Controlled Oxidation of Alkyl Monolayers Grafted onto Flat Si (111) in

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Controlled Oxidation of Alkyl Monolayers Grafted onto Flat Si(111) in an Oxygen Plasma of Low Power Density D. Aureau,† W. Morscheidt,‡ A. Etcheberry,§ J. Vigneron,§ F. Ozanam,† P. Allongue,† and J.-N. Chazalviel†,* Physique de la Matie`re Condenseé, Ećole Polytechnique, CNRS, route de Saclay, 91128 Palaiseau, France, Laboratoire de Geńie des Proce´de´s Plasmas et Traitements de Surface, Ećole Nationale Supe´rieure de Chimie de Paris, 11 rue Pierre et Marie Curie, 75005 Paris, France, and Institut LaVoisier, UniVersite´ de Versailles-St Quentin en YVelines, 45 aVenue des E´tats-Unis, 78000 Versailles, France ReceiVed: April 28, 2009; ReVised Manuscript ReceiVed: June 30, 2009

Alkyl monolayers anchored to an atomically flat hydrogenated (111)-oriented silicon surface through covalent Si-C bonding have been submitted to an oxygen plasma treatment of controlled power density. The chemical state of the surface was monitored in situ in real time in the plasma cell using multiple-internal-reflection infrared spectroscopy and ex situ by X-ray photoelectron spectroscopy. While plasma treatments of moderate power densities (∼0.1-1 W/cm3) lead to complete removal of the organic monolayer, using very-low power densities (∼10 mW cm-3) leads to selective oxidation of the organic chains with negligible etching of the monolayer and a very limited oxidation of the silicon surface. Successive formation of alcohol, ketone, and carboxyl functions is observed before reaching a quasi steady state on an hour time scale. Modelization of the plasma shows that the dominant reactive species at these low power densities is the singlet oxygen molecule (1∆g). A molecular-scale reaction mechanism is proposed, and a simple kinetic model is derived. A major conclusion is that reaching a quasi steady state is attributable to the very low amount of atomic oxygen in the plasma and to the densification of the layer upon incorporation of oxygen-bearing groups. 1. Introduction The organic functionalization of solid supports is a step of uttermost importance in many applications such as hydrophobic coatings1 or patterning,2 thin-layer processing,3 organic lightemission,4 photovoltaic devices,5 surface passivation6 or electronic functionalization.7 In sensor applications, the performance and selectivity of the device crucially depend on our ability to create sites to selectively anchor chemical or biomolecular probes for a specific probe-target coupling and to simultaneously control the physicochemical properties of the remaining surface.8 In many instances, the functionalization requires several steps. In this context, plasma treatments appear as a highly attractive method to chemically modify organic monolayers by introduction of different chemical functions. Plasma treatments have long been used to modify the surface of organic polymers, in particular to control their wettability.9-13 A variety of plasmas (O2, N2, Ar, H2O, NH3, and so forth) were explored. Several types of reactions can be induced by the plasma from the creation of new chemical functions (oxygenor nitrogen-bearing groups) to modifications of the hydrocarbon skeleton (cross-linking). These chemical modifications have been investigated using X-ray photoelectron spectroscopy (XPS) and infrared (IR) spectroscopy. More recently, the complexity of these reactions has motivated studies on simpler “model” systems. In particular, a few studies have addressed the plasma treatment of short organic chains on surfaces. Self-assembled monolayers (SAM) of thiols on gold and silver14,15 and monolayers obtained by silanization on silicon oxide16-18 have been * To whom correspondence should be addressed. Tel: +33-1-69 33 46 63. Fax: +33-1-69 33 47 99. E-mail: [email protected]. † École Polytechnique. ‡ École Nationale Supe´rieure de Chimie de Paris. § Universite´ de Versailles-St Quentin en Yvelines.

considered. The effect of oxidizing plasmas (O2,16,17 N2+O2,14 Ar+O2,15 CO218) was studied. The reactions on organic monolayers appear to be less complex than those encountered in the oxidative plasma treatment of organic polymers, which has allowed for studies of the mechanisms in some more detail. However, up to now there is a limited number of such studies, and the formation of new species (C-O, CdO, O-CdO) was found to be limited by the competition with a degradation of the layer.14-18 In the present work, an oxygen plasma was used in very mild conditions in order to selectively create functional groups on the uppermost surface region of alkyl monolayers grafted on Si(111). A dc plasma was chosen because it allows for a lower power density than radiofrequency or microwave plasmas. For the sake of a quantitative study, we started from well-defined alkyl monolayers grafted on atomically smooth hydrogenated Si(111) surfaces.19 The monolayers were prepared either by thermal hydrosilylation20 or by electrochemical treatment of this surface in a Grignard solution21 (the samples will hereafter be designated as SiCnH2n+1, with n ) 10 or 2). The mechanism and kinetics of oxidation of the organic chains were investigated in detail in situ by IR spectroscopy and ex situ by XPS. Also, since selective oxidation of the monolayers was sought, special attention was paid to the oxidation of the interface between the silicon substrate and the organic layer. The paper is organized in two main sections. The experimental section puts special emphasis on the plasma cell for in situ infrared measurements. The results and discussion section is divided into subsections, focusing on the effect of plasma power density on surface modification, the silicon oxidation, the kinetics of selective monolayer oxidation, and the oxidation mechanisms deduced from a quantitative analysis of the monolayer modification.

10.1021/jp903892z CCC: $40.75  2009 American Chemical Society Published on Web 07/20/2009

Alkyl Monolayers Grafted onto Flat Si(111)

Figure 1. Picture (a) and scheme (b) of the special cell.

2. Experimental Methods 2.1. General Information. Decene (97%), ethylmagnesium bromide (3 M in diethylether), and ammonium sulfite monohydrate (92%) were purchased from Aldrich. All cleaning (H2O2 30%, H2SO4 96%, acetic acid 100%) and etching (NH4F 40%) reagents were of VLSI grade and supplied by Merck. Organic solvents (THF, CH2Cl2) were of HPLC purity and supplied by Carlo Erba. Ultrapure water is provided by a Millipore station, which ensures a resistivity of 18.2 MΩ cm. 2.2. Preparation of Atomically Smooth H-Terminated Silicon Surfaces.22 The silicon samples were cut from [111]oriented p-type double-side polished silicon wafers (0.2° misorientation toward [112j], float zone, F) 30-40 Ωcm), shaped as ATR prisms (15 × 15 × 0.5 mm3, approximately 45° bevels) for infrared spectra. The samples were initially cleaned in “piranha”, a 3:1 mixture of concentrated H2SO4 (98%) and H2O2 (30%) in order to remove all organic contaminations. This wet oxidation was followed by copious rinsing with ultrapure water. The cleaned silicon samples were then chemically etched during 12 min in oxygen-free 40% NH4F (ca. 0.05 mol L-1 ammonium sulfite was added to the etching solution) to remove native oxide and obtain atomically smooth H-terminated surfaces with flat terraces of ca. 100 nm width (consistent with the expectation for 0.3 nm high steps and 0.2° misorientation). After etching, the surface was rinsed with ultrapure water. 2.3. Formation of Decyl Monolayers on Silicon by Thermal Hydrosilylation. 23 A one-step procedure was used to prepare well-defined decyl monolayers on H-Si (111) via direct thermal hydrosilylation of decene. The neat decene 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 grafted surface was then rinsed in THF and CH2Cl2. 2.4. Formation of Ethyl Monolayers on Silicon by Electrochemical Grafting.21 The hydrogenated silicon sample was transferred to an electrochemical cell in a glovebox under purified nitrogen. The PTFE parallelepipedic cell was equipped with a U-shaped Cu counterelectrode, designed so as to minimize ohmic drop in the electrolyte (diethylether + 3 M C2H5MgBr). Anodic current (1 mA cm-2) was passed during 5 min. The cell was emptied, rinsed with bromobutane and then with clean diethyl ether before being taken out from the glovebox. The sample was dismounted, further rinsed (ethanol and water), and dried. 2.5. Plasma Treatment. The organic modifications of the monolayer and of the silicon-molecule interface during the plasma treatment were characterized in real time in a specially designed IR cell equipped with BaF2 windows that are transparent to wave numbers higher than 800 cm-1 (Figure 1). This approach allowed us to monitor the chemical modifications of the interface with great sensitivity, because the sample was not moved all along the treatment, which avoids contributions of

J. Phys. Chem. C, Vol. 113, No. 32, 2009 14419 bulk silicon absorption from appearing in the spectra. The dc glow discharge was produced between two stainless-steel electrodes (diameter 6 mm) separated by 16 mm. The active volume was ca. 1 cm3 and the electrical input power was adjustable from 5 mW (0.5 kV, 10 µA) to 500 mW (1 kV, 0.5 mA) (high-voltage electrical hazard). The flow rate of oxygen was fixed at 1 sccm to maintain a pressure of 0.5 Torr in the plasma chamber. Furthermore, it is known that the degradation rate of alkyl species and the oxidation of silicon substrates are particularly high when the substrate is directly in the plasma discharge where it is submitted to the ion and electron bombardments.24 Therefore, in the present experiments the substrate was located in the flowing afterglow region of the oxygen plasma, approximately 6 cm below the electrodes. Before introduction of the silicon prism, the plasma chamber was cleaned for several minutes with a 500 mW cm-3 oxygen plasma to remove organic contaminants inside the chamber. 2.6. In Situ Infrared Spectroscopy. ATR-FTIR spectra were recorded in s and p polarizations, successively, using a Bomem MB100 FTIR spectrometer equipped with a liquid-nitrogen cooled MCT photovoltaic detector. Thanks to the small size of our ATR samples, the optical path length in silicon is reduced, which minimizes silicon absorption below 1500 cm-1 and makes it possible to collect data over an extended spectral range (900-4000 cm-1, 4 cm-1 resolution). This allows us to simultaneously investigate the modification of the alkyl chains and the possible silicon oxide formation. The average number of reflections is ∼30. The spectra recorded at the various steps of the processing are ratioed to a reference spectrum (recorded at the beginning or at the end of the experiment). In this way, all the displayed spectra are given in absorbance (computed using natural logarithm) per reflection. Every 2 min, one spectrum was recorded (corresponding to 100 scans). In order to extract information on the kinetics of the phenomena, the different regions of all the infrared spectra have been fitted as a combination of bands. 2.7. Ex Situ XPS Measurements. Selected samples were studied by XPS after stopping the plasma treatment at selected times. Just after stopping the plasma, the chemical state of the sample was characterized by in situ infrared measurements, which enables us to compare IR and XPS characterizations on the same sample. To minimize adventitious contamination, the sample was then rapidly transferred (through air) into an inert atmosphere, and after a few hours it was installed inside the UHV chamber of the spectrometer. The adventitious contamination resulting from this transfer, as estimated from spectra on SiH (not shown) or SiC10H21 surfaces (to be shown later) corresponds to a fraction of monolayer. The XPS spectrometer was a Thermo Electron VG ESCALAB 220i XL model. Data were acquired using an Al KR1 monochromatic X-ray excitation. The ultimate overall resolution used in this work gives a fwhm for the Si2p1/2 and Si2p3/2 lines equal to 0.31 eV. The data were exploited using the Thermo Electron “Avantage XPS software”. In the present work, the photoelectrons are collected perpendicularly to the sample surface. The detection was performed in a constant analyzer energy mode (CAE), using a pass energy of 8 eV for ultrahigh-resolution configuration or 20 eV for the high-resolution configuration used as the routine procedure. The spectrometer calibration was performed using the Thermo Electron procedure and was completed by a self-consistent check on sputtered copper and gold samples based on the ASTM E90294 recommendation. The binding energy position of the Au4f7/2 line was 84 eV.

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Figure 2. In situ time evolution of infrared spectra in p-polarization for a SiC10H21 surface exposed to an oxygen plasma of power density 10 mW cm-3 (a-e) and subsequently 500 mW cm-3 (f-i). The time indicated next to each spectrum represents the time elapsed from the time of switching on the 10 mW cm-3 plasma (a-e) or the 500 mW cm-3 plasma after 3 h at 10 mW cm-3 (f-i). The reference spectrum was taken before plasma treatment. Spectra (b-i) have been vertically shifted for clarity.

2.8. AFM Characterizations. The surface morphology was inspected by AFM in contact mode (Molecular Imaging microscope, Phoenix, AZ) in a nitrogen atmosphere using Si3N4 cantilevers (Nanoprobes, spring constant 0.12 N m-1). 3. Results and Discussion 3.1. Influence of Plasma Power Density: Evidence for Two Distinct Regimes. Figure 2 presents wide range in situ FTIR spectra (900-3000 cm-1) of a decyl monolayer grafted on Si(111) (“SiC10H21 surface”), during exposure to an oxygen plasma of varying power density. Spectra a-e were recorded at low power density (10 mW cm-3, total time: 3 h). Spectra f-i were subsequently acquired after increasing the power density to 500 mW cm-3 (total time: 3 h) on the same sample. The reference spectrum was recorded before turning on the discharge. With this choice for the reference spectrum, negative bands correspond to species that are removed during the treatment. Positive bands correspond to species that are created. A. Analysis of the Spectra. In spectra a-e, the negative bands between 2700 and 3000 cm-1 (attributed to νC-H vibrations) and the negative band around 1460 cm-1 (δCH2) indicate that a loss or a transformation of the alkyl chains takes place. The associated progressive increase of the positive band between 1700 and 1800 cm-1 (attributed to νCdO) indicates that at least part of the aliphatic chains are in fact oxidized to create carbonyl-containing species. It is emphasized that silicon oxidation remains very limited in the initial stages of the treatment. In fact the bands in the region 900-1200 cm-1 (associated with νSi-O) appear only after 10 min of plasma treatment and remain very weak during the first 30 min. After 3 h at 10 mW cm-3, their shape remains ill-defined as compared to that of a dense SiO2 layer. At this stage of the modification, the plasma power density was increased to 500 mW cm-3. Spectra f-i present the same characteristic IR bands in the same three spectral regions as

Aureau et al. above. In the region 2700-3000 cm-1, the magnitude of the negative bands is now compatible with a complete loss of the CH2 groups. In the region 1700-1800 cm-1, the CdO band that appeared during the treatment at 10 mW cm-3 quickly vanishes. In the region 900-1200 cm-1, the increase of the positive νSi-O bands is attributed to silicon oxide formation. In p-polarization, the νSiO spectrum can be analyzed as the superposition of 4 characteristic bands.25 The main peaks at 1060 and near 1230 cm-1 correspond to the TO and LO modes of νasSiO in Si-O-Si. The band at 940 cm-1 is attributed to nonbridging oxygens. The shoulder near 1150 cm-1 is usually ascribed to disorder. At the end of the treatment, the shape of the band (in particular the clear LO-TO splitting) is that of a well-defined silicon oxide layer. Therefore, the 500 mW cm-3 treatment results in a removal of the organic layer and a complete oxidation of the surface. Control experiments where a plasma of 500 mW cm-3 was directly applied to a virgin SiC10H21 sample led to essentially similar results. The characteristic IR bands in the three spectral regions (ppolarization) were fitted as superpositions of an appropriate number of pseudo-Voigt functions (4 for νasSiO, 1 or 2 for νCdO, 5 for νCH, as described in ref 26), and the corresponding band intensities were extracted. The time evolution of the LO mode of the νasSiO vibration is presented in Figure 3a. The variations of the νCdO absorption and the sum of the characteristic νC-H band intensities are given in Figure 3b. At 10 mW cm-3, the silicon oxide absorption bands increase slowly and level off after 100 min (Figure 3a), suggesting that a stable steady state has been reached. By comparison, the decrease of the CH bands intensity in the region 2700-3000 cm-1 and the rise of the CdO band intensity (fitted with a single pseudo-Voigt function) are much faster. This is a primary indication that the organic monolayer is selectively oxidized within the first 50 min. Increasing the plasma power density (from 10 to 500 mW cm-3) leads to a rapid silicon oxide uptake (Figure 3a) and an equally fast disappearance of both the CH and CdO bands (they are totally disappearing). This suggests that the surface oxidation is accompanied with the destruction of the organic layer by the 500 mW cm-3 plasma. B. Plasma Modelization. The above results demonstrate that two different plasma regimes must be considered (selective monolayer oxidation at 10 mW cm-3 and total surface oxidation at 500 mW cm-3) depending on the power density of the plasma. In order to account for these observations, the plasma composition was simulated in the geometry of our cell. A plasma is a complex mixture of ionized and neutral species with different energies, whose relative concentrations depend on the injected electrical power in a nontrivial manner. A global plasma model was adapted to investigate the chemistry of the oxygen discharge.27 This model is based on the solution of a stationary electron Boltzmann equation coupled to balance equations for each species including the vibrational kinetics equations of O2. The reactions deal with the electron impact processes (the excitation-de-excitation of O2, O, and metastable states, the dissociation and dissociative attachment of O2 and the ionization of O2 and O) and the collision between heavy species (leading to the relaxation of electronic energy, ion conversion, mutual recombination, or ozone formation and destruction). The set of equations is closed using the energy balance that relates the absorbed power with the electron energy distribution function, the electron density and the reduced electric field. Finally, our model involves 36 vibrational levels of O2 and 11 excited species, O2(1∆g), O2(1Σg), O3, O(3P), O(1P), O(1S), O+, O2+, O-, O2- and e-. Figure 4 gives the power-density dependence of


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Figure 3. Time evolution of the integrated p-polarization absorbance of the LO mode of the νSi-O vibration (a), of the νCdO band and of the νC-H massif (b) for a SiC10H21 surface under oxygen plasma.

Figure 4. Simulated power-density dependence of the molar fraction of active species in an oxygen plasma generated between the two electrodes. The remainder consists of ground-state dioxygen3Σg (not represented). The geometry is that of the IR cell used in this work. See text for more details about simulation.

the molar fractions of the main active species generated between the two electrodes. Besides ground-state dioxygen O2(3Σg), atomic oxygen O(3P) and molecular dioxygen in the singlet state O2(1∆g) are found to be the two dominant species. As shown in Figure 4, the concentration of atomic oxygen is nearly proportional to the injected electrical power, while the concentration of O2(1∆g) remains weakly dependent on plasma power density. In addition, at 10 mW cm-3 the molar fraction of O2(1∆g) is much larger than that of O(3P), while the two molar fractions are in comparable amounts at 500 mW cm-3 (crossover at about 700 mW cm-3). In our geometry and operating conditions (see Experimental Methods), these two active species are most probably the ones to be considered at the sample surface, because they need about half a second to reach the sample region, a delay smaller than their lifetime (in the range of seconds, the loss processes being very slow at glass walls28-30). A critical examination of Figures 3 and 4 strongly suggests that the presence of two distinct behaviors depending on power density is associated with this change in plasma composition. The rate of surface reactions is expected to depend on the molar fraction of the reactive species in the plasma. At 10 mW cm-3, atomic oxygen is present in very small concentration and the observed selective oxidation of the organic chains plausibly arises from the dominant singlet molecular oxygen species O2(1∆g). On the other hand, silicon oxidation and decomposition of the organic layers at high power density can clearly be attributed to the presence of the energetic atomic oxygen O(3P).

Atomic oxygen is well known for its capability to create radicals on organic chains by the abstraction of one hydrogen atom, according to the reaction RH + O(3P) f R• + •OH.29-33 The formation of such radicals could be an efficient path toward oxidation of the organic chains immobilized on the silicon surface. However, atomic oxygen can also attack the alkyl chains at C-C bonds, which probably accounts for the organic layer decomposition, and the competition between selective oxidation and destruction of the layers, as reported in the literature.14-18 In the present work, the use of unusually low power densities, minimizing the concentration of atomic oxygen, provides an unprecedented capability to functionalize the organic layer while minimizing layer degradation and silicon oxidation. Reaching a quasi steady state under these conditions also makes the oxidation process much more controlled and reproducible than when modifying the layer during a critical very short duration, as required at larger power densities.14-18 3.2. Plasma Power-Density Dependence of Silicon Oxidation. In this section, we analyze in details the oxide layer formed along the plasma treatment. According to the infrared spectrum (Figure 2), the oxide formed at 10 mW cm-3 appears quite different from that formed at 500 mW cm-3. The oxidation induced by the 10 mW cm-3 plasma treatment can also be studied from the XPS Si2p spectrum. Figure 5 shows spectra obtained for SiC10H21 surfaces before starting the plasma treatment and in the steady state obtained after a 2 h exposure to the plasma at 10 mW cm-3. Below 102 eV, the Si2p spectra appear similar and can be analyzed as the superposition of two doublets, each one with the normal spin-orbit splitting of 0.62 eV, a common line width of 0.53 eV at full width half maximum (fwhm), and an intensity ratio of 0.5. The weaker doublet, shifted to higher energy by 0.44 eV, is attributed to the outer silicon layer bonded to hydrogen or carbon atoms. Two contributions have been added (101, 103 eV) in Figure 5b to fit the XPS spectra and are attributed to small amounts of silicon oxide. The low intensity of those peaks as compared to previous works where formation of a complete oxide layer has been studied by XPS34,35 confirms that very small amounts of SiOx (effective thickness around 1 Å) are formed by the 10 mW cm-3 plasma. The thickness of the oxide can be obtained from infrared spectroscopy by a simple method using the absorption maximum for the TO band in s-polarization.36 At low power density, an average oxide thickness of 0.8 Å is estimated within the steady state region of Figure 3a, which is in fair agreement with XPS. The ill-defined spectral shape is compatible with the presence of oxide patches of various spatial extents and/or of a disordered

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Figure 5. XPS spectra of the Si2p region for a SiC10H21 surface before (a) and after (b) an exposure of 2 h to oxygen plasma (10 mW cm-3).

Figure 6. Time evolution of p-polarized infrared spectra in the ranges 2800-3020 cm-1 (a) and 1600-1900 cm-1 (b) for a SiC10H21 surface under oxygen plasma (10 mW cm-3) at the early stages of the treatment. (a) From top to bottom: t ) 0, 2, 4, 6, 10, 20, and 30 min. The reference spectrum was taken after exposure of the surface for 3 h to the 500 mW cm-3 oxygen plasma. (b) From bottom to top: 10, 16, 26, 36, 56, 96, and 180 min. The reference spectrum was taken before switching on the plasma. Panels a and b correspond to the same experiment (note the different time scale of the evolution for the two spectral regions).

layer of substoichiometric oxide. Furthermore, since the organic monolayer is still present in these conditions, the oxide layer is most probably composed of SiO2 islands preferentially formed from point defects in the organic layer. Assuming 4 Å high oxide islands (a value close to the characteristic distance between two SiO2 units in vitreous silica, and slightly higher than the height of a double atomic step on a Si(111) surface), the corresponding surface coverage would remain limited to 0.2. Note that this estimate is consistent with the XPS data, though the agreement can only be semiquantitative due to the difficulty to define a thickness unambiguously at such low oxidation levels. The infrared spectra in Figure 2 show that switching the plasma to 500 mW cm-3 leads to the appearance of a welldefined LO-TO splitting, as already mentioned. One also observes a weak negative band at 1015 cm-1. The fact that it is negative indicates that the corresponding species are removed or modified by the 500 mW cm-3 plasma. We attribute this band to isolated Si-O-Si vibrators corresponding to small amounts of silicon suboxide plausibly formed during sample preparation26 [this band is absent when special care is taken to avoid any oxidation during decyl grafting (unpublished results)]. Qualitatively, the main IR (positive) band with LO-TO splitting seen in spectra f-i of Figure 2 and the disappearance of the initial oxide (negative peak at 1015 cm-1) suggest that the oxygen plasma at 500 mW cm-3 is responsible for the formation of a compact silicon oxide layer. Using the same method as above, the thickness of this oxide is estimated to be 4 Å after 3 h at 500 mW cm-3. AFM observations suggest that the oxide

layer is uniform since the initial terrace structure of the hydrogenated or grafted surface is preserved. Only the roughness measured on the terraces appears to be somewhat increased (see Supporting Information). 3.3. Kinetics of Selective Monolayer Oxidation in LowPower-Density Plasma, Analyzed by Real-Time IR Spectroscopy. A. Analysis of the IR Results. Figure 6a presents narrow spectra corresponding to the CH groups (bands in the range 2700-3000 cm-1) recorded during the early steps of a low-power-density plasma treatment (10 mW cm-3). For convenience, the reference spectrum is that of the completely oxidized surface after 3 h in the high-power-density plasma (500 mW cm-3). It corresponds to the situation where the initial organic monolayer was entirely oxidized and removed. The attribution of the different vibration modes is given in the figure. At times shorter than 10 min, a decrease of the intensities of νsCH2 and νasCH2 is observed while the intensities of the νsCH3 and νasCH3 bands remain essentially constant. This indicates that the terminal methyl groups remain intact, while some CH2 units along the alkyl chains undergo chemical transformations. However, at longer times, the different types of νCHx become hardly separable. Only a broad positive contribution is seen. The corresponding time evolution of the νCdO band (region 1700-1800 cm-1) is presented in Figure 6b. The νCdO band is asymmetric and appears centered at ca. 1715 cm-1. With increasing time, the peak broadens and its maximum shifts to higher wave numbers (up to ca. 1740 cm-1). As for the C-H bands, the CdO band does not evolve after 2 h of the 10 mW cm-3 treatment.


Figure 7. p-polarized infrared spectra in the range 1600-1900 cm-1 for a SiC2H5 surface under oxygen plasma (10 mW cm-3) at different times. The reference spectrum was taken before switching on the plasma.

However, the change of shape of the νCdO band suggests that it corresponds to several distinct species in variable amounts. This idea is further supported by the analysis of the νCdO band (region 1700-1800 cm-1) for the plasma treatment of a SiC2H5 surface (Figure 7). In that case, the same behavior as with SiC10H21 is observed in the three characteristic spectral regions (νSiO, νCH, νCdO), including for the reaching of a near steady state, but the νCdO band clearly presents two contributions suggestive of two types of species containing CdO groups. Here, the possible position of the CdO groups is limited by the number of carbon atoms in the chain (2 carbon atoms). This band is then seen to exhibit two distinct contributions at around 1720 and 1750 cm-1. This suggests that the broad CdO band obtained in Figure 6b can be decomposed into two similar contributions. The fitting of the CdO band for the SiC10H21 case can actually be refined by analyzing it as the superposition of two bands. Figure 8 presents a typical fit obtained with two peaks (fixed positions and widths, Figure 8a) and the evolution (Figure 8b) of the CdO bands (1720 cm-1, fwhm 40 cm-1, squares; 1750 cm-1, fwhm 70 cm-1, dots). The species associated with the vibration at 1720 cm-1 are formed first, reach a maximum, and are partially replaced by the species associated with the vibration at 1750 cm-1, which become dominant after 20 min of treatment. The constant intensities of νsCH3 and νasCH3 during the early steps of the plasma treatment (Figure 6a) indicate that the first CdO functions appear on secondary carbons. The fact that the formation of these functions is not associated with the removal of end groups (CH3) suggests that they arise from ketones rather than aldehydes. Extra pieces of information for identifying the distinct carbonyl species can be sought from the lower wavenumber range 900-1500 cm-1 (see Figure 2. An expanded view of this region can be found as Supporting Information). Among various overlapping features, one can distinguish a sharp positive band around 1270 cm-1 (see Figure 2, spectra b-f) and a broader band around 1410 cm-1, which both appear during the 10 mW cm-3 plasma treatment. The evolution of the 1270 cm-1 band was also plotted in Figure 8 (open circles) and appears to be conspicuously similar to that of the ν2CdO band. The bands at 1270 and 1410 cm-1 are strong indications in favor of carboxyl groups (rather than, e.g., esters). The band at 1270 cm-1 is associated with the stretching mode νC-OH in carboxylic acids. Its evolution suggests that ν2CdO is actually associated with carboxylic acids. B. Simplified Kinetic Models. Figure 9a compares the time evolution of the intensities of the νCdO band and of the νsCH2

J. Phys. Chem. C, Vol. 113, No. 32, 2009 14423 band. The latter band appears to be the most relevant one to study the early times of plasma oxidation because the CH2 groups disappear first and the symmetric mode is little overlapping with the other νCHx contributions (and consequently yields the most reliable piece of information). Here the νCdO band has been fitted by a single broad pseudo-Voigt function with variable position and width. The absorbance values extracted from the analysis of the experimental spectra are plotted using square (νCdO) and dot (νsCH2) symbols. If CH2 species are converted into oxidized CdO species during the treatment, the correlated evolution of the νCdO and νsCH2 signals should be understood using an appropriate kinetic model. The experimental variation of the CH2 band intensity is fairly fitted by an exponential decay (Figure 9a), suggestive of first-order kinetics

[CH2] ) [CH2]0 exp(-k1t)

(1)

where k1 is the corresponding first-order rate constant. In a naive approach, one might postulate that one CdO group appears each time one CH2 disappears. However, this postulate is clearly not valid, because the initial growth of the CdO band is very slow and suggestive of a second-order mechanism. The introduction of an intermediate surface species X then appears necessary, the formation of CdO resulting from the succession of two firstorder reactions k1

k2

(2)

CH2 f X f CdO This scheme leads to the following equations:

[X] )

k1[CH2]0 [exp(-k1t) - exp(-k2t)] k2 - k1

(

[CdO] ) [CH2]0 1 -

1 [k exp(-k1t) k2 - k1 2

(3)

)

k1 exp(-k2t)]

(4)

The simulated curves (lines in Figure 9a) are in good agreement with experiment with k1 ) 0.1 min-1 and k2 ) 0.05 min-1. The above model can be refined in order to account for the distinct behaviors of the ketone and carboxyl bands ν1CdO and ν2CdO (Figure 8). A straightforward refinement is assuming a cascade mechanism k1

k2

k3

CH2 f X f ketone f carboxylic acid

(5)

The resulting equations, describing the time dependence of the various concentrations, are given as Supporting Information, and a typical result in fair agreement with experiment is shown as Figure 9b. Note that this refinement does not bring any change to the values of k1 and k2 of the first approach. Therefore, a first analysis of the kinetics of the oxidation suggests that the formation of oxidized CdO species takes place through the formation of a chemical intermediate. Such a hypothesis is also in full agreement with chemical interpretation. Oxidation of carbon to degree 1 (alcohol) would be expected in a first step. Unfortunately, a characteristic infrared signature of such species, possibly present in the region 900-1500 cm-1 (see, e.g., weak

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Figure 8. (a) Typical fit of the CdO region of a SiC10H21 surface after 1 h under a 10 mW cm-3 oxygen plasma. Symbols are experimental data. The solid line corresponds to the fit obtained with two peaks. (b) Time evolution of the integrated absorbances of νCdO [ν1CdO, squares (blue online), and ν2CdO, dots (red online), bands centered at 1720 and 1750 cm-1, respectively] and νC-OH (band centered at 1270 cm-1, open circles) for a SiC10H21 surface under 10 mW cm-3 oxygen plasma.

Figure 9. (a) Time evolution of the integrated absorbance of νsCH2 and νCdO vibrations for a SiC10H21 surface under a 10 mW cm-3 oxygen plasma. The symbols are experimental data. The values have been scaled to unity (at t ) 0 for νsCH2 and at t ) 100 min for νsCdO). The solid lines are calculated curves according to eqs 1-4, with the parameter values k1 ) 0.1 min-1 and k2 ) 0.05 min-1. (b) Separate evolution of the ν1CdO (c1) (squares, blue online) and ν2CdO (c2) (dots, red online) bands, fitted with the refined “cascade” model (eq 5 and Supporting Information; k1, k2 same as above, k3 ) 0.09 min-1). The points are experimental (left-hand scale) and the curves are calculated (right-hand scale).

bands in Supporting Information at 1110 and 1320 cm-1, which may be attributed to secondary alcohols) can hardly be analyzed quantitatively due to the many overlapping bands, including those of silicon oxide. 3.4. Quantification of the Oxidized Carbon Atoms Generated with the Low-Power-Density Plasma. At this stage, several points remain unsatisfactory in the simulation as well as in the experimental data. The agreement between simulation and real-time IR experiments remains qualitative, especially at long reaction times. This probably means that the assumption of a simple cascade mechanism is an oversimplification. Also, allowing conversion of all of the CH2’s to carboxyl groups is not realistic (there can be at most one carboxyl group per chain, and the presence of esters appears unlikely). On the other hand, the infrared technique is very powerful as it allows for realtime kinetic studies, but it suffers from a relative insensitivity to the intermediate X, and from the fact that the IR absorption cross sections depend on the local environment. For example, the IR absorption cross section of the νCdO band may vary typically by a factor of 2 between different carbonyl species.37 That of the νCHx bands may vary by up to an order of magnitude, depending on the environment of the CHx group.38 In the present case, it means that the strong decrease of the νsCH2 band intensity may be due not only to a loss of CHx

species, but also to a decrease in the IR cross section of the remaining CH2’s, due to the formation of oxygen-rich functions on the neighboring carbon sites. The need for a more quantitative “calibration” of the various species present on the surface has motivated investigations using XPS, which in many respects appear as a complementary technique to IR spectroscopy. In particular, XPS gives a more directly quantitative information about the various oxidized species, since the yield of a core level is essentially independent of the oxidation state of the element considered. A. Analysis of the XPS Results. C1s spectra obtained in the early stages of the monolayer oxidation are shown in Figure 10. Before any exposure to the plasma (Figure 10a), the C1s spectrum may be decomposed into the following three bands: the main one, centered at 285 eV, is associated with aliphatic carbons (with C-C and C-H environment), the weak shoulder at 284 eV is associated with carbon atoms involved in a Si-C bond,21,39 and the satellite at 286.5 eV is attributed to adventitious contamination.39 On the other extreme end, after 2 h treatment (Figure 10e) the spectrum analysis requires the superposition of five characteristic peaks. The peaks located at 285 and 284 eV are related to the residual alkyl chain backbone and its linkage to the silicon surface. The three other bands around 286.5, 288, and 289.5 eV are respectively associated



Figure 10. XPS spectra (normalized by the Si2p intensities) of the C1s region for a SiC10H21 surface under oxygen plasma (10 mW cm-3) at times t ) 0 (a), 3 min (b), 6 min (c), 30 min (d), and 2 h (e).

with the different oxidized (C-O), (CdO) and (O-CdO) species. The same five contributions are found at intermediate times (Figure 10, spectra b-d) in variable relative intensities (except for the position of the 284 eV contribution, the other parameters were left free in the fit). Figure 11 shows the time evolution of the integrated intensities of the different C1s peaks as a function of time. The integrated intensities have been normalized to the Si2p spectrum (we assume here that the attenuation of the Si2p spectrum does not change, because the organic monolayer remains essentially intact in terms of thickness, as it will be verified a posteriori, and the photoelectron attenuation length is also marginally affected by the oxidation state of the organic chains). These normalized intensities were converted to an average number of carbon atoms per chain by applying a simple multiplicative scale

factor, chosen so that the initial state corresponds to 10 carbon atoms (case of a SiC10H21 surface). The ordinate in Figure 11 then represents the average number of carbon atoms of each kind in a chain. In agreement with the IR results, the layer is seen to reach a near steady state after 10-30 min. The faster evolution appearing for the XPS data (Figure 11), as compared to IR (Figure 9), is assigned to a further oxidation of reactive intermediate species (such as hydroperoxides, see hereafter) during the storage time (a few hours) between the plasma treatment and the XPS investigation. The evolutions of CdO and O-CdO (Figure 11a), attributed to ketones and carboxylic acids, are similar to those observed by infrared (Figure 8b). Most interestingly, the presence and the evolution of the C-O intermediate provide a direct experimental support to the main assumption of the above kinetic model (see Section 3.3).

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Aureau et al. B. Molecular-Scale Reaction Mechanisms. The schemes of Figure 12 summarize the successive chemical states that at this stage are plausibly encountered in the course of the progressive oxidation of C10H21 chains exposed to a 10 mW cm-3 oxygen plasma. The oxidation of the molecular chains starts with the formation of secondary alcohols, which accounts for the preservation of the CH3 terminal groups in Figure 6a at times shorter than 10 min, then some of these alcohols are oxidized to ketones and finally to carboxylic acids. The large number of remaining unoxidized carbons, even when carboxylic acids are formed, suggests that functionalization occurs near the top of the chains (though CH2 is more readily oxidized than CH3). The obtained steady state appears resistant and contains all of the different oxidized species. On a molecular scale, the above chemical states have to be accounted for in light of the plasma composition. In the 10 mW cm-3 oxygen plasma, as calculated by the simulation, the concentration of O(3P) is very weak, which suggests that the functionalization, at least in the first 30 min of plasma treatment, is attributable to molecular oxygen species only (Figure 4). On this basis, it appears difficult to imagine that secondary alcohols could be formed in a single step. As a matter of fact, excited states of molecular oxygen, especially O2(1∆g), are known to react with C-H to create hydroperoxides,40 preferentially on secondary or tertiary carbon atoms (consistent with a beginning of the oxidation on CH2 rather than on CH3), as depicted in the following scheme:

Figure 11. Time evolution of the integrated intensities (normalized by the Si2p intensity and scaled to the initial value of 10 carbons per chain) of the C1s peaks during the first half-hour of oxygen plasma treatment of a SiC10H21 surface. The points at 120 min have been added to show the slowing down of the evolution at longer times.

However, the most striking results from XPS are that these C-O species survive in a significant concentration even after more highly oxidized species have been formed. It is also worth noticing that within the first 30 min, more than 50% of the carbons have been left unmodified (Figure 11b). The loss of one carbon per chain on average (Figure 11b) is only semiquantitative, due to our neglect of escape-depth effects (a length that may change upon oxidation of the layer). In any case, the loss appears minor. It is also important to notice that the number of carbon atoms involved in Si-C bonding remains almost constant throughout the plasma treatment (Figure 11b). After correction for the photoelectron escape depth ∼35 Å,39 we find, as expected, one C-Si bond per chain. The XPS data indicate therefore that a selective oxidation of the aliphatic chains may be achieved with negligible etching of the monolayer and no significant oxidation of the substrate along the plasma treatment at 10 mW cm-3. However, a prolonged exposure of the sample to the same plasma leads to partial monolayer degradation. After 120 min, the loss amounts to ∼3 carbon atoms per chain (see points at 120 min in Figure 11), and a maximum of two carbonyls (CdO + O-CdO) per chain can be obtained. This conclusion is consistent with the comparable intensities of the CdO bands obtained during the oxidation of SiC10H21 (Figure 6b) and SiC2H5 (Figure 7) surfaces. Note however that all the last numbers are uncorrected for escape-depth effects due to the unknown effect of layer oxidation on the photoelectron meanfree path and the unknown repartition in depth of the oxidized species in the ∼1 nm thick organic layer.

These hydroperoxides could be the first intermediate produced during plasma treatment. Such species could rearrange or react further to give alcohols, ketones, acids, or more complicated products (Figure 13). The X species evidenced above and identified to C-O could then consist of these hydroperoxides as well as of secondary alcohols, or most probably of both. C. Kinetic Model ReWisited. The near steady state obtained in the very low power regime, and especially the survival of unoxidized carbons in an important concentration, suggests that the layer oxidation becomes limited by steric hindrance (increase in the concentration of bulky functions COH, CdO, COOH in the layer) and stops when the surviving C-Hs are no longer accessible for the O2(1∆g) species. The survival of carbons in a low oxidation state (C-O species) is also attributed to the same limitations. In an attempt to make our simple kinetic model more realistic, we have incorporated the following additional features: (i) the rate constants k1, k2, k3, are actually proportional to the amount of free space available around the chains. In the initial state, we can assume that this free space amounts to a fraction ε ∼ 10-1 of the chain volume [that is, a unit of volume V has an available space V(1 + ε)]. The replacement of a CH2 unit by an oxidized species leads to a “swelling” δ. If δ > ε, at some stage of oxidation, no free space is left, the rate constants vanish, and the system gets “frozen”. (ii) The formation of a carboxyl group leads to the loss of the chain end (as suggested by the last reaction in Figure 13). The mathematics for the incorporation of these features are given as Supporting Information. A typical result is shown as Figure 14. Though the model remains a crude approximation of the many reaction pathways (Figure 13) and ignores the detailed repartition of the oxidized functions in the depth of the modified layer, it is seen to reproduce the main experimental features, that is, the sequence of the formed



Figure 12. Schematic representation of the successive oxidation steps of SiC10H21 layers during the early times of exposure to oxygen plasma.

for producing singlet dioxygen free of atomic oxygen might be of interest for an improved control of layer modification. 4. Conclusion

Figure 13. Sketches of some possible rearrangements and subsequent reactions for hydroperoxides. The numbers in the figure for the three rearrangements are estimated values of ∆H (in kJ/mol), as deduced from tabulated data for various hydroperoxides, diols, ketones and carboxylic acids.41

Figure 14. Simulated time evolution of the concentrations (number per chain) of the carbons of each type from the kinetic model taking into account the etching of chain ends and the slowing down of the kinetics due to decreasing free space between the oxidized chains (k1 ) 0.1 min-1, k2 ) 0.2 min-1, k3 ) 1 min-1, k4 ) 0.1 min-1, ε ) 0.07, δ ) 0.2, see details in Supporting Information). Compare with the experimental results in Figure 11.

species CH f C-O f CdO f O-CdO with the correct concentrations (compare with Figure 11), a concentration of carboxyl groups lower than 1 per chain, and the reaching of a steady state of modification. This result strongly suggests that, although the molecular mechanism is more complicated than a simple cascade mechanism, steric hindrance limitations are a key factor for reaching a steady state in which the various oxidized species are all present in significant amounts and are located at the outer part of the molecular layer. The residual evolution observed at long times (see point at 120 min in Figure 13), not accounted for by the model, can be attributed to the presence of a very small concentration of atomic oxygen in the plasma (Figure 4), which opens new reaction pathways leading to degradation of the layer. This suggests that alternate means

We have demonstrated that selectiVe oxidation of alkyl monolayers at a silicon surface can be obtained with an oxygen plasma of a very low power density (∼10 mW cm-3). In this regime the etching out of organic fragments is negligible on a time scale of a few minutes and the oxidation of the underlying silicon substrate remains in the submonolayer range. The low content of atomic oxygen in this low-power-density plasma as well as the cell geometry are the key factors for the selective oxidation of the organic chains, assigned to singlet molecular oxygen. Quantitative studies using XPS and in situ infrared spectroscopy support a reaction mechanism in which a steady state is obtained. Remarkably, only the outer part of the layer is oxidized in this state and the layer exhibits secondary alcohol, ketone, and carboxylic acid functions. Such a self-limited oxidation can be attributed to steric hindrance limitations. Using a low power density oxygen plasma provides a convenient method for trimming the surface wettability of grafted organic layers, and it opens a new route to the preparation of surfaces for the immobilization of controlled amounts of various chemical functions. Furthermore, it had been found that grafting organic chains bearing terminal chemical functions somewhat lowers the surface concentration in grafted chains,26 which is detrimental to the passivation and long-term stability of the surface. The possibility of functionalizing the outer part of a dense alkyl monolayer grafted at a silicon surface without deteriorating the silicon/monolayer interface represents a new route for obtaining controlled functional layers of a higher density. Alkyl monolayers functionalized with carboxyl groups by treatment in an oxygen plasma of low power density could then be used as substrates to immobilize probe biomolecules17,42,43 and elaborate biosensors of improved stability and reliability. Acknowledgment. The authors are highly indebted to Professor K. Hassouni for organizing a key discussion on the plasma theoretical aspects of this work, acting here as a bridge between the surface-science and plasma communities. Supporting Information Available: AFM characterization of the surfaces after plasma oxidation, expanded view of an IR spectrum in the 900-1900 cm-1 region, simple kinetic model for a three-step cascade conversion, and refined kinetic model. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Jaynes, W. F.; Boyd, S. A. Clays Clay Miner. 1991, 39, 428–436. (2) Abbott, N. L.; Folkers, J. P.; Whitesides, G. M. Science 1992, 257, 1380–1382. (3) Ando, M.; Kawasaki, M.; Imazeki, S.; Sasaki, H.; Kamata, T. Appl. Phys. Lett. 2004, 85, 1849–1851. (4) Nuësch, F.; Si-Ahmed, L.; François, B.; Zuppiroli, L. AdV. Mater. 1997, 9, 222–225.

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Controlled Oxidation of Alkyl Monolayers Grafted onto Flat Si (111) in

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