydrogen bonding or electrostatic interactions.1013 In addition, an amino group is known to interact with a silicon atom in APTES, giving rise to the promotion of siloxane condensation between APTES and the surface as well as between APTES molecules.10,14,15 As a result, APTES films are known to adopt different structures and thicknesses depending on deposition conditions.1623 This is a significant issue and demands further investigation of the surface
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structures of APTES films because the availability of reactive surface amino groups is crucial to many applications.49 Infrared-visible sum frequency generation vibrational spectroscopy (SFG) has proven to be a powerful technique that provides the requisite surface and structural sensitivity needed to interrogate surfaces and interfaces that are not easily accessible by conventional spectroscopic techniques. As a second-order nonlinear optical process, SFG is forbidden in media that possess inversion symmetry but is allowed at surfaces and interfaces where inversion symmetry is necessarily broken. The intensity of the SFG signal, ISFG, is proportional to the square of the surface nonlinear susceptibility 2 A q ð2Þ ð2Þ ð2Þ ð1Þ ISFG jχNR þ χR j2 ¼ χNR þ ω ω þ iΓ IR q q
∑
with Aq, ijk ¼ n
^ ^k 3 ^nÞæ ∑ aq, lmn Æð^i 3^lÞð^j 3 mÞð
ð2Þ
lmn
Received: February 23, 2011 Revised: April 2, 2011 Published: April 08, 2011 5171
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Figure 1. SFG spectra (ssp polarization combination) in the CH stretching region from APTES films deposited on quartz in anhydrous toluene solutions for (a) 1, (b) 4, and (c) 24 h before curing and (d) 24 h after curing. The thicknesses of these films were measured to be 18 ( 2, 70 ( 3, 133 ( 5, and 102 ( 3 Å, respectively. (2) where χ(2) NR, χR , Aq, ωq, Γ, aq,lmn, and Æ æ are the nonresonant contribution, resonant contribution, oscillator strength, resonance frequency, width, absolute magnitude of hyperpolarizability, and average over the orientation distribution, respectively. The oscillator strength (Aq) is related to the absolute magnitude of the hyperpolarizability of the vibrational resonance (aq,lmn), the number density of contributing oscillators (n), and an orientationally averaged coordinate transformation.24 For decades, SFG has been implemented to extract interfacial chemical and physical pictures of both small (e.g., water, acetonitrile, and carbon monoxide) and large chemical species (e.g., self-assembled monolayers, polymers, and proteins) under a diverse range of environmental conditions.2531 In this work, we utilized SFG, Fourier transform infrared spectroscopy (FTIR), ellipsometry, and fluorescence measurements to investigate the effect of the deposition time and curing on the structure and reactivity of APTES films on solid substrates. The APTES films used in this work were produced on either clean silicon wafers or infrared-grade silica discs in 2% anhydrous toluene solutions for varied deposition times (1, 4, and 24 h). Ellipsometric thicknesses were measured to be 18 ( 2, 70 ( 3, and 133 ( 5 Å for films deposited for 1, 4, and 24 h, respectively. When cured at 100 C, the thickness of an APTES film deposited for 24 h was reduced to 102 ( 3 Å. Figure 1 shows SFG spectra in the CH stretching region taken at the airsolid interface for APTES films prepared on quartz under the same preparation conditions. SFG spectra were collected under the ssfgsvispir polarization combination by overlapping 532 nm visible and tunable infrared (IR) laser pulses on the sample surfaces. The solid lines represent least-squares fits of the raw data to eq 1 to obtain oscillator strengths (Aq), peak positions (ωq), and widths (Γ). (Fitting parameters are listed in Table S1 in the Supporting Information.) All SFG spectra in Figure 1 contain vibrational modes around 2850, 2875, 2925, and 2950 cm1. They are assigned to the CH2 symmetric stretching (CH2(s)), CH3 symmetric stretching (CH3(s)), CH3 Fermi resonance (CH3(F)), and CH3 asymmetric stretching (CH3(a)) modes, respectively.2628,3234 The SFG signal intensities of the CH2(s) and CH3(s) modes at 2850
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Figure 2. SFG spectra (ssfgsvispir polarization combination) in the NH and OH stretching regions from the same APTES films as in Figure 1. The thicknesses of the APTES films are (a) 18 ( 2, (b) 70 ( 3, (c) 133 ( 5, and (d) 102 ( 3 Å, respectively.
and 2875 cm1 increased with the thickness of the APTES films but suddenly decreased after curing. The SFG signal intensity for a given vibrational mode is proportional to the number density (n) and ordering of contributing oscillators at the interface as shown in eq 2.35,36 Therefore, the enhancement of the SFG signal intensity of CH3(s) at 2875 cm1 in Figure 1 is primarily attributed to the increased number of ethoxy groups on APTES films as the films become thicker for an extended deposition time. In the case of the CH2(s) mode at 2850 cm1, the SFG signal intensity originates from the CH2 groups in ordered ethoxy groups in APTES before curing. However, the CH2 groups in the aminopropyl group are mainly responsible for the SFG signal intensity at 2850 cm1 after curing (Figure 1d). The relative magnitude of the oscillator strength of CH3(s) with respect to CH2(s), ACH3(s)/ACH2(s), increases as the film becomes thicker. This indicates that the orientation and ordering of ethoxy groups on the surface change as the number density of ethoxy groups increases, although details are not elucidated from our data.37 Similar changes in the spatial orientation and ordering of molecules (or structural moieties) were observed when the surface number density changed at various interfaces.27,37,38 After curing, a significant portion of ethoxy groups are removed from the surface via siloxane condensation as indicated by the attenuated SFG signal intensity of the CH3(s) mode at 2875 cm1 (Figure 1d). Figure 2 shows SFG spectra in the NH stretching region taken from the same APTES films at the airsolid interface. The most striking fact is that the NH stretching (NH(s)) mode at around 3290 cm1 was not clearly observed from APTES films prior to curing regardless of the film thickness (Figure 2ac), but its intensity was dramatically enhanced after curing (Figure 2d). Under the ssfgsvispir polarization combination, SFG does not access molecular vibrations that are parallel to the surface or randomly oriented at the interface.24 From our SFG spectra shown in Figures 1 and 2, it appears that the hydrophobic ethoxy groups existing in APTES films are well ordered whereas the hydrophilic amino groups are buried and/or randomly oriented on the surface of the APTES film before curing. However, curing induces structural changes in APTES films, and a new class of amino groups is present on the surface. 5172
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Figure 3. FTIR spectra of APTES films produced in anhydrous toluene solutions with deposition times of (a) 1, (b) 4, and (c) 24 h before curing and (d) 24 h after curing. FTIR data was collected via the grazing-angle attenuated total reflection (GATR) method.
To obtain further structural information, complementary FTIR spectra of APTES films deposited on silicon wafers under the same experimental conditions were obtained by the grazing-angle attenuated total reflection (GATR) method.20,21,39 Unlike the SFG spectra in Figures 1 and 2, all FTIR spectra show similar features in the CH stretching region ranging from 2800 to 3000 cm1. (FTIR spectra are available in the Supporting Information.) In addition, the NH(s) mode from amino groups in APTES films at around 3290 cm1 is not observed in FTIR spectra regardless of the film thickness and curing conditions.20,21,39 Figure 3 shows FTIR spectra in the range of 1800 to 1270 cm1 from the APTES films deposited on silicon wafers under the same deposit conditions. A vibrational mode at around 1655 cm1 is responsible for the presence of an imine group formed by the oxidation of an amino group, and its intensity is closely related to the film thickness.16,17,20,21,40,41 Alternatively, this mode was also assigned to an NH2 bending mode.4244 Two dominant vibrational modes are found at around 1575 and 1485 cm1, and their intensity increases as the film becomes thicker.10,17,20,21,41 These two modes as well as a mode at around 1330 cm1 arise from amino groups in physisorbed and/or partially condensed APTES. The presence of ethoxy groups in APTES is confirmed by two modes at around 1440 and 1390 cm1 arising from the symmetric and asymmetric deformation modes of CH3 groups.10,20,21 Our FTIR data indicate that amino groups in APTES are not neutral amino groups but are involved in reactions with adjacent chemical species in several ways, producing different structures at the interface before curing. A proton transfer from acidic silanols in hydrolyzed APTES and/or silanols from silicon oxides to adjacent basic NH2 groups may give rise to the formation of SiO 3 3 3 NH3þ species as was proposed previously.1113 The second possibility is the formation of bicarbonated amino groups (HCO3 3 3 3 NH3þ) with water and carbon dioxide in air.45 Because the NH(s) mode is not clearly observed in our SFG spectra (Figure 2ac), these
Figure 4. Schematic representation of proposed structures of APTES thin films on silicon substrates. (a) Before the films are cured, ordered ethoxy groups in unhydrolyzed APTES are aggregated on the surface, but protonated amino groups are either buried or randomly oriented. (b) However, most ethoxy groups are removed by siloxane condensation, and the surface is covered with well-ordered and reactive neutral amino groups.
protonated amino groups are either buried or randomly arranged at the interface. This is supported by our independent SFG experiments at the airliquid interface using aqueous solutions (1.0 M) of methylamine (CH3NH2), ammonia (NH3), methylammonium chloride (CH3NH3Cl), and ammonium chloride (NH4Cl). SFG spectra from solutions of neutral methyl amine and ammonia contain the NH(s) mode at around 3330 cm1, but those from solutions containing charged amino groups (methylammonium chloride and ammonium chloride) did not produce a noticeable SFG signal from the NH(s) mode. (SFG spectra are available in the Supporting Information.) After the films were cured, the relative intensities of vibrational modes at around 1575 and 1485 cm1 were significantly reduced (Figure 3d). This indicates that a substantial amount of loosely bound APTES containing protonated amino groups was removed from the surface via either evaporation or condensation. In addition, a new vibrational mode at around 1550 cm1 was observed. We assume that this mode at ∼1550 cm1 arises from a new class of amino groups (neutral amino groups, NH2) existing in condensed APTES nets formed by curing at the elevated temperature. Unlike the protonated amino groups (NH3þ), these neutral surface amino groups in the cured APTES film appear to be well ordered at the airsolid interface, as was identified by our SFG data (Figure 2d). In a final set of experiments, the reactivity of these two types of surface amino groups on APTES films was estimated by 5173
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Langmuir fluorescence measurements. Surface amino groups in APTES films were conjugated by carboxyfluorescein (λexcitation = 494 nm and λemission = 521 nm). The fluorescence intensity from the APTES film prepared for 24 h (thickness 133 ( 5 Å) is approximately 3- and 1.2-fold greater than that from the APTES films prepared for 1 h (thickness 18 ( 2 Å) and 4 h (thickness 70 ( 3 Å), respectively. After the films were cured, the fluorescence intensity was enhanced by ca. 1.5-fold, although the thickness of the film was reduced. Our results suggest that ordered neutral amino groups after curing are more reactive than protonated amino groups present on the surface before curing, which was observed by FTIR and SFG data. From the SFG data, FTIR spectra, thickness measurements, and fluorescence measurements, it was found that APTES films on silicon substrates adopt different structures depending on the deposition time and curing. APTES films grow by both covalent and noncovalent adsorption of APTES containing incompletely hydrolyzed ethoxy groups and protonated amino groups. Hydrophobic ethoxy groups are aggregated on the surface, but a majority of hydrophilic protonated amino groups are either buried in the film or randomly oriented at the interface, making no significant contributions to the SFG signal intensity (Figure 4a). After the films are cured, most ethoxy groups are removed via siloxane condensation and the surface is covered with a new class of well-ordered neutral amino groups that are more reactive than buried and/or randomly oriented protonated amino groups (Figure 4b). Currently, studies including temperature-dependent structures of APTES thin films, chemical analyses of evaporated species from APTES thin films at the elevated temperature, and structural investigations of films formed by different aminating agents such as aminopropyldimethylethoxysilane (APDES) via FTIR, SFG, and a mass-selective detector coupled with a thermogravimetric analyzer are underway in our laboratory.
’ ASSOCIATED CONTENT
bS
Supporting Information. A description of SFG theory and data collection procedures, sample preparation and complementary experimental procedure, SFG fitting parameters, FTIR spectra in the range of 38002600 cm1, and SFG spectra of methylamine (CH3NH2), ammonia (NH3), methylammonium chloride (CH3NH3Cl), and ammonium chloride (NH4Cl) at the airliquid interface. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Tel: 716-878-5114. Fax: 716-878-4028. E-mail: kimj@buffalostate. edu.
’ ACKNOWLEDGMENT This work was supported by startup funds from the SUNY Research Foundation and Department of Chemistry, Buffalo State, SUNY, and the Director, Office of Science, Office of Basic Energy Sciences of the U.S. Department of Energy under contract no. DE-AC02-05CH11231. ’ REFERENCES (1) Netzer, L.; Sagiv, J. J. Am. Chem. Soc. 1983, 105, 674–676.
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