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Quantification of Silane Molecules on Oxidized Silicon: Are there Options for a Traceable and Absolute Determination? P. M. Dietrich,*,† C. Streeck,‡ S. Glamsch,†,∥ C. Ehlert,†,§ A. Lippitz,† A. Nutsch,‡ N. Kulak,∥ B. Beckhoff,‡ and W. E. S. Unger† †

Bundesanstalt für Materialforschung und -prüfung (BAM), Unter den Eichen 87, 12205 Berlin, Germany Physikalisch-Technische Bundesanstalt (PTB), Abbestr. 2-12, 10587 Berlin, Germany § Institut für Chemie, Universität Potsdam, Karl-Liebknecht-Straße 24-25, 14476 Potsdam, Germany ∥ Institut für Chemie und Biochemie, Freie Universität Berlin, Fabeckstr. 34/36, 14195 Berlin, Germany Downloaded by CENTRAL MICHIGAN UNIV on September 15, 2015 | http://pubs.acs.org Publication Date (Web): September 10, 2015 | doi: 10.1021/acs.analchem.5b02846

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S Supporting Information *

ABSTRACT: Organosilanes are used routinely to functionalize various support materials for further modifications. Nevertheless, reliable quantitative information about surface functional group densities after layer formation is rarely available. Here, we present the analysis of thin organic nanolayers made from nitrogen containing silane molecules on naturally oxidized silicon wafers with reference-free total reflection X-ray fluorescence (TXRF) and X-ray photoelectron spectroscopy (XPS). An areic density of 2−4 silane molecules per nm2 was calculated from the layer’s nitrogen mass deposition per area unit obtained by reference-free TXRF. Complementary energy and angle-resolved XPS (ER/ARXPS) in the Si 2p core-level region was used to analyze the outermost surface region of the organic (silane layer)−inorganic (silicon wafer) interface. Different coexisting silicon species as silicon, native silicon oxide, and silane were identified and quantified. As a result of the presented proof-of-concept, absolute and traceable values for the areic density of silanes containing nitrogen as intrinsic marker are obtained by calibration of the XPS methods with reference-free TXRF. Furthermore, ER/AR-XPS is shown to facilitate the determination of areic densities in (mono)layers made from silanes having no heteroatomic marker other than silicon. After calibration with reference-free TXRF, these areic densities of silane molecules can be determined when using the XPS component intensity of the silane’s silicon atom.

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reactions, e.g., multilayer formation and polymerization, occur readily due to the presence of three reactive OH groups located on the silane’s central silicon atom.9−11,19 The application of silane precursor molecules with three alkyl residues and only one reactive moiety located in the headgroup (cf. Scheme 1) is one way to circumvent these known problems.19,21,22 From previous work, it is known that these trialkylsilanes exclusively generate (sub)monolayers because they form only one siloxane bond (Si−O−Si).19,23−26 This bond tethers the silane to the silicon oxide substrates (Sisilane−O−Sisurface), representing a key step during (mono)layer formation.9−11 Among the vast number of functional end groups available for silane molecules, the amino group is one of the most popular because of its versatile chemistry.27,28 The reactivity of amines toward various other functional groups provides numerous opportunities of further transformations of the aminosilane-functionalized substrates, e.g., to prepare tailor-

ilane molecules with different functionalities have been used for decades1−9 to customize many different materials (polymers, metals, semiconductors, glasses, etc.) by fine-tuning the surface and its properties as needed for the final application. The popularity of silane-based surface chemistry originates mainly from the broad applicability to various surfaces, the multitude of commercially available silanes, and the apparent ease of use.9−16 Despite many years of research on organosilane-based surface modifications, some fundamental aspects of the underlying chemistry are not fully understood. For example, Naik et al.17 reported recently that surface hydroxyl groups (Si−OH) play only a minor role in monoalkylsilane monolayer formation on silicon oxide surfaces, which is contrary to the idealized and commonly accepted reaction mechanism.9−11 Reproducible formation of real monolayers using monoalkylsilanes is quite complex because of the high susceptibility of the process to reaction conditions like temperature, solvent, silane concentration, nature of the silane, reaction time, and presence of water.9−11,18−20 Therefore, a very careful control of all relevant reaction parameters is mandatory because side © XXXX American Chemical Society

Received: July 27, 2015 Accepted: September 2, 2015

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DOI: 10.1021/acs.analchem.5b02846 Anal. Chem. XXXX, XXX, XXX−XXX

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Downloaded by CENTRAL MICHIGAN UNIV on September 15, 2015 | http://pubs.acs.org Publication Date (Web): September 10, 2015 | doi: 10.1021/acs.analchem.5b02846

Analytical Chemistry

For common organic materials, a reliable analysis with a lower information depth limit of about 1 nm can be reached.58−60 XPS yields the relative atomic composition of the analyzed surfaces. A calibration of XPS by reference-free TXRF spectroscopy enables absolute XPS measurements with traceability back to SI base units, here kg (mass) or mol (amount of substance). A proof-of-concept will be demonstrated in this report. Herein, we present a study of thin layers made from trialkylsilanes carrying amine or amide functionalities to investigate the potential of reference-free TXRF to achieve (i) a label-free quantification of surface functional group densities of silane monolayers and (ii), based thereon, a traceable calibration of quantitative XPS. For this purpose, we chose two different nitrogen containing precursor molecules for nanolayer formation, 3-aminopropyldimethylmethoxysilane (1) and N-(3-(methoxydimethylsilyl)propyl)benzamide (2), as shown in Scheme 1.

made materials such as biointerfaces, biomimetic surfaces, or biosensors.13,15,16 However, its reactivity can also cause some side reactions of the amino group after silane film formation, e.g., oxidation reactions.29−32 For their application, knowledge on the amount of accessible amino groups on the surface can be of particular importance. These reactive amino groups are essential for all following surface reactions, e.g., attachment of biomolecular entities (probes) on a biosensor.33 However, it is still a challenge to quantify these surface functional groups in an absolute manner. Most of the existing approaches are of indirect nature and rely on a preliminary labeling step of the surface functional groups and a detection of the label afterward with, e.g., spectroscopic (XPS, FT-IR, NMR, UV−vis, and fluorescence), mass spectrometric (SIMS, ICP-MS), or radiometric methods.11,24,32,34−48 In view of these restrictions, reference-free TXRF analysis,49 which relies on calibrated instrumentation as well as on knowledge on atomic fundamental parameters instead of any calibration samples or reference materials, had to be evaluated in the current work as complementary method to quantify surface functional group densities in organic nanolayers without any need for labels. Reference-free TXRF provides direct access to the mass per unit area of selected elements in a thin surface layer such as nitrogen on the functionalized wafer substrates which corresponds to the amino groups on the surface. Then, in a second step the calibration of XPS data with reference-free TXRF as a physically traceable method was employed to achieve the absolute quantification of surface-bound organic molecules on silicon oxide surfaces. TXRF analysis is a well-established method to monitor lowest levels of surface contaminations.49,50 Chemical traceability in TXRF analysis is based on the addition of an internal standard in minute liquid depositions to derive the mass deposition of an element of interest.50,51 By using monochromatic synchrotron radiation in the soft X-ray range and an ultrahigh vacuum setup including window-less detectors, even light elements on a substrate surface, e.g., carbon or nitrogen, can be excited effectively.52,53 That high experimental effort facilitates quantitative TXRF analyses of organic nanolayers.52−57 However, to the best of our knowledge TXRF is used rarely to quantify the amount of adsorbed or covalently bound molecules in organic nanolayers.34,54−56 We present the approach of reference-free TXRF for a straightforward quantification of surface functional group densities using the nitrogen mass deposition of silane monolayers obtained from the detected count rates of the characteristic N Kα fluorescence line. In this case, the main advantage of nitrogen is its use as an intrinsic heteroatomic marker of the silane molecules which provides a way to quantify the amount of bound silanes by both TXRF and XPS analyses. The other elements, like silicon, oxygen, and carbon, representing the silanes are not well suited for this purpose as they are also present in the substrate (Si and O) or on the substrate as inevitable hydrocarbon layer (CHx). In parallel, the silane films are analyzed with laboratory XPS as well as energy- and angle-resolved XPS (ER/AR-XPS)58−60 to obtain more details about their chemical composition. This approach allows a chemical analysis of the outermost layer of thin organic materials with an exceptional surface sensitivity realized through the variation of emission angle (θ) and excitation energy (hν), delivered by a synchrotron radiation source, and reveals information on in-depth chemical variations.

Scheme 1. Amino- (1) and Benzamidosilane (2) Used for Monolayer Formation on Oxidized Silicon Wafers

Both silane layers were prepared by deposition conditions resulting in monolayer formation as reported recently.24−26,61 The benzamide group was selected to induce intermolecular hydrogen-bonding and minimize possible side reactions related to the presence of free amines, e.g., with the Si surface itself or with other silanes.22,29,32,62−68 Additionally, the benzamide moiety allows investigating a potential influence of the terminal group’s steric demand on the surface functional group density. The presented study extends our previous work on dualmode fluorescence and XPS labeling of nitrogen-functionalized surfaces for biosensing applications.34,69 Now reference-free TXRF analysis as a primary method complemented by XPS is applied to develop and establish a reliable procedure to quantify the absolute amount of surface-bound amine- and amidefunctionalized silane molecules without the need for labeling.

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EXPERIMENTAL SECTION Materials. All reagents were obtained from commercial suppliers and used without further purification unless otherwise stated (benzoyl chloride (99%, Sigma-Aldrich), 3-aminopropyldimethylmethoxysilane (1, 97%, Acros), toluene (>99.8%, AppliChem), and triethylamine (>99.5%, Fluka)). Silicon samples were obtained by cutting pieces of 2.5 mm × 1.5 mm from Si(100) wafers (p-type, 1−10 Ω cm, CrysTec, Berlin, Germany). Sample Preparation. Silicon wafer pieces (2.5 cm × 1.5 cm) were immersed in isopropyl alcohol (10 min), dried in a stream of nitrogen, and put in an UV ozone cleaning system (PR-100, UVP Inc.) for 30 min. Afterward, they were immersed in a 2% (v/v) solution of silane 1 or 2 in toluene for 19 h at 70 °C.24−26 Postsilanization cleaning was done by sonication in toluene (20 mL), dichloromethane (20 mL), and ethanol (20 mL) for 5 min each. Finally, the samples were rinsed with B


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= 30) was used in combination with a Shirley background. The Si 2p core-level spectra were fitted with the three Si 2p peak components Si0 (bulk), SiO2 (native oxide), and R3Si(O)1 (silane). The relative uncertainty for XPS quantification of atomic fractions is given in Table S4, Supporting Information.

ethanol, dried in a stream of N2, and stored under inert atmosphere in a gastight aluminum pouch. A freshly cleaned wafer piece was used as unmodified reference sample. Reference-Free Total Reflection X-ray Fluorescence Analysis. Reference-free TXRF measurements were carried out at a plane grating monochromator beamline in the laboratory of the PTB at the synchrotron radiation facility BESSY II (Berlin, Germany) providing monochromatized undulator radiation in the soft X-ray range between 78 and 1870 eV.70 Measurements were performed using excitation radiation having a photon energy of 1060 eV. Here, an ultrahigh vacuum measurement chamber was used as the experimental endstation.71 Its 9-axis manipulator allows for accurate adjustments of both the sample position and the angle of incidence. Using a window-less energy dispersive silicon-drift detector (SDD), which is placed at an angle of 90° with respect to the incoming beam, the samples were excited in total reflection geometry at an angle of incidence with respect to the sample surface of 1.0°. This angle of incidence is approximately 70% of the critical angle of external total reflection at the SiO2/ Si wafer substrate for an incoming beam with the chosen excitation energy. The relative uncertainty for TXRF quantification of elemental mass deposition is given in Table S2, Supporting Information. X-ray Photoelectron Spectroscopy. Laboratory XPS measurements were carried out with a Kratos Analytical AXIS Ultra DLD photoelectron spectrometer. XPS spectra were recorded using monochromatized Al Kα excitation at pass energies of 80 eV for survey and 20 eV for high-resolution corelevel spectra using the charge neutralizer. The electron emission angle was 60° (with respect to the surface normal) and the source-to-analyzer angle was 60°. The binding energy scale of the instrument was calibrated following a Kratos Analytical procedure, which uses ISO 15472 binding energy (BE) data.72 Spectra were taken by setting the instrument to the hybrid lens mode and the slot mode providing approximately a 300 μm × 700 μm analysis area. The binding energy scale was furthermore corrected for charging,73 using an electron binding energy of 285.0 eV for the C 1s level of aliphatic hydrocarbon.74 Synchrotron Radiation XPS. (SR-XPS) measurements were carried out with a Scienta 3000 energy analyzer at the endstation of the HE-SGM monochromator dipole magnet beamline (CRG) at Helmholtz-Zentrum Berlin (synchrotron radiation source BESSY II, Berlin, Germany). High-resolution core-level SR XPS spectra (O 1s, N 1s, C 1s, and Si 2p) were recorded in FAT (fixed analyzer transmission) mode at pass energy of 50 eV. The excitation energies were varied as followed: 620, 500, 385, and 210 eV, respectively. That approach is based on the acquisition of photoelectron spectra at a constant kinetic energy of 100 eV with the help of a tunable synchrotron X-radiation source. This ensures a constant XPS 95%information depth z95 of about 1 nm for all elements in the thin organic layers, thereby alleviating many of the quantification problems inherent to the analysis of inhomogeneous layers by laboratory XPS.58−60 This is important because the “silane layer on silicon wafer” samples are inhomogeneous in depth. Binding energy (BE) scales were referenced to the Si0 2p3/2 peak binding energy set to 99.0 eV measured at the respective excitation energy. SR XPS spectra were measured at an emission angle of 60°. All high resolution Si 2p, C 1s, N 1s, and O 1s core-level spectra were analyzed using CasaXPS (Casa Software). In the curve fitting of the core-level spectra, a Gaussian/Lorentzian product function peak shape model (G/L

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RESULTS AND DISCUSSION In reference-free TXRF analysis, the absolute mass deposition per unit area of relevant constituents of the studied surface, here C, N, and O, is derived from the emitted intensity of the respective characteristic X-ray fluorescence radiation per incident radiant power or flux. The basic requirement for this measurement approach is a radiometrically calibrated instrumentation in order to determine the incoming photon flux by means of a photodiode, to derive emitted intensities from detected count-rates using the efficiency of the energydispersive detector in conjunction with knowledge on the effective solid angle of detection defined by either the detector diaphragm or an additional calibrated aperture placed at a wellknown distance to the sample, and to deconvolve spectral information by means of physically modeled detector response functions. In addition, reliable knowledge on atomic fundamental parameters such as fluorescence yields, transition probabilities and photoelectric cross sections is required for reference-free TXRF analysis.49,75 Surfaces functionalized with silane 1 and 2 together with a pristine Si wafer substrate (SiO2/ Si blank) were analyzed by means of reference-free TXRF at the plane grating monochromator beamline in the laboratory of PTB at the synchrotron radiation facility BESSY II.76 Figure 1a shows the characteristic C Kα, N Kα, and O Kα lines in the TXRF spectra of silane layers 1 and 2 as well as of the pristine SiO2/Si substrate (blank). Figure 1b shows the deconvolution of the TXRF spectrum of the pristine SiO2/Si wafer employing

Figure 1. (a) TXRF spectra of silane layers 1 and 2, respectively, and for comparison, the spectrum of a pristine SiO2/Si wafer (blank). (b) TXRF spectrum of the blank showing the spectral deconvolution based on detector response functions for the characteristic lines and physical background modeling. C


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Table 1. Quantification Results of Silane Layers 1 and 2 Obtained from Reference-Free TXRF and Laboratory XPS Analysis TXRF

XPS

XPS calibration factor

layer

nitrogen (ng/cm)

silane areic density (molecules/nm2)

thickness a (nm)

nitrogen (at.−%)

lab-XPS [N 1s] (molecules/nm2 × at.−%)

1 2

5.9 ± 1.8 8.9 ± 2.7

2.5 ± 0.8 3.8 ± 1.1

0.6 ± 0.2 1.4 ± 0.5

1.9 ± 0.6 2.6 ± 0.8

1.3 ± 0.5 1.5 ± 0.6

b

SR-XPS [Si 2p] (molecules/nm2 × peak area−%) 0.09 ± 0.02 0.09 ± 0.02

a


The silane layer thickness was estimated according to ref 79 from silane areic density values as obtained from TXRF, the silane’s density, and its molar mass. For details, see eq (1) in the Supporting Information. bThis calibration factor is only valid for the used SR-XPS conditions of z95 = 1.2 nm, hν = 210 eV, and θ = 60°.

The number of functional groups, here amines or amides, in the organic layers under investigation is directly related to the determined mass deposition of nitrogen as nitrogen is only present in the silane molecules, but not in the silicon substrate itself. Absolute surface densities of (2.5 ± 0.8) and (3.8 ± 1.1) silane molecules per nm−2 were obtained for the organic nanolayer 1 and 2, respectively, using Sherman’s equation for TXRF and are traceable to the SI.86 They support earlier published data. Values in the order of 1 to 4 silane molecules per nm2 were reported for thin layers prepared from aminopropylsilanes on silica surfaces that were, however, determined by nontraceable approaches.24,26,43,44,87−89 From these silane surface densities, a silanization reaction efficiency of 50% for aminosilane 1 and of 75% for amidosilane 2, respectively, can be estimated from experimental and calculated surface silanol group (Si−OH) densities of silica surfaces, which are about 5 Si−OH per nm2.90−92 In addition, average layer thicknesses of (0.6 ± 0.2) nm for 1 and (1.4 ± 0.5) nm for 2 were estimated from the silane’s density, molecular mass and the areic density of silane molecules on the silicon oxide surface determined by TXRF.79 These thicknesses correspond very well with experimental and theoretical values of single monolayers of aminosilane 1 and amidosilane 2.26,44,89 Complementary information about relative elemental and chemical composition of the nanolayers was obtained from laboratory XPS analysis. Nitrogen could be detected by XPS in the monolayers made from 1 and 2, respectively (cf. Tables 1 and S2). The presence of nitrogen as intrinsic heteroatomic marker of the silane molecules provides a way to quantify the amount of bound silane. The N 1s XPS peak area was used to calibrate the relative data from laboratory XPS with the silane surface density obtained from reference-free TXRF yielding comparable calibration factors of (1.3 ± 0.5) molecules × nm2 × at.−%−1 for the aminosilane 1 and (1.5 ± 0.6) molecules × nm−2 × at.−%−1 for the amidosilane 2 monolayer, respectively. Thus, absolute areic density data for adsorbed organic molecule as silanes on analyzed surfaces can be obtained from XPS measurements after calibration. And a standardized and traceable quantification routine for organic functional surface layer characterization by using XPS is possible. Beyond that, a traceability chain for quantitative XPS data to the SI base unit kg (or mol) through quantitative TXRF given as mass (or amount of substance) per area unit is possible, too. In addition to the determination of areic densities of silanes on an oxidized Si wafer, the samples were investigated in-depth by X-ray energy and angle-resolved XPS (ER/AR-XPS). This approach allows an exceptional surface sensitivity as result of varying the excitation energy (hν) and electron emission angle (θ).58−60 Here, the layer stack composed of silane layer // native oxide layer // silicon substrate was analyzed by ER/AR-

detector response functions at the photon energies of the relevant characteristic fluorescence lines and also physical background modeling convolved with the detector response behavior. The photon energy axis is defined by the lifetime correlated zero line on the left-hand side and a well-known, preselectable scaling factor. Figure 1a shows that the intensity of the N Kα line, serving for the quantification of the silane layers, is about 2 orders of magnitude smaller than the one of the O Kα line, thus strongly calling for a reliable spectral deconvolution of the fluorescence lines by means of detector response functions that had been experimentally determined and physically modeled.77,78 This approach works very well for the O Kα line in the measured spectrum of the pristine wafer substrate (SiO2/Si blank) and, in addition, provides a spectral background allowing for a stable fit of the less intense N Kα line in the TXRF spectra of the silane layer samples. The dominant O Kα fluorescence line originates from the native oxide layer on top of the silicon substrate and corresponds to a (1.7 ± 0.5) nm thick layer of native silicon oxide (SiO2) being well in line with the typical literature values ranging from 1 nm up to 3 nm.80−82 The C Kα line of the SiO2/Si blank’s TXRF spectrum in Figure 1a shows that a significant amount of carbon was found on the pristine wafer (SiO2/Si blank) due to formation of an inevitable adventitious hydrocarbon layer as a result of sample handling in laboratory air.83 In addition to the C Kα, N Kα, and O Kα lines in the TXRF spectrum of the pristine wafer substrate, the F Kα and a low intense Cu Lα fluorescence lines are included into the spectral deconvolution. These two elements were not detected in the XPS measurements, as their mass deposition might be below the XPS limit of detection. The minute amounts of fluorine and copper may originate from the wafer manufacturing process.84,85 The spectral background below the N Kα line, mainly due to the lower-energetic part of the SDD response associated with the strong O Kα line, leads to a limit of detection of the nitrogen mass deposition of approximately 0.7 ng × cm−2 which corresponds to 0.3 silane molecules per nm2. For future measurements, an excitation energy below the oxygen K-edge may improve the sensitivity for nitrogen detection. The TXRF quantification results expressed as absolute nitrogen mass deposition per unit area are summarized in Table 1 and show significant amounts of deposited nitrogen on the Si wafer as a result of layer formation with silane molecules 1 and 2. Nitrogen was below the limit of detection on the SiO2/ Si blank (for complete TXRF quantification results, see Table S1, Supporting Information). The uncertainties given for the analytical TXRF quantification results are combined uncertainties of measurement involving both experimental and fundamental parameter related contributions (for more details, see Table S2, Supporting Information) D


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−1.5 eV for R3Si(O)1 in relation to the SiO2 component is in principal agreement with the shift of −2.0 eV obtained from calculated ionization potentials of a benzamidosilane 2 layer on a quartz surface (for more details, see Table S5 and Figure S7, Supporting Information). The Si 2p core-level spectrum of monolayer 1 reveals SiO2 as the main species next to R3Si(O)1 and a negligible Si0 contribution with relative component peak areas of 69%, 28%, and 3%, respectively. From that, it becomes obvious that the silane-related R3Si(O)1 species can be resolved here as a real single component peak due to the exceptional surface sensitivity that can be achieved with the applied ER/AR-XPS method whereas the same approach yields data with rather high uncertainties when laboratory XPS is applied.23−26 Different component peak areas of 54%, 43%, and 3% for SiO2, R3Si(O)1, and Si0, respectively were deduced from Si 2p core-level spectra of the amidosilane 2 monolayer at z95 = 1.2 nm (cf. Figure 2b). From that, it can be concluded that in the case of the amidosilane 2 monolayer the silane component is a major component in the XPS spectra of the outer surface region whereas for the aminosilane 1 layer the SiO2 peak component is the predominant species in the XPS spectra taken at z95 = 1.2 nm. The finding of a higher silane component peak area at highest surface sensitivity (z95 = 1.2 nm) for the amidosilane 2 monolayer is supported by a higher amount of adsorbed nitrogen for that sample compared to the aminosilane 1 layer as detected by TXRF and laboratory XPS. In the layer of amidosilane 2, the benzamide group prevents possible side reactions of the nitrogen atom in contrast to the aminosilane layer with its free amino group. Additional intermolecular hydrogen-bonding between adjacent amide groups can improve molecular assembly and thus a higher density of silane molecules on the surface than for the corresponding amine groups results.68 As a consequence of the differentiation of organic silicon atoms in silane molecules from inorganic Si atoms in the substrate, another possible calibration of quantitative XPS has been identified and tested. Here we can use the silane-specific R3Si(O)1 component peak area at maximum surface sensitivity and calibrate it with the silane surface density from TXRF analysis giving a calibration factor of (0.09 ± 0.02) molecules × nm−2 × peak area−%−1 for both monolayers. In principle, this ER/AR-XPS approach utilizing the silane’s silicon atom allows, after a careful calibration with TXRF, the absolute measurement of silane surface densities irrespective of a functional end group with intrinsic heteroatomic marker atoms. That is a substantial advantage compared to the above presented alternative of using the N 1s signal to calibrate laboratory XPS measurements. Currently, we are working on the extension of the presented calibration of quantitative XPS with TXRF data to a wider range of surface functional group densities to obtain an improved calibration for absolute XPS measurements that are traceable to the SI.


XPS focusing on variations of different silicon species in the Si 2p core-level region.93 The analyzed depth, measured as the XPS 95% information depth z95,94 was 1.2 nm using an excitation energy of 210 eV and an electron emission angle of 60° with respect to the surface normal.58−60 In comparison, an information depth of z95 ∼ 6 nm is obtained by laboratory XPS at hν = 1486.7 eV and θ 60°. Figure 2a shows the fitted high-resolution Si 2p core-level spectrum taken on the aminosilane 1 monolayer in the most

Figure 2. Fit of Si 2p core-level spectra of silane layers made from (a) aminosilane 1 and (b) amidosilane 2 taken at z95 = 1.2 nm (hν = 210 eV, θ = 60°) including the Si 2p components Si0, R3Si(O)1, and SiO2. The experimental values are shown as circles and the sum curve of the single components of the fit is shown as black line. For simplicity reasons, the R3Si(O)1 and SiO2 components were fitted with one singlet only as usual instead of using a Si 2p1/2 Si 2p3/2 doublet. Corresponding Si 2p spectra from the same samples measured with laboratory XPS at z95 ∼ 6 nm (hν = 1486.7 eV, θ = 60°) are shown in Figure S6, Supporting Information.

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surface sensitive mode characterized by an information depth of z95 = 1.2 nm (hν = 210 eV, θ = 60°).95 Elemental silicon Si0 at a binding energy of 99.0 eV represents the bulk region and a second peak originating from the native oxide (SiO2) layer thereon occurs at a binding energy of 103.2 eV.81,96−98 In accordance with the measured peak position of 101.7 eV, the third component is assigned to the silane’s central silicon atom R3Si(O)1 that is bound to the surface by a single siloxane bond (Sisilane−O−Siwafer).23−26,99−101 The measured chemical shift of

CONCLUSIONS In conclusion, we could demonstrate in a proof-of-concept that reference-free TXRF is a well suited method for a direct and absolute quantification of surface functional group densities of organic nanolayers with intrinsic heteroatoms as nitrogen on silicon oxide surfaces. These silane surface densities give a direct measure of reactive silanol groups existing on the surface E


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synchrotron radiation beamtime at the HE-SGM beamline. Special thanks are due to Prof. P. Saalfrank for fruitful discussions. Furthermore, the authors want to thank Dr. A. Nefedov (Karlsruhe Institute of Technology, KIT) from the HE-SGM Collaborate Research Group and Dr. O. Schwartzkopf, Dr. A. Vollmer, Dr. C. Jung, and M. Mast (all BESSY II) for support during our activities at HZB.

prior to the reaction with the silane. We used two different monolayers made from trialkylsilanes with one terminal amine (1) or benzamide (2) group, respectively. From the total nitrogen mass deposition measured by reference-free TXRF, a range for the areic density of 2 to 4 silane molecules per nm2 was calculated covering both monolayer systems (amine and amide). However, the areic density of amidosilane 2 on the surface is slightly higher as inferred independently from the amount of silane specific nitrogen or silicon by TXRF, laboratory XPS, and synchrotron XPS. X-ray energy and angle-resolved X-ray photoelectron spectroscopy (ER/AR-XPS) utilizing Si 2p core-level spectra was applied to analyze the three component layer stack: silane layer // native oxide layer // silicon substrate. It was found that the outer surface region (∼1 nm) of monolayers 1 and 2 is dominated by peak components SiO2 and R3Si(O)1, which originate from native oxide and surface bound silane molecules. Moreover, calibration of laboratory and synchrotron XPS data with the help of reference-free TXRF spectrometry as a primary method could be demonstrated. This approach delivers traceability to the SI base units kilogram (mass) or mole (amount of substance). Basically, this methodology allows absolute quantitative XPS of silanized surfaces even for silane molecules with no other heteroatom available than silicon. In the future, the presented approach using a combination of X-ray-based spectroscopy methods, XPS and TXRF, for absolute quantitative determination of surface functional group densities will be extended to other silane monolayers. Especially for intended low surface functional group densities, binary mixtures of silane molecules, to dilute the functional silane with a nonfunctional one, will be prepared to accomplish traceable calibration of XPS data with reference-free TXRF.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.5b02846. Synthesis and characterization of benzamidosilane 2, further experimental details and data from TXRF, labXPS, and ER/AR-XPS as well as the calculation of Si 2p ionization potentials (PDF).

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*P. M. Dietrich. Phone: +49-30-8104-3533. Fax: +49-30-81041827. Email: [email protected]. Author Contributions

The paper was written through contributions of all authors. All authors have given approval to the final version of the paper. Funding

This work is funded by the European Union through the European Metrology Research Program (EMRP). The EMRP is jointly funded by the EMRP participating countries within EURAMET and the European Union. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Thanks are due to D. Treu for operation of the Kratos Axis Ultra XPS instrument. We thank HZB for the allocation of F


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