Chemical reactivity of hydrogen-terminated crystalline silicon surfaces

Current Opinion in Solid State and Materials Science 9 (2005) 66–72

Chemical reactivity of hydrogen-terminated crystalline silicon surfaces Rabah Boukherroub

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Institut de Recherche Interdisciplinaire, Cité ScientiWque, Avenue Poincaré – BP 60069, 59652 Villeneuve d’Ascq Cedex, France Institut d’Electronique, de Microélectronique et de Nanotechnologie, Cité ScientiWque, Avenue Poincaré – BP 60069, 59652 Villeneuve d’Ascq Cedex, France

Abstract Chemical functionalization of hydrogen-terminated silicon surfaces holds considerable promise from both fundamental and applied research aspects. This article covers a selection of examples concerning the proposed strategies for chemical grafting of diVerent organic functionalities and further immobilization of biological molecules on the surface through covalent bonding. From the fundamental view point, the reaction mechanism is discussed in terms of electron–hole pair excitons generation or formation of delocalized radical cations at the silicon surface for the light-induced surface hydrosilylation. The electronic properties of the silicon/organic monolayer interface are investigated in details and direct detection of DNA hybridization using electrochemical means is presented. © 2006 Elsevier Ltd. All rights reserved. Keywords: Hydrogen-terminated crystalline silicon; Organic monolayers; Silicon hybrids; Reaction mechanism; Electrical characterization; Biomolecule immobilization; Detection

1. Introduction Organic functionalization of hydrogen-terminated crystalline silicon surfaces is a very active research area from both fundamental and applied aspects. From the fundamental view point, the chemistry of the surface Si–H bonds and understanding the reaction mechanism involved in the grafting processes will oVer opportunities for designing new reactions in organic and organometallic synthesis. A myriad of approaches to form organic monolayers chemically tethered to silicon is useful for diVerent applications ranging from surface passivation to organic electronics and biotechnology. Using the well-established microfabrication techniques will allow to prepare and to integrate various microcomponents into the devices such as biological microelectromechanical systems (bioMEMS) with potential applications in microscale, high throughput biosensing and medical devices. Furthermore, chemical grafting of organic species directly on hydrogen-terminated silicon surface (with no intervening *

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oxide layer) will allow to fully take advantage of the electronic properties of the semiconductor to develop sensitive devices for the detection of biomolecular interactions. The review article covers a selection of papers published in the last 18 months. It is divided in several parts and deals with the: (i) new techniques developed for integrating organic materials on hydrogen-terminated silicon surface, (ii) progress in understanding the reaction mechanism, (iii) electrical and electronic characterization of the organic monolayer/silicon interface, (iv) covalent immobilization of biomolecules on the surface, (v) label free detection of biomolecular interactions on the chemically-functionalized silicon surfaces. 2. New routes for organic monolayers formation on hydrogen-terminated silicon surfaces There is a continuous need and search for new methods to assemble organic monolayers on hydrogen-terminated silicon surfaces under mild conditions. This is mainly motivated by the ease and the adaptability of the method to large scale integration, but also to overcome the instability and incompatibility of large variety of chemical and biological

R. Boukherroub / Current Opinion in Solid State and Materials Science 9 (2005) 66–72

active moieties borne by the organic molecules to assemble on the surface with the existing techniques. Zuilhof et al. have used visible light (447–658 cm¡1) to graft various unsaturated molecules (1-alkenes and 1-alkynes) on Si–H surfaces at room temperature [*1,*2]. A systematic study of the eVect of the type, and concentration of doping and silicon orientation was undertaken. It was shown that the monolayer formation on Si(1 1 1) is more eYcient than on Si(1 0 0) and the covalent attachment follows the rate order: highly doped n 7 lowly doped n > lowly doped p > highly doped p [*2]. The reaction time is in the order of 10 h (compared to 2 h for UV irradiation) to achieve a highly dense organic monolayer, but the technique is very mild and will allow to graft organic molecules bearing labile groups. The hydrosilylation reaction of activated alkynes on Si(1 1 1)–H was examined by Liu et al. They reported an easy and eYcient reaction that proceeds at room temperature for 24– 40 h with 31–56% surface coverage [3]. Methyl and acyl chloride terminated monolayers were produced when a bare silicon is scribed in the presence of mono and diacid chlorides [4]. The reaction of aryldiazonium salts with Si–H surfaces [poly Si or single crystal (1 1 1) or (1 0 0), p-doped, n-doped, or intrinsic] was revisited by Stewart et al. [5]. They showed that the reaction takes place at open circuit potential (OCP) at room temperature with a surface coverage of 65%. Surface exposition to the diazonium salt for times higher than two hours resulted in the formation of multilayers. Moreover, an elegant method to generate aryldiazonium salts in situ and their subsequent reaction with hydride-terminated silicon surfaces was developed by the same group [*6]. It consists on the in situ conversion of aryldiethyltriazenes into aryldiazonium salts using 2% HF. The method allows carrying out the grafting process in air since HF insures a continuous hydride passivation of the surface. The procedure was successfully applied to covalently attach SWNTs to a silicon surface and thus to prepare a carbon nanotube–molecule-silicon junctions [7]. In a recent report, Dirk et al. proposed a new technique to assemble organic mono and multilayers on Si(1 0 0)–H based on electroreduction of iodonium salts precursors [8]. Bunimovich et al. have extended the chemistry of hydroquinone-terminated monolayers to silicon surfaces and demonstrated the potential of such a termination for molecular attachment via Michael addition of thiol terminated molecules [9]. Reaction of alcohols under UV irradiation was investigated in diVerent solvents and the structural characteristics of the resulting monolayers were compared to those obtained with aldehydes under the same conditions. The best monolayers were formed when dichloromethane was used as solvent for both alcohols and aldehydes. However, monolayers with lower coverage were formed when Si–H surfaces were exposed to ambient light or kept in the dark in the presence of alcohols and aldehydes [10]. Chemical reaction of simple and functional oleWns with hydrogen-terminated Si(1 1 1) surfaces in the presence of

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0.1 mol% of a free radical: 2,2,6,6-tetramethylpiperidinooxy (TEMPO) and its derivatives at room temperature yields organic monolayers covalently attached to the surface. The presence of oxygen in the XPS spectra of the resulting surfaces was assigned to TEMPO molecules bonded to the silicon surface [11]. A recent report by Buriak’s group demonstrated the use of tetraalkylammonium, tetraalkyl/ arylphosphonium reagents and alkyl pyridinium salts as sources of organic moieties for the formation of organic monolayers covalently attached to hydrogen-terminated surfaces through Si–C bonds under electrochemical conditions [*12]. The result addresses the importance of the competition that may account for surface alkylation during electrochemical reduction of alkynes and alkyl halides in the presence of the alkylammonium salts. 3. Acid-terminated silicon surfaces The acid functional group is particularly interesting because of its chemical versatility. It can be easily coupled with alcohols, thiols and amines to prepare simple and activated esters, thioesters and amides under mild chemical conditions. Several reports appeared in the literature dealing with acid incorporation on silicon surfaces. The thermal reaction of undecylenic acid with p-type Si(1 1 1)–H was examined by Li et al. [13]. Self-assembled layers of undecylenic acid were deposited on Si(1 1 1)–H at elevated temperatures (150–200 °C) and the resulting surfaces were characterized using AFM and FT–IR spectroscopy. AFM analysis showed the formation of multilayers (islands with the same height) on the surface via layer by layer deposition mechanism through weak interactions such as Van der Waals forces and hydrogen bonding. The AFM results were corroborated by FT-IR spectroscopy by the presence of two peaks at 1712 and 1650 cm¡1 assigned to CBO and CBC vibrations, respectively. The presence of the peak at 1650 cm¡1 was unambiguously attributed to the adsorbed acid molecules on the surface. The reaction of undecylenic acid was investigated on hydrogen-terminated crystalline silicon n-type Si(1 1 1)–H under photochemical conditions [**14]. The resulting monolayers were covalently grafted to the silicon surface through the Carbon–Carbon double bond with no detectable bonds between the carboxylic acid groups and silicon. The acid-terminated surface was converted to an activated ester by reaction with NHS/ EDAC and then coupled to various primary amines through amide bond formation. Given the oxophilic nature and the Lewis acid character of silicon sites on the surface, it was expected that the hydroxyl group (OH) of the acid will react solely or in a competing pathway with the Si–H bonds to yield silylester. Detailed kinetic studies using ATR-FTIR and contact angle measurements were undertaken for the photochemical (350 nm) reaction of undecylenic acid with Si(1 1 1)–H [15]. It was found that 90 min UV irradiation time was suYcient to prepare good quality acid-terminated silicon surfaces. HREELS analysis of the acid-terminated surface shows the key bands

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corresponding to the carboxy groups: CBO stretching at 1715 cm¡1 and O–H bending vibrations at 940 cm¡1. An adequate rinsing was, however, required in order to remove any physisorbed contaminants on the surface [**16]. Rinsing with common solvents (THF, CH2Cl2, TCE) leaves unwanted physisorbed material on the acid surface as evidenced by AFM imaging and ATR-FTIR spectroscopy. A Wnal rinse with hot acetic acid was found necessary to prepare smooth atomically Xat and perfectly free of organic contaminants. A surface coverage of 35% was estimated by a quantitative analysis using ATR-FTIR spectroscopy. Direct assembly of acid-terminated organic monolayers on Si(1 1 1)–H was achieved using undecylenic acid in the presence of 0.1 mol% of 4-(decanoate)-2,2,6,6-tetramethylpiperidinooxy (TEMPO-C10) at room temperature without UV light [17]. The acid terminal groups were activated with triXuoroacetic anhydride and triethylamine to yield an anhydride-terminated surface, which was coupled in a controlled fashion with primary amines using soft lithography. Carboxylic acid-terminated silicon surfaces were prepared by direct cathodic electrografting (CEG) and thermal treatment with ,-bifunctional molecules such as bromo and iodododecanoic acid and 10-undecynoic acid [18]. The obtained results suggest that better surface coverage was obtained for the cathodic reaction of hydrogen-terminated silicon surface with 10-undecynoic acid compared to the thermal reaction. 4. Preparation of polymer-silicon hybrids The group of Kang has published a series of papers dealing with the synthesis of well-deWned polymer brushes on silicon surfaces by surface-initiated atom transfer radical polymerization (ATRP). Monolayers covalently grafted onto silicon surfaces with terminal -bromoester [19], benzyl chloride [20,21], and aniline moieties [22] were prepared and used for the immobilization of a wide range of polymer and diblock copolymer brushes via surface initiated ATRP. Furthermore, halogen-terminated Si surfaces (Si–X, X D Cl, Br) prepared by the reaction of hydride-terminated Si(1 1 1) surfaces with PCl5 and N-bromosuccinimide, respectively were used as eVective initiators to synthesize well-deWned polymer-Si hybrids by surface-initiated ATRP [23]. The resulting polymer-Si hybrids are potentially useful as stimuli-responsive modiWers for cell adhesion [24], for the direct immobilization of glucose oxidase with a higher conformational freedom and enzyme activity [25], and as anti-fouling and antithrombogenic surfaces [26]. In a diVerent approach, semicrystalline, slightly branched thin polyethylene layers covalently anchored to hydrideterminated Si(1 1 1) surfaces were formed by thermal decomposition of methoxymethyl magnesium chloride at a temperature above ¡20 °C [27]. Polypyrrole dots and lines with sub-200 nm resolution were locally generated using electrochemical dip-pen nanolithography on pyrolle-terminated silicon surfaces [28].

5. Reaction mechanism Understanding the reaction mechanism is primordial for designing new chemical reactions on the surface and tailoring the surface properties in a controlled and reproducible fashion. Even though there is a parallel between molecular chemistry and chemical reaction occurring at the semiconductor surface, there is a major contribution of the semiconductor band gap to take into account during chemical processes at the surface. For instance, the band gap can inXuence the reaction pathway even at zero current. The thermal reaction of Grignard reagents with Si(1 1 1)–H is doping dependent and it was demonstrated that the remaining alkyl halide impurities (from which the Grignard reagent was synthesized) present in the solution plays a key role in the grafting process [29]. STM studies on partially reacted H–Si(1 1 1) with 1decene using visible light (447 nm) activation [**30] revealed that the attachment proceeds via a propagating radical chain reaction. Since the photons used in these studies are signiWcantly low of energy to induce homolytic bond cleavage, it was suggested that photoexcitation is most likely responsible for the generation of active sites on the Si surface. However, the authors did not exclude the generation of radical species from the impurities in solution, which account for the observed radical chain mechanism. By extending the wavelength range (447–658 nm), the authors provided a clear cut regarding the reaction mechanism taking place without any Si–H or Si–Si homolytic bond cleavage [*2]. To explain the observed radical chain propagation, a new reaction mechanism was proposed. It is based on the formation of delocalized radical cations at the silicon surface upon excitation. The resulting surface is susceptible to nucleophilic attack followed by a Si–Si bond cleavage in a concerted manner. The generation of Si radical accounts for the initiation step and thus for the radical chain propagation. These observations were corroborated by the recent work by Chabal et al. [**31]. To elucidate the reaction mechanism of the UV-induced hydrosilylation, the authors have combined the well-deWned silicon surfaces: atomically Xat H–Si(1 1 1) and H–SiO2/Si(1 0 0) [obtained by hydrogenation reaction of SiO2/Si(1 0 0) with triethoxysilane] and infrared spectroscopy. They have studied the hydrosilylation reaction of the above surfaces with 3-[2⬘-(1H-inden-3⬙yl)ethyl]-5-(4⵮-vinylphenyl)-1H-indene ‘indene ligand’ under UV irradiation in chlorobenzene. They have demonstrated that the immobilization of the indene ligand takes place on the oxide free surfaces only, suggesting electron–hole pair excitons as key to the mechanism of UV-induced surface reaction. 6. Electrical characterization of the organic monolayer/ silicon interface Considerable eVorts were devoted to electrical properties evaluation of the silicon/organic monolayer interface. Interfaces with low density of active electronic defects is a crucial

R. Boukherroub / Current Opinion in Solid State and Materials Science 9 (2005) 66–72

issue for building biosensors in which the detection scheme uses the electrical properties of the semiconductor, and for a perspective development of molecular electronics. Electron transfer through organic monolayers formed on n-type Si(1 1 1)–H under thermal conditions was investigated using current sensing atomic force microscopy (CSAFM) [32]. The I–V curves showed strong force and chain length dependences. Organic monolayers with terminal electroactive groups covalently bonded to Si(1 0 0)–H surfaces through Si–C [33] and Si–O–C [34] bonds were prepared and their electrochemical properties were evaluated using cyclic voltammetry. The monolayer grafted via Si–C bonds showed a faster and more reversible electron transfer than the monolayer bounded through Si–O–C bonds. However, silicon oxide growth was observed upon increase of the number of cycles and hinders the electron transfer. Other classes of monolayers consisted of zinc porphyrin formed on Si(1 0 0) via Si–C bonds showed that both electron transfer and charge dissipation rates decrease monotonically as the surface coverage and the length of the linker increase [35]. The electronic properties of mixed methyl/carboxyl-terminated alkyl monolayers on Si(1 1 1) surfaces were evaluated using electrochemical impedance measurements. The organic monolayer/silicon interface showed excellent electronic properties with a very low density of gap states. The measurements also evidenced that the acid-terminal groups promote the penetration of water in the outer part of the organic Wlm resulting in an increase of the dielectric constant of mixed organic monolayers with increasing the acid content [**16]. Independently, electrical properties (I–V and C–V) of organic monolayers covalently bound to nand p-type Si(1 0 0) were studied by forming MIS (metal– insulator–semiconductor) diodes using a mercury probe. It was demonstrated that all layers showed better insulating properties than samples with 2 nm thick SiO2 and alkyl monolayers on n-type silicon form more ideal diodes than those on p-type silicon [36]. Electronic properties of octadecyl-terminated monolayers on Si(1 1 1)–H surfaces (prepared by UV irradiation) were evaluated as a function of type and doping level. The silicon/monolayer/metal junction formed on low-doped n- and p-type, and highly doped p-type substrate displayed leakage currents as low as »10¡7 A cm¡2 while monolayers formed on highly doped ntype silicon were disordered and exhibited larger leakage current densities (>10¡4 A cm¡2) [37]. A recent study based on the combination of p-polarized backside reXection absorption infrared spectroscopy (pb-RAIRS), and I–V and C–V measurements showed that alkoxy Wlms directly attached to Si(1 1 1)–H surfaces are displaced during metal evaporation [38]. It is clear from this study that the electrical properties of the monolayers will depend on their stability during metal deposition. The electronic properties of a methyl-terminated Si(1 1 1) surface were investigated using high-resolution synchrotron photoelectron spectroscopy. Electronically, the surface was close to the Xat-band conditions with a surface dipole of ¡0.4 eV [*39].

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7. Immobilization of biomolecules on silicon surfaces 7.1. DNA and peptides Several approaches are described for DNA and peptide immobilization on silicon surfaces. Direct synthesis of oligonucleotides using automated solid-phase DNA synthesis on Si(1 1 1) surfaces terminated with a primary alcohol was reported by Lie et al. and the hybridization eYciency (47%) was evaluated by electrochemical means [40]. Single strand DNA tethered to a primary amine was immobilized and patterned in a controlled fashion on acid-terminated Si(1 1 1) surfaces with limited non-speciWc adsorption and its hybridization reaction was evaluated using Xuorescence spectroscopy. The surface chemistry employed provides good stability under hybridization/denaturation conditions [**14]. DNA patterns with submicron features were formed on Si(1 0 0)–H surface in a two-step process. This has been achieved using UV light in interference patterns onto Wlms of undecylenic acid N-hydrosuccinimide ester on Si(1 0 0)–H followed by exposure of the surface motifs to DNA tethered to hexyl amine [41]. Site-speciWc immobilization of peptides was undertaken on semicarbazide functionalized Si(1 1 1) surfaces. The preparation of peptide microarray relied on the chemoselective reaction of glyoxylyl peptides with semicarbazide function. The resulting peptide chips were characterized using AFM and Xuorescence spectroscopy. AFM imaging displayed a grain-like structure on the peptide-terminated surfaces, while the semicarbazide-functionalized surface showed atomically Xat terraces comparable to the initial hydrogen-terminated surface [42]. 7.2. Proteins There is a considerable interest for the incorporation of anti-fouling components on the silicon surface such as oligo(ethylene glycols) through Si–C bond formation and active groups to allow the covalent immobilization of bioNSrecognition elements. Biofouling is crucial for preventing and suppressing the non-speciWc adsorption of biomolecules on the surface during device operation and for designing biocompatible coatings. Direct photochemical irradiation (254 nm) of Si(1 1 1)–H surface in the presence of unprotected triethylene glycol undecen-1-ene for 3 h led to the chemical grafting of the triethylene glycol units. The reaction took place at both the vinyl and the OH groups of the starting organic molecule even though the vinyl group reacts three times faster (75% of the molecules were binded through the vinyl group while 25% via the terminal O atom) [43]. The resulting surfaces were exposed to Xuorescently labled proteins and their behavior was compared to diamond and gold surfaces terminated with triethylene glycol units. It was found that non-speciWc adsorption can be reduced by at least 60% on silicon, by 70% on diamond, and by 90% on gold surfaces. Mixed monolayers composed of reactive amino groups and ethylene glycol units where the

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amino groups were linked to biotin, were prepared on silicon and exposed to avidin. The presence of triethylene glycol moieties reduces the non-speciWc adsorption by a factor 8 for a mixed monolayer obtained using 30% of t-Boc 10aminodec-1-ene and 70% triethylene glycol undecen-1-ene. Similarly, organic monolayers with terminal oligo(ethylene glycol) groups covalently linked through Si–C bonds to the surface were obtained by photo-induced hydrosilylation of methoxy-protected -oligo(ethylene glycol)--alkenes [CH2BCHA(CH2)9–(OCH2CH2)n–OCH3, n D 3 (Me– EG3), 6 (Me–EG6), and 9 (Me–EG9)] on atomically Xat Si(1 1 1)–H. 30% monolayer adsorption of Wbrinogen was estimated using XPS on the Me–EG3-terminated silicon surfaces compared to 2% monolayer on Me–EG3-terminated thiolate SAMs on gold. However, both the Me–EG6 and Me–EG9 Wlms on Si(1 1 1) resisted 97% of protein adsorption, comparable to SAMs on gold of similar composition [44]. Moreover, Me–EG7 Wlms were prepared on hydrogen-terminated Si(1 1 1) and Si(1 0 0) surfaces and their stability and resistance to protein adsorption was compared [45]. While both Me–EG7-terminated Si(1 1 1) and Si(1 0 0) surfaces reduced adsorption of Wbrinogen by 97–98%, the monolayers formed on Si(1 1 1) displayed a higher stability than those on Si(1 0 0) under a wide range of conditions. Furthermore, micrometric [45] and nanometric [46] sized protein arrays were fabricated on the Me–EG7 Wlms using conductive AFM anodization lithography. A more systematic study showed that hydroxyl-terminated EG3 monolayers are more eVective than the methylated Me–EG3 monolayers at reducing non-speciWc adsorption of proteins on Si(1 1 1) surfaces, while longer EG chains are more eYcient than shorter EG3 molecules [*47]. Monolayers terminated with Me–EG3 were prepared on silicon under thermal conditions; XPS and X-ray reXectivity studies suggested monolayers with low grafting density (compared to those obtained with simple alkenes) and high disorder. The orientational disorder accounts for the suboptimal anti-fouling properties of these monolayers [48]. Moreover, the same authors showed that monolayers terminated with t-BDMS (t-BDMS-EG4) can be easily converted to the corresponding hydroxyl EG4 termination under mild conditions. The OH terminal groups were further coupled to free amines using carbonyl diimidazole (CDI), and disuccinimidyl carbonate (DSC) [49]. 8. Direct detection of DNA hybridization on chemically-modiWed silicon surfaces Electrochemical detection of DNA hybridization was performed on 12-mer ferrocenyl-containing oligonucleotides synthesized on Si(1 1 1) electrodes using automated solid-phase techniques. A positive shift of +34 mV was observed upon hybridization with complementary strands when measured in aqueous buVer, while a shift of +130 mV was detected when the solvent was THF [50]. Direct electrical detection of DNA hybridization on DNA-modiWed silicon surfaces was demonstrated using

impedance spectroscopy. The detection was realized without addition of redox agents in the solution by operating at open circuit potential and using impedance measurements at high frequencies, where the overall impedance is dominated by silicon and the resistance of the molecular layers [51]. 9. Conclusions and perspectives There are several advantages associated with silicon substrates compared to other surfaces: (i) crystalline silicon is a well-deWned surface, (ii) ease and control of the preparation of atomically Xat hydrogen-terminated silicon surfaces, (iii) compatibility of the Si–H bonds terminating the surface with the main organic and organometallic reactions and a wide variety of organic functional groups, (iv) formation of highly stable organic monolayers covalently bonded to the silicon surface through Si–C bonds, (v) use of the well-established micro and nanofabrication methods for the integration of chemical and biochemical functionality into microelectronic platforms, (vi) take advantage of intrinsic properties of silicon to detect molecular events occurring on the surface. However, there are some limitations to overcome before reaching stable and reusable devices based on organic monolayer/silicon hybrids and using the electronic properties of silicon for electrical detection of biomolecular events on the surface. The main drawback is the number of Si–H bonds remaining unsubstituted after the chemical process. Because of the steric hindrance on the surface the maximum surface coverage reachable is 50%. The remaining Si–H bonds on the surface are not completely preserved against oxidation, which introduces electronic active surface defects on the surface. Another drawback for using silicon/organic monolayer as a biochip substrate and Xuorescence for the detection scheme of biomolecular interactions on the surface is the Xuorescence quenching in the close vicinity of the semiconductor. Finally, the improvement of the monolayers quality to resist biofouling is a real challenge to design selective and sensitive devices for monitoring biomolecular interactions. The Weld of chemical functionalization and assembly of organic monolayers on hydrogen-terminated silicon surfaces will remain very active for the coming years. This will be mainly driven by potential applications of such hybrid structures in diVerent Welds ranging from molecular electronics to biosensors, biological microelectromechanical systems (bioMEMS) and nanotechnology. There is a huge progress in the elaboration of new grafting techniques under mild conditions and steps towards understanding the reaction mechanism involved during the grafting process. Organic monolayers on atomically Xat silicon surfaces oVer the possibility for immobilization and imaging nanoobjects on the surface with a high accuracy and will be very beneWcial for interfacing the silicon surface with high-k oxides for the new generation of silicon devices.

R. Boukherroub / Current Opinion in Solid State and Materials Science 9 (2005) 66–72

Acknowledgements The Centre National de la Recherche ScientiWque (CNRS) and the Nord-Pas-de Calais region are gratefully acknowledged for Wnancial support. References The papers of particular interest have been highlighted as: * of special interest; ** of very special interest. [*1] Sun QY, de Smet LCPM, van Lagen B, Wright A, Zuilhof H, Sudhölter EJR. Covalently attached monolayers on hydrogen-terminated Si(1 0 0): extremely mild attachment by visible light. Angew Chem Int Ed 2004;43:1352–5. [*2] Sun QY, de Smet LCPM, van Lagen B, Giesbers M, Thüne PC, van Engelenburg J, et al. Covalently attached monolayers on crystalline hydrogen-terminated silicon: extremely mild attachment by visible light. J Am Chem Soc 2005;127:2514–23. [3] Liu Y, Yamazaki S, Yamabe S, Nakato Y. A mild and eYcient Si(1 1 1) surface modiWcation via hydrosilylation of activated alkynes. J Mater Chem 2005;15:4906–13. [4] Lua YY, Fillmore WJJ, Yang L, Lee MV, Savage PB, Asplund MC, et al. First reaction of a bare silicon surface with acid chlorides and a one-step preparation of acid chloride terminated monolayers on scribed silicon. Langmuir 2005;21:2093–7. [5] Stewart MP, Maya F, Kosynkin DV, Dirk SM, Stapleton JJ, McGuiness CL, et al. Direct covalent grafting of conjugated molecules onto Si, GaAs, and Pd surfaces from aryldiazonium salts. J Am Chem Soc 2004;126:370–8. [*6] Chen B, Flatt AK, Jian H, Hudson JL, Tour JM. Molecular grafting to silicon surfaces in air using organic triazenes as stable diazonium sources and HF as a constant hydride-passivation source. Chem Mater 2005;17:4832–6. [7] Flatt AK, Chen B, Tour JM. Fabrication of carbon nanotube-molecule-silicon junctions. J Am Chem Soc 2005;127:8918–9. [8] Dirk SM, Pylypenko S, Howell SW, Fulghum JE, Wheeler DR. Potential-directed assembly of aryl iodonium salts onto silicon (1 0 0) hydride terminated and platinum surfaces. Langmuir 2005;21:10899–901. [9] Bunimovich YL, Ge G, Beverly KC, Ries RS, Hood L, Heath JR. Electrochemically programmed, spatially selective biofunctionalization of silicon wires. Langmuir 2004;20:10630–8. [10] Hacker CA, Anderson KA, Richter LJ, Richter CA. Comparison of Si–O–C interfacial bonding of alcohols and aldehydes on Si(1 1 1) formed from dilute solution with ultraviolet irradiation. Langmuir 2005;21:882–9. [11] Arafat SN, Dutta S, Perring M, Mitchell M, Kenis PJA, Bowden NH. Mild methods to assemble and pattern organic monolayers on hydrogen-terminated Si(1 1 1). Chem Commun 2005:3198–200. [*12] Wang D, Buriak JM. Electrochemically driven organic monolayer formation on silicon surfaces using alkylammonium and alkylphosphonium reagents. Surf Sci 2005;590:154–61. [13] Li YJ, Tero R, Nagasawa T, Nagata T, Urisu T. Deposition of 10undecenoic acid self-assembled layers on H–Si(1 1 1) surfaces studied with AFM and FT-IR. Appl Surf Sci 2004;238: 238–41. [**14] Voicu R, Boukherroub R, Bartzoka V, Ward T, Wojtyk JTC, Wayner DDM. Formation, characterization, and chemistry of undecanoic acid-terminated silicon surfaces: patterning and immobilization of DNA. Langmuir 2004;20:11713–20. [15] Asanuma H, Lopinski GP, yu HZ. Kinetic control of the photochemical reactivity of hydrogen-terminated silicon with bifunctional molecules. Langmuir 2005;21:5013–8.

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