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Organic monolayers on Si(111) by electrochemical method
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Electrochimica Acta, Vol. 43, Nos 19±20, pp. 2791±2798, 1998 # 1998 Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain S0013-4686(98)00020-6 0013±4686/98 $19.00 + 0.00
Organic monolayers on Si(111) by electrochemical method P. Allongue,a* C. Henry de Villeneuve,a J. Pinson,b F. Ozanam,c J. N. Chazalvielc and X. Wallartd Laboratoire de Physique des Liquides et EÂlectrochimie, CNRS, UPR 15, ESPCI, 10 rue Vauquelin, 75005 Paris, France
a
Laboratoire d'EÂlectrochimie MoleÂculaire, CNRS, URA 438, Universite Paris 7, 2 Place Jussieu, 75005 Paris, France
b
Laboratoire de Physique de la MatieÁre CondenseÂe, CNRS - UMR 1254, EÂcole Polytechnique, 91128 Palaiseau, France
c
d
IEMN, CNRS, UMR 9929, Avenue PoincareÂ, B.P. 69, 59655 Villeneuve d'Ascq Cedex, France (Received in Newcastle 21 January 1998)
AbstractÐThis work details the formation of organic monolayers on Si(111) by electrochemical methods. We show that grafting of phenyl groups is possible by reduction of N2±Ar±X cations where the substituent X may be Br, NO2, COOH, CN, CnH2n + 1 (n = 1, 4, 12). Characterizations show that the electrochemical process is self stopped after completion of the ®rst monolayer whose structure is (2 1) close packed in the case X = Br and CH3 as observed by STM. The stability and passivating properties of ®lms are also investigated. # 1998 Published by Elsevier Science Ltd. All rights reserved
INTRODUCTION There is great potential interest in connecting electronically an organic material with a semiconductor like Si which is widely used in microelectronics. Low temperature dielectric ®lms of controlled thickness, templates for nanolithography and formation of a seed layer to grow thick polymer ®lms are among the envisioned applications. Integrated sensors, by choice of the functional group on top of molecules as well as electroluminescence, with suitable molecules, constitute other possibilities. Organic thin ®lms on silicon generally refer to SAM (self assembled monolayers) formed on Si/ SiO2 substrates [1±3]. These organic ®lms, for instance obtained by reacting a R±Si±Cl3 solution with a speci®cally treated OH-terminated SiO2 overlayer, are however not in electrical contact with the semiconductor substrate [Fig. 1(a)], which precludes some of the above applications. If one excludes the UHV approach (molecular adsorption on dangling *Author to whom correspondence should be addressed.
bonds), the only possibility to directly bind a molecule on Si is using a radical reaction so as to abstract the existing ligands (e.g. H, Cl) and form a new chemical bond [Fig. 1(b)]. In fact, Si is a covalent material and no dangling bond is available out of a UHV chamber. From a technological point of view silicon is obviously an important substrate. From a fundamental point of view, it is also the best material to start with because almost perfect H- [4±6] or Clterminated [7] (111) atomically ¯at surfaces may be prepared. Such a chemical homogeneity of the surface is favorable for performing a uniform surface radical reaction and ¯at terraces are necessary for the internal order of monolayers. Upon choosing a molecular process several other considerations must be born in mind: (i) the radical reaction itself should preserve the initial surface microstructure, (ii) the maximum molecular packing will depend, not only on the size of the molecule but also on the surface orientation and hybridization of the bridging atoms because covalent bonding means rigid and directional bonds and ®nally, (iii) since one deals with electronic properties, the Si-molecule
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P. Allongue et al. are quoted against SSE. Reagent grade chemicals and bidistilled water were used for solutions. XPS measurements were performed at IEMN (Lille) with a spectrometer PHI-5600 (monochromated AlKa source, resolution 0.45 eV on Ag3d5/2). FTIR was performed ex-situ within a multiple internal re¯ection geometry as exposed elsewhere [13, 14]. STM images were acquired in-situ with the microscope described previously [18]. Unless otherwise speci®ed, the gray scale was proportional to heights with heights decreasing from white to black. RESULTS
Fig. 1. Dierent routes for molecular ®lm formation on Si surfaces. (a) SAM formation on SiO2. No electrical contact exists between the molecule and the substrate. (b) Radical reaction on a H- or Cl- terminated surface. A direct contact between the molecule and the substrate is available.
bond should be the least possible polarized to reduce the density of states [8]. Only few groups have addressed the question of direct molecular grafting on Si surfaces. Using a chain of chemical radical reactions Lindford and Chidsey [9, 10] obtained a robust (2 1) alkyl layer on Si(111), but with Si±OH groups on remaining sites. Bansal et al. [11, 12] also grafted alkyl chains with a Grignard reaction on a Cl-terminated Si(111) surface prepared by reacting an H±Si(111) surface on PCl5. Electrochemistry is an alternative way to generate radicals. The group of Chazalviel [13±15], modi®ed porous silicon surfaces with -OCH3 and CH3 groups by anodization in organic solvents. In the last case a coverage of 0.8 was achieved. On ¯at Si(111) a (1 1) layer is expected. Isopropanol groups -OCH(CH3)2 were also anodically attached and observed by STM in a mixture of NaOH/ isopropanol [8]. Quite recently, our group introduced for the ®rst time the electrochemical reduction of aryl diazonium salts on H±Si(111) electrodes as a grafting technique [16, 17]. The present paper provides new XPS, FTIR and preliminary STM characterizations which pertain to the formation of molecular monolayers terminated by various functional groups. Data also con®rm the stability of the modi®cation against usual etching solutions of Si and examine the passivating behavior of layers against ambient oxidation.
Electrochemical results Figure 2 shows a typical cyclic voltammogram (CV) obtained with H-terminated n-Si(111) electrodes in aqueous diazonium N2±Ar±X solutions (X = Br in Fig. 2). The supporting electrolyte was 0.1 M H2SO4+2% HF to ensure that the surface remained free of oxide prior to the reduction. On the ®rst negative going sweep a broad wave is generally observed (curve a), except for X = NO2, CN or COOH where the simultaneous reduction of the functional group results in the superposition of a second wave [16]. In all cases, the initial reduction wave disappears on the second negative going sweep (dotted line) as if the electrode was completely inhibited. However, ultrasonic rinsing in organic solvents (methanol, ether and methanol again) nearly restores the initial cathodic wave with a cathodic delay depending on the molecule size. A shift of the hydrogen evolution reaction (HER) is also systematically observed. The eect of the layer on charge transfer is in fact quite obvious in the case of the HER. In Fig. 3(a), the substituent X was an alkyl chain of dierent length [X = CnH2n + 1 with n = 1, 4, 12]. The solution was the supporting electrolyte (0.1 M H2SO4+2% HF). With respect to the H-terminated surface (dotted line), the shift DU of the HER, as measured for a current density of 1 mA/cm2, increases with increasing chain length and amounts to 0.5 V for n = 12 [Fig. 3(b)]. Notice that the experiments of Fig. 3 constitute a simple experimental test to verify that surfaces have actually been modi®ed.
EXPERIMENTAL Silicon substrates were cut from 1 O cm n-type Si(111) wafers and cleaned in hot trichlorethylene, acetone and methanol prior to stripping the oxide in 2% HF (1 min) and etching in 40% NH4F (6 min). Electrochemical measurements were conducted in a 3-electrode cell with a mercury saturated sulfate electrode (SSE) as a reference of potential and a Pt counter electrode. All potentials
Fig. 2. Cyclic voltammograms of H±Si(111) in a solution of N2±Ar±Br in 0.1 M H2SO4+2% HF. First (bold line) and second (broken line) negative going sweeps.
Organic monolayers on Si(111)
Fig. 3. (a) Cyclic voltammograms of modi®ed surfaces in 0.1 M H2SO4+2% HF (samples rinsed in solvents). The bold lines correspond to Ar±X modi®cations with X = CnH2n + 1. The value of n is indicated in the ®gure. In the case of n = 12, the same curve was obtained after dipping the sample for 2±10 min in 40% HF, 40% NH4F and NaOH. (b) Variations DU(n).
Figure 3(b) studies also the robustness of the modi®cation against dierent etching solutions. In the case X = C12H25, the voltammogram of the as modi®ed sample, was not altered after exposure of the modi®ed surface to 40% HF or 40% NH4F or even NaOH for 2±10 min. With shorter chains (n = 1, 4) the test was also quite successful after the HF dip, with no change in DU. Dipping surfaces, still with n = 1, 4 in 40% NH4F or NaOH (2 min) gave less good results since DU decreased after these treatments. This points to the role of the chain length on the protecting properties of layers.
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two absorption bands (1350 and 1525 cmÿ1), which are related to symmetric and asymmetric nNO2 modes and (ii) the prominent peak at 1605 cmÿ1 and weaker ones at 1185 and 1115 cmÿ1, which are attributed to ring deformation modes. The ®rst band is known to be enhanced by substituents like NO2 [19]. The survival of NO2 related IR bands, with unchanged intensity after the HF rinse, agrees with Fig. 3(A) and pertains to the stability of the layer in HF. The narrow band at 1120 cmÿ1 observed in spectrum (b) is consistent with Si±phenyl bonding [19]. Though weak, this band was resolved with other substituents such as X = Br and CN. It was however impossible to further con®rm this assignment by looking at the region 650± 750 cmÿ1 with our experimental set-up. There were not much dierences with s-polarized light. The behavior of the Si±H related band is quite instructive. The negative sharp band at 2083 cmÿ1 implies that terrace monohydrides [4] have been removed from the surface. This narrow band was quenched under s-polarized light, which con®rmed the good quality of the initial surface. The remaining positive broad band 02100 cmÿ1 proves that some Si±H species are left on the surface after modi®cation, particularly step mono- and dihydrides from terrace edges (bands at 2071 cmÿ1 and 02090±2130 cmÿ1). Some terrace monohydrides are also presumably left since the integrated background of the positive band around 2100 cmÿ1 is quantitatively larger on the initial H±Si(111) surface. No polarization eect was noticed for this broad band. These two points will be discussed in more detail below. Also noticeable is the quenching of the broad positive band around 1040 cmÿ1 after the HF rinse. We attribute it to the removal of physisorbed molecules and/or counterions rather than to oxide stripping. An oxide layer, even incomplete, would have given a polarization eect in spectrum (a) due to TO±LO splitting. This was not observed. The most likely explanation is trapping of BFÿ 4 counterions in
FTIR results FTIR spectra (p-polarization) are shown in Fig. 4 for a sample modi®ed by an Ar±NO2 layer. This functional group was considered because of its intense IR absorption bands. The reference spectrum was that of the H-terminated surface (prepared by etching in 40% NH4F). After modi®cation the surface was rinsed in acetone and methanol (spectrum a) and further exposed to 48% HF for 4 min (spectrum b). Positive bands therefore correspond to newly formed species and negative bands to removed species. The same sample was studied throughout the process for a quantitative comparison of spectra, with the disadvantage however that some contamination may appear at each step. The presence of Ar±NO2 molecules on the surface is ascertained by several positive peaks: (i) the
Fig. 4. p-Polarized FTIR spectra of a surface modi®ed by an Ar±NO2 layer. The reference spectrum was that of the H±Si(111) surface. (a) As modi®ed surface (rinsed in solvents); (b) same as (a) but after subsequent dip in 40% HF (4 min).
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or on top of the layer, since the diazonium solution gives rise to similar weak IR bands in the same region. Finally, the positive at 1425 cmÿ1 ®nds no obvious assignment. It was not systematically resolved and is attributed to contaminations appearing throughout the experimental procedure. High resolution XPS The derivatization and aging of the Ar±Br modi®ed surface was studied by XPS. Figure 5 presents high resolution Si2p and Br3d XPS spectra. All spectra correspond to the same sample. Curves (a± b) were recorded, respectively, 24 h and 6 months after preparation (ambient storage). Curves (c) were obtained after dipping the aged sample for 2 min in 40% HF. The take o angle of photoelectrons was 258 from the surface plane to increase surface sensitivity. The position and shape of the Si2p doublet [Fig. 5(A)] were quite similar to those of a H-terminated surface. A ®t required however an additional contribution shifted by 0.3±0.4 eV, on the modi®ed surface as well as on the H-terminated one, which makes a clear statement about surface binding such
Fig. 5. High resolution Si2p (A) and Br3d (B) XPS peaks on the Ar±Br modi®ed surface (rinsed in solvents). The detector is 258 from the surface plane. Curve (a): sample stored 24 h at ambient; curve (b): same sample but after 2 months at ambient and curve (c): same as (b) but after subsequent dip in 40% HF (2 min).
as Si±C bond formation dicult (expected chemical shift <1 eV [20]). The absence of contribution with a chemical shift 01±2 eV also gives clear evidence that Si±O±C bond formation may be excluded. The intensity of the satellite peak at 0103±103.5 eV indicates the presence of some oxide, whose average thickness increases after 6 months [Fig. 3(A), curves a and b]. Correspondingly to this observation, Fig. 5(B) shows that the Br coverage remains identical upon aging (curves a±b), when accounting for the 7% decrease of the incident photon ¯ux as determined on calibration samples. Dipping the aged sample in 40% HF removed all oxide and 50% of the Br [Fig. 5(A) and (B), curves c]. STM observations In-situ imaging, in the grafting solution itself, was not considered in this study because of the formation of excess reaction products (see the next section). Figure 6 presents STM images of an Ar±Br modi®ed surfaces image under potential control in an indierent electrolyte (2 M NaOH). This approach was preferred to ambient imaging to avoid surface degradation upon imaging as has been reported in the case of H±Si surfaces [21]. NaOH was considered because high resolution imaging is quite easy with tungsten tips in this solution. The cathodic polarization applied to the sample and necessary to image it [22] was such that the hydrogen evolution current was 100 to 200 mA/cm2. It was veri®ed that the modi®cation was resistant for hours in these conditions. The tunneling current was 00.5 nA. This was the upper limit to image the modi®ed surface. Figure 6(a) and (b) are large scale STM images of the Ar±Br modi®ed surface. With the gray scale proportional to heights [Fig. 5(a)] the surface looks very much the same as before any modi®cation, with atomically smooth terraces and bilayer high steps (3.1 AÊ). Enhancing the gray scale on each of the terraces [Fig. 5(b)] reveals nevertheless some distinct dierences with respect to the naked surface. Very shallow defects (depth < 1 AÊ, typical diameter 30 AÊ) are visible from place to place on otherwise atomically smooth terraces. On the atomic scale, Fig. 6(c) shows that the structure is uniform over two adjacent terraces separated by one bilayer high step. The grey scale was applied to each of the two terraces. Instead of the usual (3.8 3.8 AÊ) hexagonal lattice of unreconstructed (1 1)-Si(111) [8], a nearly rectangular (3.8 6.6 AÊ) unit cell is resolved. The corrugation was 00.1±0.2 AÊ. Observations were quite similar on Ar±CH3 modi®ed surfaces. DISCUSSION Layer structure Figure 7 presents the molecular model of the layer. Benzene rings are attached vertically to the Si
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Fig. 6. STM images of Ar±Br modi®ed Si(111) surfaces (rinsed in solvents). (a) and (b) Large scale image (400 800 AÊ2) of the same portion of the surface with two gray scales (see text). (c) 161 181 AÊ2 atomically resolved image.
atoms as suggested by FTIR (Fig. 4). This bonding was anticipated from the sp3 hybridization of Si surface atoms and sp2 hybridization of carbon atoms on the aryl ring. The remaining sites are terminated by hydrogen to agree with FTIR (see the positive broad band at 2100 cmÿ1 in Fig. 4). Minimization of Van der Waals interactions
between molecules, according molecular mechanics [23], promotes the rotation of rings by 308 from Si close packed Si atomic rows. Ring stack into rows. The unit cell of the p structure is rectangular: 3.84 6.65 AÊ (i.e. a a 3, a = 3.84 AÊ) in agreement with Fig. 6(c) where Br atoms are probably imaged. With respect to the Si lattice the struc-
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Fig. 7. Molecular model for the Ar±Br layer using computation based on molecular mechanics.
ture is however (1 2) and the coverage 0.5. Each molecule occupies 025 AÊ2, a value close to maximum packing since several works found values in the range 21.8±25.2 AÊ2 [3, 24±26]. The monolayer thickness is consistent with larger scale STM images [Fig. 6(a)] which do suggest a uniform thickness. There is indeed no islands nor protrusion visible on modi®ed terraces. This is also in accordance with previous results [16, 17], such as the attenuation of the Si2p peak (with a take o angle of photoelectrons at 908) and RBS measurements. The shallow depressions resolved in Fig. 6(b) indicate however that some defects exist in the layer. They probably correspond to disordered region such as grain boundaries between domains. In conclusion, Fig. 7 is supported by much data. A puzzling contradiction remains however when considering the absence of polarization dependence in Fig. 4. IR bands related to nNO2 (1350 and 1525 cmÿ1), /Si±phenyl (1120 cmÿ1) and remaining /Si±H bonds (broad band at 2100 cmÿ1) should indeed be modi®ed under s-polarized light. Surface roughening induced by the modi®cation could have been a straightforward explanation. Figure 6(a) indicates that this is not the case. We therefore tentatively suggest the following interpretation:
physisorbed molecules (see next section), counter ions or moieties modify the local environment and aect the IR response of the dierent bands. It is indeed well known that the /Si±H stretch modes are greatly perturbed by putting the surface in contact with a liquid [13±15]. Deposition of Ag on H± Si(111) also enhances the IR response of terrace / Si±H [27]. The same eect is seemingly observed here because the integrated intensity of the Si±H band (02100 cmÿ1) is greater after than before the modi®cation. Reaction model Observations may be accounted for by the simpli®ed reaction scheme: . ÿ4 ÿ Ar±X N2 1 N/ N±Ar±X eÿ ÿ . . /Si±H Ar±Xÿ ÿ4/Si ÿ H±Ar±X
2
. . /Si Ar±Xÿ ÿ4/Si±Ar±X ÿ
3
. where Ar±X designs an aryl radical. The reaction in equation (1) is the well accepted reaction of reduction of diazonium salts and was initially studied on glassy carbon and HOPG (highly oriented pyrolitic graphite) [24±26]. This reaction occurs at low
Organic monolayers on Si(111) overvoltage, before the evolution of molecular hydrogen (Fig. 2). Molecular grafting itself is a two step process. The initial step is abstraction of one H . atom from the Si surface by a ®rst Ar±X radical (equation (2)). The second step is the reaction . between a second Ar±X radical and the surface . radical /Si , to form the covalent bond /Si±Ar±X (equation (3)). It should be mentioned that the technique of modi®cation is successful because . of the diculty of generating the radical anion ÿAr±Br by further . reduction of the aryl radical Ar±X [28]. This makes the aryl radicals suciently long lived to react with the surface. It will however been shown in the next section, that molecular grafting is not the only radical reaction and that other ones occur.
Charge transfer at modi®ed surfaces There are several indications that the organic layers aect the interfacial transfer kinetics. This is particularly visible in the case of the hydrogen evolution reaction [Fig. 3(b)] where the shift DU increases with the chain length, since electron tunneling becomes more dicult as the chain length increases. This result resembles very much the case of thiol SAM's on Au(111) [29]. The internal chain±chain interactions have also probably a bene®cial eect on shielding defects such as disordered regions. In the case of a redox reaction such as the reduction of the diazonium salt, changes in kinetics are much less obvious, since the initial wave (Fig. 2) is almost identical to that observed on the H-terminated surface, except for a small cathodic shift. On unrinsed surfaces, the inhibition observed on the second potential sweep (Fig. 2, dotted curve) stems from physisorbed molecules generated during the electrochemical process. In fact, the model (equations (1)±(3)) has neglected some side reactions. According to the net reaction: ÿ4 ÿ Si±H 2N N±Ar±X 2eÿ ÿ Si±Ar±X H±Ar±X
4
the formation of one aryl monolayer consumes 124 mC/cm2 (2 1.610ÿ193.9 1014 C/cm2], which is less than the 250±375 mC/cm2 found by voltammetry (Fig. 2) and indicates radical losses. Protonation in solution and/or dimer formation are likely reactions. Together with the reaction in equation (2) these reaction paths produce species which may be insoluble and therefore adsorb on top of the molecular layer. This could explain the inhibition of charge transfer stem from the physisorption of such molecules. That molecules do not adsorb on the silicon itself and hinder grafting may come from the fact that the radicals are generated in the close vicinity of the electrode surface and
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that radical losses (by protonation, dimerization) occur further away from the surface. Stability and aging behavior of layers Layers are quite stable in air since no loss in Br is detected after 6 months [see Fig. 5(B), curve b]. Stability is however not synonym of complete passivation against ambient oxidation. The main information is the limited loss in Br upon stripping the oxide on the aged sample [Fig. 5(B), curve c]. By comparison freshly prepared layers are insensitive to HF (Figs 3 and 4). This indicates that the molecular ®lm is located on top of the oxide on the aged surface. This further suggests that only 050% of the surface has been oxidized after 6 months since the Br coverage is divided by a factor 2 [Fig. 5(B), curve c]. This is an encouraging result because the Si±H sites seem to be stabilized by the close-packed ultrathin organic layer (thicknessÇ 6 AÊ in the case of the Ar±Br layer). By comparison, a native oxide layer grows within 800 h (33 days) on H±Si(111) [30]. In solutions which etch the substrate itself (case of NaOH and 40% NH4F), the protecting properties of layers seem to improve with the length of the molecule. However, this remains dicult to quantify and probably depends on the order of the layer, i.e. on the quality of the initial surface. The aging (Fig. 5) and protecting (Fig. 3) behaviors of layers suggest the following. In air, oxide formation stems from moistures and oxygen. The process starts at defects of the layer, probably the ones observed in Fig. 6(a) [Fig. 8(A)]. Step edges are also favorable sites [30], because molecular grafting does not take place there. After nucleation, oxide islands laterally expand at the expense of the substrate and under the organic layer [Fig. 8(B) and (C)]. Dipping the age sample in HF removes therefore the oxide together with the molecules attached on top [Fig. 8(D)]. If the oxidation of the modi®ed surface was uniform no Br signal should have been detected in Fig. 5(B). In the case of chemical etching, the scenario is similar with initiation of the attack at above defects followed by local anisotropic etching. Step edges are of course also favorable sites for etching [18]. The eect of the length of the alkyl substituent on the stability of layers is interesting (Fig. 3). Shielding of defects by the mobility of alkyl chains seems a likely explanation. Playing with the same process might therefore greatly improve the protection of surfaces against ambient oxidation. Work in this direction is underway. CONCLUSIONS Electrochemical molecular derivatization of Si surfaces has been demonstrated. The primary advantage of the method, based on the electrochemical reduction of aryl diazonium salts, is its
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Fig. 8. Schematic model explaining the oxide growth on the Ar±Br modi®ed surface. Step A: oxidation starts at defects in the layer; steps (B±C): lateral expansion of oxide islands and step D: same as (C) but after HF rinse.
simplicity and the possibility to choose a wide variety of functional groups and perform the process from an aqueous solution. Functional groups include so far Br, NO2, CN, NH2, COOH and CnH2n + 1 (n = 1, 4, 12). Another advantage of this method is that it preserves the surface structure on the atomic scale (cathodic reaction). The modi®cation itself is quite stable and it is able to slow down native oxide growth at ambient. REFERENCES 1. A. Ulman, in Ultrathin Organic Film. Academic Press, San Diego, 1991. 2. A. Ulman, Chem. Rev. 96, 1533 (1996). 3. A. Ulman, Adv. Mater. 2, 573 (1990). 4. G. S. Higashi and Y. J. Chabal, in Handbook of Semiconductor Wafer Cleaning Technology, ed. W. Kern. Noyes Publications, Park Ridge, 1993. 5. G. J. Pietsch, U. KoÈhler and M. Henzler, J. Appl. Phys. 73, 4797 (1993). 6. H. E. Hessel, A. Feltz, U. Memmert and R. J. Behm, Chem. Phys. Lett. 186, 275 (1991). 7. M. R. Lindford, P. Fenter, P. M. Eisenberger and C. E. D. Chidsey, Mater. Res. Soc. Proc., Boston Fall Meeting. 1998, in press. 8. P. Allongue, Phys. Rev. Lett. 77, 1986 (1996). 9. M. R. Lindford and C. E. D. Chidsey, J. Am. Chem. Soc. 115, 12631 (1993). 10. M. R. Lindford, P. Fenter, P. M. Eisenberger and C. E. D. Chidsey, J. Am. Chem. Soc. 117, 3145 (1995). 11. A. Bansal, X. Li, I. Lauermann, N. S. Lewis, S. Yi and W. H. Weinberg, J. Am. Chem. Soc. 117, 3145 (1995). 12. A. Bansal, X. Li, I. Lauermann, N. S. Lewis, S. Yi and W. H. Weinberg, J. Am. Chem. Soc. 118, 7225 (1996). 13. M. Warntjes, C. Veillard, F. Ozanam and J. N. Chazalviel, J. Electrochem. Soc. 142, 4138 (1995).
14. C. Veillard, M. Warntjes, F. Ozanam and J. N. Chazalviel, Proc. 4th Int. Symp. on Advanced Luminescent Materials, PV 95±25, eds. D. J. Lockwood, P. M. Fauchet, N. Koshida and S. R. J. Brueck. The Electrochemical Society Softbounds, 1995, p. 250. 15. T. Dubois, F. Ozanam and J. N. Chazalviel, Book of Abstracts of the 3rd European Workshop on Electrochemical Processing of Semiconductors (EWEPS `96). Meudon, Nov. 1996. 16. C. Henry de Villeneuve, J. Pinson and P. Allongue, J. Phys. Chem. 101, 2415 (1997). 17. C. H. de Villeneuve, J. Pinson, F. Ozanam, J. N. Chazalviel and P. Allongue, Mater. Res. Soc. Proc. Boston, 1998, in press. 18. P. Allongue, V. Kieling and H. Gerischer, J. Electrochem. Soc. 140, 1008 (1993). 19. L. J. Bellamy, in Infra Red Spectra of Complex Molecules. Methuen, London, 1958. 20. Elmer Perkin (ed.), Handbook of X-Ray Photoelectron Spectroscopy. 1992. 21. J. A. Dagata, J. Schneir, H. H. Harary, J. Benett and W. Tseng, J. Vac. Sci. Technol. B 9, 1384 (1991). 22. P. Allongue, in Advances in Electrochemical Sciences and Engineering, Vol. 4, Chap. 1, eds. H. Gerischer and C. W. Tobias. VCH, Weinheim, 1995, p. 3. 23. Software Alchemy 2000, Tripos, St. Louis, U.S.A. 24. M. Delamar, R. Hitmi, J. Pinson and J. M. Saveant, J. Am. Chem. Soc. 114, 5883 (1992). 25. P. Allongue, M. Delamar, B. Desbat, O. Fagebaume, R. Hitmi, J. Pinson and J. M. Saveant, J. Am. Chem. Soc. 119, 201 (1997). 26. Y. C. Liu and R. L. Mc Crerry, J. Am. Chem. Soc. 117, 11254 (1995). 27. M. Gruynters, Y. J. Chabal and P. Dumas, to be published. 28. J. M. Saveant, New J. Chem. 16, 131 (1992). 29. J. A. M. Sondag-Huethorst and L. G. L. Fokkink, J. Electroanal. Chem. 367, 49 (1994). 30. U. Neuwald, H. E. Hessel, A. Feltz, U. Memmert and R. J. Behm, Appl. Phys. Lett. 60, 1307 (1992).
Electrochimica Acta, Vol. 43, Nos 19±20, pp. 2791±2798, 1998 # 1998 Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain S0013-4686(98)00020-6 0013±4686/98 $19.00 + 0.00
Organic monolayers on Si(111) by electrochemical method P. Allongue,a* C. Henry de Villeneuve,a J. Pinson,b F. Ozanam,c J. N. Chazalvielc and X. Wallartd Laboratoire de Physique des Liquides et EÂlectrochimie, CNRS, UPR 15, ESPCI, 10 rue Vauquelin, 75005 Paris, France
a
Laboratoire d'EÂlectrochimie MoleÂculaire, CNRS, URA 438, Universite Paris 7, 2 Place Jussieu, 75005 Paris, France
b
Laboratoire de Physique de la MatieÁre CondenseÂe, CNRS - UMR 1254, EÂcole Polytechnique, 91128 Palaiseau, France
c
d
IEMN, CNRS, UMR 9929, Avenue PoincareÂ, B.P. 69, 59655 Villeneuve d'Ascq Cedex, France (Received in Newcastle 21 January 1998)
AbstractÐThis work details the formation of organic monolayers on Si(111) by electrochemical methods. We show that grafting of phenyl groups is possible by reduction of N2±Ar±X cations where the substituent X may be Br, NO2, COOH, CN, CnH2n + 1 (n = 1, 4, 12). Characterizations show that the electrochemical process is self stopped after completion of the ®rst monolayer whose structure is (2 1) close packed in the case X = Br and CH3 as observed by STM. The stability and passivating properties of ®lms are also investigated. # 1998 Published by Elsevier Science Ltd. All rights reserved
INTRODUCTION There is great potential interest in connecting electronically an organic material with a semiconductor like Si which is widely used in microelectronics. Low temperature dielectric ®lms of controlled thickness, templates for nanolithography and formation of a seed layer to grow thick polymer ®lms are among the envisioned applications. Integrated sensors, by choice of the functional group on top of molecules as well as electroluminescence, with suitable molecules, constitute other possibilities. Organic thin ®lms on silicon generally refer to SAM (self assembled monolayers) formed on Si/ SiO2 substrates [1±3]. These organic ®lms, for instance obtained by reacting a R±Si±Cl3 solution with a speci®cally treated OH-terminated SiO2 overlayer, are however not in electrical contact with the semiconductor substrate [Fig. 1(a)], which precludes some of the above applications. If one excludes the UHV approach (molecular adsorption on dangling *Author to whom correspondence should be addressed.
bonds), the only possibility to directly bind a molecule on Si is using a radical reaction so as to abstract the existing ligands (e.g. H, Cl) and form a new chemical bond [Fig. 1(b)]. In fact, Si is a covalent material and no dangling bond is available out of a UHV chamber. From a technological point of view silicon is obviously an important substrate. From a fundamental point of view, it is also the best material to start with because almost perfect H- [4±6] or Clterminated [7] (111) atomically ¯at surfaces may be prepared. Such a chemical homogeneity of the surface is favorable for performing a uniform surface radical reaction and ¯at terraces are necessary for the internal order of monolayers. Upon choosing a molecular process several other considerations must be born in mind: (i) the radical reaction itself should preserve the initial surface microstructure, (ii) the maximum molecular packing will depend, not only on the size of the molecule but also on the surface orientation and hybridization of the bridging atoms because covalent bonding means rigid and directional bonds and ®nally, (iii) since one deals with electronic properties, the Si-molecule
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P. Allongue et al. are quoted against SSE. Reagent grade chemicals and bidistilled water were used for solutions. XPS measurements were performed at IEMN (Lille) with a spectrometer PHI-5600 (monochromated AlKa source, resolution 0.45 eV on Ag3d5/2). FTIR was performed ex-situ within a multiple internal re¯ection geometry as exposed elsewhere [13, 14]. STM images were acquired in-situ with the microscope described previously [18]. Unless otherwise speci®ed, the gray scale was proportional to heights with heights decreasing from white to black. RESULTS
Fig. 1. Dierent routes for molecular ®lm formation on Si surfaces. (a) SAM formation on SiO2. No electrical contact exists between the molecule and the substrate. (b) Radical reaction on a H- or Cl- terminated surface. A direct contact between the molecule and the substrate is available.
bond should be the least possible polarized to reduce the density of states [8]. Only few groups have addressed the question of direct molecular grafting on Si surfaces. Using a chain of chemical radical reactions Lindford and Chidsey [9, 10] obtained a robust (2 1) alkyl layer on Si(111), but with Si±OH groups on remaining sites. Bansal et al. [11, 12] also grafted alkyl chains with a Grignard reaction on a Cl-terminated Si(111) surface prepared by reacting an H±Si(111) surface on PCl5. Electrochemistry is an alternative way to generate radicals. The group of Chazalviel [13±15], modi®ed porous silicon surfaces with -OCH3 and CH3 groups by anodization in organic solvents. In the last case a coverage of 0.8 was achieved. On ¯at Si(111) a (1 1) layer is expected. Isopropanol groups -OCH(CH3)2 were also anodically attached and observed by STM in a mixture of NaOH/ isopropanol [8]. Quite recently, our group introduced for the ®rst time the electrochemical reduction of aryl diazonium salts on H±Si(111) electrodes as a grafting technique [16, 17]. The present paper provides new XPS, FTIR and preliminary STM characterizations which pertain to the formation of molecular monolayers terminated by various functional groups. Data also con®rm the stability of the modi®cation against usual etching solutions of Si and examine the passivating behavior of layers against ambient oxidation.
Electrochemical results Figure 2 shows a typical cyclic voltammogram (CV) obtained with H-terminated n-Si(111) electrodes in aqueous diazonium N2±Ar±X solutions (X = Br in Fig. 2). The supporting electrolyte was 0.1 M H2SO4+2% HF to ensure that the surface remained free of oxide prior to the reduction. On the ®rst negative going sweep a broad wave is generally observed (curve a), except for X = NO2, CN or COOH where the simultaneous reduction of the functional group results in the superposition of a second wave [16]. In all cases, the initial reduction wave disappears on the second negative going sweep (dotted line) as if the electrode was completely inhibited. However, ultrasonic rinsing in organic solvents (methanol, ether and methanol again) nearly restores the initial cathodic wave with a cathodic delay depending on the molecule size. A shift of the hydrogen evolution reaction (HER) is also systematically observed. The eect of the layer on charge transfer is in fact quite obvious in the case of the HER. In Fig. 3(a), the substituent X was an alkyl chain of dierent length [X = CnH2n + 1 with n = 1, 4, 12]. The solution was the supporting electrolyte (0.1 M H2SO4+2% HF). With respect to the H-terminated surface (dotted line), the shift DU of the HER, as measured for a current density of 1 mA/cm2, increases with increasing chain length and amounts to 0.5 V for n = 12 [Fig. 3(b)]. Notice that the experiments of Fig. 3 constitute a simple experimental test to verify that surfaces have actually been modi®ed.
EXPERIMENTAL Silicon substrates were cut from 1 O cm n-type Si(111) wafers and cleaned in hot trichlorethylene, acetone and methanol prior to stripping the oxide in 2% HF (1 min) and etching in 40% NH4F (6 min). Electrochemical measurements were conducted in a 3-electrode cell with a mercury saturated sulfate electrode (SSE) as a reference of potential and a Pt counter electrode. All potentials
Fig. 2. Cyclic voltammograms of H±Si(111) in a solution of N2±Ar±Br in 0.1 M H2SO4+2% HF. First (bold line) and second (broken line) negative going sweeps.
Organic monolayers on Si(111)
Fig. 3. (a) Cyclic voltammograms of modi®ed surfaces in 0.1 M H2SO4+2% HF (samples rinsed in solvents). The bold lines correspond to Ar±X modi®cations with X = CnH2n + 1. The value of n is indicated in the ®gure. In the case of n = 12, the same curve was obtained after dipping the sample for 2±10 min in 40% HF, 40% NH4F and NaOH. (b) Variations DU(n).
Figure 3(b) studies also the robustness of the modi®cation against dierent etching solutions. In the case X = C12H25, the voltammogram of the as modi®ed sample, was not altered after exposure of the modi®ed surface to 40% HF or 40% NH4F or even NaOH for 2±10 min. With shorter chains (n = 1, 4) the test was also quite successful after the HF dip, with no change in DU. Dipping surfaces, still with n = 1, 4 in 40% NH4F or NaOH (2 min) gave less good results since DU decreased after these treatments. This points to the role of the chain length on the protecting properties of layers.
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two absorption bands (1350 and 1525 cmÿ1), which are related to symmetric and asymmetric nNO2 modes and (ii) the prominent peak at 1605 cmÿ1 and weaker ones at 1185 and 1115 cmÿ1, which are attributed to ring deformation modes. The ®rst band is known to be enhanced by substituents like NO2 [19]. The survival of NO2 related IR bands, with unchanged intensity after the HF rinse, agrees with Fig. 3(A) and pertains to the stability of the layer in HF. The narrow band at 1120 cmÿ1 observed in spectrum (b) is consistent with Si±phenyl bonding [19]. Though weak, this band was resolved with other substituents such as X = Br and CN. It was however impossible to further con®rm this assignment by looking at the region 650± 750 cmÿ1 with our experimental set-up. There were not much dierences with s-polarized light. The behavior of the Si±H related band is quite instructive. The negative sharp band at 2083 cmÿ1 implies that terrace monohydrides [4] have been removed from the surface. This narrow band was quenched under s-polarized light, which con®rmed the good quality of the initial surface. The remaining positive broad band 02100 cmÿ1 proves that some Si±H species are left on the surface after modi®cation, particularly step mono- and dihydrides from terrace edges (bands at 2071 cmÿ1 and 02090±2130 cmÿ1). Some terrace monohydrides are also presumably left since the integrated background of the positive band around 2100 cmÿ1 is quantitatively larger on the initial H±Si(111) surface. No polarization eect was noticed for this broad band. These two points will be discussed in more detail below. Also noticeable is the quenching of the broad positive band around 1040 cmÿ1 after the HF rinse. We attribute it to the removal of physisorbed molecules and/or counterions rather than to oxide stripping. An oxide layer, even incomplete, would have given a polarization eect in spectrum (a) due to TO±LO splitting. This was not observed. The most likely explanation is trapping of BFÿ 4 counterions in
FTIR results FTIR spectra (p-polarization) are shown in Fig. 4 for a sample modi®ed by an Ar±NO2 layer. This functional group was considered because of its intense IR absorption bands. The reference spectrum was that of the H-terminated surface (prepared by etching in 40% NH4F). After modi®cation the surface was rinsed in acetone and methanol (spectrum a) and further exposed to 48% HF for 4 min (spectrum b). Positive bands therefore correspond to newly formed species and negative bands to removed species. The same sample was studied throughout the process for a quantitative comparison of spectra, with the disadvantage however that some contamination may appear at each step. The presence of Ar±NO2 molecules on the surface is ascertained by several positive peaks: (i) the
Fig. 4. p-Polarized FTIR spectra of a surface modi®ed by an Ar±NO2 layer. The reference spectrum was that of the H±Si(111) surface. (a) As modi®ed surface (rinsed in solvents); (b) same as (a) but after subsequent dip in 40% HF (4 min).
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or on top of the layer, since the diazonium solution gives rise to similar weak IR bands in the same region. Finally, the positive at 1425 cmÿ1 ®nds no obvious assignment. It was not systematically resolved and is attributed to contaminations appearing throughout the experimental procedure. High resolution XPS The derivatization and aging of the Ar±Br modi®ed surface was studied by XPS. Figure 5 presents high resolution Si2p and Br3d XPS spectra. All spectra correspond to the same sample. Curves (a± b) were recorded, respectively, 24 h and 6 months after preparation (ambient storage). Curves (c) were obtained after dipping the aged sample for 2 min in 40% HF. The take o angle of photoelectrons was 258 from the surface plane to increase surface sensitivity. The position and shape of the Si2p doublet [Fig. 5(A)] were quite similar to those of a H-terminated surface. A ®t required however an additional contribution shifted by 0.3±0.4 eV, on the modi®ed surface as well as on the H-terminated one, which makes a clear statement about surface binding such
Fig. 5. High resolution Si2p (A) and Br3d (B) XPS peaks on the Ar±Br modi®ed surface (rinsed in solvents). The detector is 258 from the surface plane. Curve (a): sample stored 24 h at ambient; curve (b): same sample but after 2 months at ambient and curve (c): same as (b) but after subsequent dip in 40% HF (2 min).
as Si±C bond formation dicult (expected chemical shift <1 eV [20]). The absence of contribution with a chemical shift 01±2 eV also gives clear evidence that Si±O±C bond formation may be excluded. The intensity of the satellite peak at 0103±103.5 eV indicates the presence of some oxide, whose average thickness increases after 6 months [Fig. 3(A), curves a and b]. Correspondingly to this observation, Fig. 5(B) shows that the Br coverage remains identical upon aging (curves a±b), when accounting for the 7% decrease of the incident photon ¯ux as determined on calibration samples. Dipping the aged sample in 40% HF removed all oxide and 50% of the Br [Fig. 5(A) and (B), curves c]. STM observations In-situ imaging, in the grafting solution itself, was not considered in this study because of the formation of excess reaction products (see the next section). Figure 6 presents STM images of an Ar±Br modi®ed surfaces image under potential control in an indierent electrolyte (2 M NaOH). This approach was preferred to ambient imaging to avoid surface degradation upon imaging as has been reported in the case of H±Si surfaces [21]. NaOH was considered because high resolution imaging is quite easy with tungsten tips in this solution. The cathodic polarization applied to the sample and necessary to image it [22] was such that the hydrogen evolution current was 100 to 200 mA/cm2. It was veri®ed that the modi®cation was resistant for hours in these conditions. The tunneling current was 00.5 nA. This was the upper limit to image the modi®ed surface. Figure 6(a) and (b) are large scale STM images of the Ar±Br modi®ed surface. With the gray scale proportional to heights [Fig. 5(a)] the surface looks very much the same as before any modi®cation, with atomically smooth terraces and bilayer high steps (3.1 AÊ). Enhancing the gray scale on each of the terraces [Fig. 5(b)] reveals nevertheless some distinct dierences with respect to the naked surface. Very shallow defects (depth < 1 AÊ, typical diameter 30 AÊ) are visible from place to place on otherwise atomically smooth terraces. On the atomic scale, Fig. 6(c) shows that the structure is uniform over two adjacent terraces separated by one bilayer high step. The grey scale was applied to each of the two terraces. Instead of the usual (3.8 3.8 AÊ) hexagonal lattice of unreconstructed (1 1)-Si(111) [8], a nearly rectangular (3.8 6.6 AÊ) unit cell is resolved. The corrugation was 00.1±0.2 AÊ. Observations were quite similar on Ar±CH3 modi®ed surfaces. DISCUSSION Layer structure Figure 7 presents the molecular model of the layer. Benzene rings are attached vertically to the Si
Organic monolayers on Si(111)
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Fig. 6. STM images of Ar±Br modi®ed Si(111) surfaces (rinsed in solvents). (a) and (b) Large scale image (400 800 AÊ2) of the same portion of the surface with two gray scales (see text). (c) 161 181 AÊ2 atomically resolved image.
atoms as suggested by FTIR (Fig. 4). This bonding was anticipated from the sp3 hybridization of Si surface atoms and sp2 hybridization of carbon atoms on the aryl ring. The remaining sites are terminated by hydrogen to agree with FTIR (see the positive broad band at 2100 cmÿ1 in Fig. 4). Minimization of Van der Waals interactions
between molecules, according molecular mechanics [23], promotes the rotation of rings by 308 from Si close packed Si atomic rows. Ring stack into rows. The unit cell of the p structure is rectangular: 3.84 6.65 AÊ (i.e. a a 3, a = 3.84 AÊ) in agreement with Fig. 6(c) where Br atoms are probably imaged. With respect to the Si lattice the struc-
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Fig. 7. Molecular model for the Ar±Br layer using computation based on molecular mechanics.
ture is however (1 2) and the coverage 0.5. Each molecule occupies 025 AÊ2, a value close to maximum packing since several works found values in the range 21.8±25.2 AÊ2 [3, 24±26]. The monolayer thickness is consistent with larger scale STM images [Fig. 6(a)] which do suggest a uniform thickness. There is indeed no islands nor protrusion visible on modi®ed terraces. This is also in accordance with previous results [16, 17], such as the attenuation of the Si2p peak (with a take o angle of photoelectrons at 908) and RBS measurements. The shallow depressions resolved in Fig. 6(b) indicate however that some defects exist in the layer. They probably correspond to disordered region such as grain boundaries between domains. In conclusion, Fig. 7 is supported by much data. A puzzling contradiction remains however when considering the absence of polarization dependence in Fig. 4. IR bands related to nNO2 (1350 and 1525 cmÿ1), /Si±phenyl (1120 cmÿ1) and remaining /Si±H bonds (broad band at 2100 cmÿ1) should indeed be modi®ed under s-polarized light. Surface roughening induced by the modi®cation could have been a straightforward explanation. Figure 6(a) indicates that this is not the case. We therefore tentatively suggest the following interpretation:
physisorbed molecules (see next section), counter ions or moieties modify the local environment and aect the IR response of the dierent bands. It is indeed well known that the /Si±H stretch modes are greatly perturbed by putting the surface in contact with a liquid [13±15]. Deposition of Ag on H± Si(111) also enhances the IR response of terrace / Si±H [27]. The same eect is seemingly observed here because the integrated intensity of the Si±H band (02100 cmÿ1) is greater after than before the modi®cation. Reaction model Observations may be accounted for by the simpli®ed reaction scheme: . ÿ4 ÿ Ar±X N2 1 N/ N±Ar±X eÿ ÿ . . /Si±H Ar±Xÿ ÿ4/Si ÿ H±Ar±X
2
. . /Si Ar±Xÿ ÿ4/Si±Ar±X ÿ
3
. where Ar±X designs an aryl radical. The reaction in equation (1) is the well accepted reaction of reduction of diazonium salts and was initially studied on glassy carbon and HOPG (highly oriented pyrolitic graphite) [24±26]. This reaction occurs at low
Organic monolayers on Si(111) overvoltage, before the evolution of molecular hydrogen (Fig. 2). Molecular grafting itself is a two step process. The initial step is abstraction of one H . atom from the Si surface by a ®rst Ar±X radical (equation (2)). The second step is the reaction . between a second Ar±X radical and the surface . radical /Si , to form the covalent bond /Si±Ar±X (equation (3)). It should be mentioned that the technique of modi®cation is successful because . of the diculty of generating the radical anion ÿAr±Br by further . reduction of the aryl radical Ar±X [28]. This makes the aryl radicals suciently long lived to react with the surface. It will however been shown in the next section, that molecular grafting is not the only radical reaction and that other ones occur.
Charge transfer at modi®ed surfaces There are several indications that the organic layers aect the interfacial transfer kinetics. This is particularly visible in the case of the hydrogen evolution reaction [Fig. 3(b)] where the shift DU increases with the chain length, since electron tunneling becomes more dicult as the chain length increases. This result resembles very much the case of thiol SAM's on Au(111) [29]. The internal chain±chain interactions have also probably a bene®cial eect on shielding defects such as disordered regions. In the case of a redox reaction such as the reduction of the diazonium salt, changes in kinetics are much less obvious, since the initial wave (Fig. 2) is almost identical to that observed on the H-terminated surface, except for a small cathodic shift. On unrinsed surfaces, the inhibition observed on the second potential sweep (Fig. 2, dotted curve) stems from physisorbed molecules generated during the electrochemical process. In fact, the model (equations (1)±(3)) has neglected some side reactions. According to the net reaction: ÿ4 ÿ Si±H 2N N±Ar±X 2eÿ ÿ Si±Ar±X H±Ar±X
4
the formation of one aryl monolayer consumes 124 mC/cm2 (2 1.610ÿ193.9 1014 C/cm2], which is less than the 250±375 mC/cm2 found by voltammetry (Fig. 2) and indicates radical losses. Protonation in solution and/or dimer formation are likely reactions. Together with the reaction in equation (2) these reaction paths produce species which may be insoluble and therefore adsorb on top of the molecular layer. This could explain the inhibition of charge transfer stem from the physisorption of such molecules. That molecules do not adsorb on the silicon itself and hinder grafting may come from the fact that the radicals are generated in the close vicinity of the electrode surface and
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that radical losses (by protonation, dimerization) occur further away from the surface. Stability and aging behavior of layers Layers are quite stable in air since no loss in Br is detected after 6 months [see Fig. 5(B), curve b]. Stability is however not synonym of complete passivation against ambient oxidation. The main information is the limited loss in Br upon stripping the oxide on the aged sample [Fig. 5(B), curve c]. By comparison freshly prepared layers are insensitive to HF (Figs 3 and 4). This indicates that the molecular ®lm is located on top of the oxide on the aged surface. This further suggests that only 050% of the surface has been oxidized after 6 months since the Br coverage is divided by a factor 2 [Fig. 5(B), curve c]. This is an encouraging result because the Si±H sites seem to be stabilized by the close-packed ultrathin organic layer (thicknessÇ 6 AÊ in the case of the Ar±Br layer). By comparison, a native oxide layer grows within 800 h (33 days) on H±Si(111) [30]. In solutions which etch the substrate itself (case of NaOH and 40% NH4F), the protecting properties of layers seem to improve with the length of the molecule. However, this remains dicult to quantify and probably depends on the order of the layer, i.e. on the quality of the initial surface. The aging (Fig. 5) and protecting (Fig. 3) behaviors of layers suggest the following. In air, oxide formation stems from moistures and oxygen. The process starts at defects of the layer, probably the ones observed in Fig. 6(a) [Fig. 8(A)]. Step edges are also favorable sites [30], because molecular grafting does not take place there. After nucleation, oxide islands laterally expand at the expense of the substrate and under the organic layer [Fig. 8(B) and (C)]. Dipping the age sample in HF removes therefore the oxide together with the molecules attached on top [Fig. 8(D)]. If the oxidation of the modi®ed surface was uniform no Br signal should have been detected in Fig. 5(B). In the case of chemical etching, the scenario is similar with initiation of the attack at above defects followed by local anisotropic etching. Step edges are of course also favorable sites for etching [18]. The eect of the length of the alkyl substituent on the stability of layers is interesting (Fig. 3). Shielding of defects by the mobility of alkyl chains seems a likely explanation. Playing with the same process might therefore greatly improve the protection of surfaces against ambient oxidation. Work in this direction is underway. CONCLUSIONS Electrochemical molecular derivatization of Si surfaces has been demonstrated. The primary advantage of the method, based on the electrochemical reduction of aryl diazonium salts, is its
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Fig. 8. Schematic model explaining the oxide growth on the Ar±Br modi®ed surface. Step A: oxidation starts at defects in the layer; steps (B±C): lateral expansion of oxide islands and step D: same as (C) but after HF rinse.
simplicity and the possibility to choose a wide variety of functional groups and perform the process from an aqueous solution. Functional groups include so far Br, NO2, CN, NH2, COOH and CnH2n + 1 (n = 1, 4, 12). Another advantage of this method is that it preserves the surface structure on the atomic scale (cathodic reaction). The modi®cation itself is quite stable and it is able to slow down native oxide growth at ambient. REFERENCES 1. A. Ulman, in Ultrathin Organic Film. Academic Press, San Diego, 1991. 2. A. Ulman, Chem. Rev. 96, 1533 (1996). 3. A. Ulman, Adv. Mater. 2, 573 (1990). 4. G. S. Higashi and Y. J. Chabal, in Handbook of Semiconductor Wafer Cleaning Technology, ed. W. Kern. Noyes Publications, Park Ridge, 1993. 5. G. J. Pietsch, U. KoÈhler and M. Henzler, J. Appl. Phys. 73, 4797 (1993). 6. H. E. Hessel, A. Feltz, U. Memmert and R. J. Behm, Chem. Phys. Lett. 186, 275 (1991). 7. M. R. Lindford, P. Fenter, P. M. Eisenberger and C. E. D. Chidsey, Mater. Res. Soc. Proc., Boston Fall Meeting. 1998, in press. 8. P. Allongue, Phys. Rev. Lett. 77, 1986 (1996). 9. M. R. Lindford and C. E. D. Chidsey, J. Am. Chem. Soc. 115, 12631 (1993). 10. M. R. Lindford, P. Fenter, P. M. Eisenberger and C. E. D. Chidsey, J. Am. Chem. Soc. 117, 3145 (1995). 11. A. Bansal, X. Li, I. Lauermann, N. S. Lewis, S. Yi and W. H. Weinberg, J. Am. Chem. Soc. 117, 3145 (1995). 12. A. Bansal, X. Li, I. Lauermann, N. S. Lewis, S. Yi and W. H. Weinberg, J. Am. Chem. Soc. 118, 7225 (1996). 13. M. Warntjes, C. Veillard, F. Ozanam and J. N. Chazalviel, J. Electrochem. Soc. 142, 4138 (1995).
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