High-Density Monolayers, Electronic Properties, and Functionalization

Invited Feature Article pubs.acs.org/Langmuir

Hybrids of Organic Molecules and Flat, Oxide-Free Silicon: HighDensity Monolayers, Electronic Properties, and Functionalization Yan Li,†,∥ Steven Calder,†,∥ Omer Yaffe,‡ David Cahen,‡ Hossam Haick,§ Leeor Kronik,‡ and Han Zuilhof*,† †

Laboratory of Organic Chemistry, Wageningen University, Dreijenplein 8, 6703 HB, Wageningen, The Netherlands Department of Materials and Interfaces, Weizmann Institute of Science, Rehovoth 76100, Israel § Department of Chemical Engineering and Russell Berrie Nanotechnology Institute, Technion - Israel Institute of Technology, Haifa 32000, Israel ‡

ABSTRACT: Since the first report of Si−C bound organic monolayers on oxide-free Si almost two decades ago, a substantial amount of research has focused on studying the fundamental mechanical and electronic properties of these Si/ molecule surfaces and interfaces. This feature article covers three closely related topics, including recent advances in achieving high-density organic monolayers (i.e., atomic coverage >55%) on oxide-free Si(111) substrates, an overview of progress in the fundamental understanding of the energetics and electronic properties of hybrid Si/molecule systems, and a brief summary of recent examples of subsequent functionalization on these high-density monolayers, which can significantly expand the range of applicability. Taken together, these topics provide an overview of the present status of this active area of research.

1. INTRODUCTION Organic/silicon hybrids are of considerable interest, both from a fundamental perspective and for possible applications in silicon-based devices, including biochips,1,2 optoelectronic devices,3,4 cantilever sensors,5,6 (bio)chemical sensors,7,8 and photovoltaic and photoelectrochemical cells.9,10 Organic monolayers on Si surfaces can be broadly divided into two categories: monolayers on native or thin Si oxide (SiOx) surfaces prepared mostly through silane chemistry and monolayers on oxide-free Si prepared mostly via Si−C and Si− O chemistry11,12 or electrochemistry.13,14 Between these two directions, research in the former is a more mature field because the SiOx platform is more chemically stable under aqueous and ambient conditions than is oxide-free Si, allowing for chemical modifications under mild conditions. In contrast, the preparation of organic monolayers on oxide-free Si is a much more difficult task. Regardless of the chemical method used, the reaction between (typical) hydrogen-terminated Si surfaces (Si−H) must take place in an oxygen- and water-free environment and usually requires elevated temperatures or irradiation with UV/vis light.11,12 Despite these difficulties, organic monolayers on oxide-free Si are well worth pursuing for a variety of reasons, particularly in the field of organic molecule-based and -assisted electronics, where thin organic layers are inserted at selected metal/ semiconductor and semiconductor/semiconductor junctions so as to affect their properties:15,16 (1) The presence of a native/ thin SiOx layer is often associated with charging effects that are © 2012 American Chemical Society

usually observed with oxidized surfaces because of charge traps.17 Without the oxide layer, these effects are absent. (2) The Si−molecule electronic coupling is significantly improved in the absence of an oxide, thereby allowing for a wider range of potential Si-based devices.12,18 (3) Semiconductor devices are usually negatively affected by surface states.19 These states are a significant source of high leakage currents in diodes and can lead to efficiency losses in solar cells20 and Fermi-level pinning.21 Fortunately, hydrogen termination is known to be one of the most efficient methods to passivate Si surfaces electrically (i.e., to reduce the density of surface states). If the passivation of the Si surface is maintained during and after the formation of a monolayer, then the electronic interplay between molecular properties (e.g., molecular dipole, conjugation, or binding group) and Si properties (e.g., doping type, doping density, and surface orientation) can be fully manifested and explored. (4) Judiciously chosen molecular monolayers may also prevent surface oxidation under ambient conditions after functionalization, which should be viewed as an issue different from the above-mentioned electrical passivation. (5) Last, the oxide-free Si/molecule interface can be much better defined than the oxide-covered one because each surfaceadsorbed molecule will be covalently attached to one of the atop Si atoms. This well-defined structure facilitates in general a Received: January 18, 2012 Revised: May 5, 2012 Published: May 15, 2012 9920

dx.doi.org/10.1021/la3010568 | Langmuir 2012, 28, 9920−9929

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Invited Feature Article

comparison between experimental results and theoretical analyses and in particular a comparison of measured electronic properties with those computed from first principles. This facilitates an understanding of the electronic consequences of chemical reactions on the surface. A significant challenge is the design and execution of reactions that lead to monolayers dense enough to achieve both electrical and chemical passivation.22 The most widely reported monolayer on the oxide-free Si surface is the simple alkyl chain. Reported methods for the attachment of alkyl chains to hydrogen-terminated silicon include reactions with alkenes through a radical process catalyzed by a diacyl peroxide initiator,23 the use of UV24 or white25 light, a halogenation/ Grignard route,18,26 thermal energy,27 Lewis acid catalysts,28 and electrochemical functionalization.29,30 However, the abovementioned preparation methods typically lead to incomplete coverage of the organic monolayer as well as to significant amounts of oxygen on the surface, unless extraordinary measures are taken. The basic premise of this feature article is that progress in the preparation of monolayers that achieve both electrical and chemical passivation opens the door to fruitful interdisciplinary research involving organic chemistry (monolayer preparation and chemical characterization), experimental physical and materials chemistry (electronic characterization, device preparation, and characteristics), and computational/theoretical chemistry (first principles calculations and a phenomenological understanding). Accordingly, section 2 provides a detailed review of progress in the preparation and study of high-density Si−C bound organic monolayers on flat silicon substrates, predominantly Si(111). (Comprehensive reviews dealing with the preparation and functionalization of monolayers on polycrystalline and porous Si surfaces can be found elsewhere11,12,22,31,32.) A characterization of the resulting monolayers, including supporting computational studies, is also presented. Section 3 discusses the progress made in both the fundamental understanding and applications of the electronic properties of hybrid Si/molecule systems. In section 4, recent examples are presented for the subsequent functionalization of high-density monolayers on Si(111), which are of interest in ongoing efforts to create silicon-based molecule-controlled devices, including chemical/biomolecular sensors.

Figure 1. (A) Preparation of a CH3-terminated silicon surface through the chlorination and alkylation process.26 (B) (a) STM image of CH3terminated Si(111), showing a highly ordered CH3-terminated surface with only minimal defects; (b) profile of the line marked as H−H′. The distance between the centers of the two maxima is the expected distance between adjacent H atoms on the same methyl group. Reproduced from ref 33. Copyright 2006 American Chemical Society.

with methyl groups vertically bound to the Si substrate.34 This computational result was in agreement with high-resolution STM experiments (Figure 1B).33 Complementary studies have shown that this methyl layer can serve as an exceptional barrier to oxidation upon exposure to ambient conditions and aqueous solvents.18,26 However, the low reactivity of the methyl group makes successive functionalization difficult. The formation of a monolayer that includes a synthetically useful functional group could allow further manipulation of the physical or chemical properties at the interface. Ring-opening metathesis polymerization has been used to produce organic overlayers that are covalently attached to Si(111) surfaces, providing molecular-level control over the thickness and electronic properties of the resulting Si/polymer contacts.35 These surfaces exhibit low surface recombination velocities because of their electrically passive Si-alkylation interfacial chemistry and the protective nature of the thick polymeric overlayer. In contrast, subsequent modification of an allylSi(111) surface by metal-catalyzed reactions, using Heck coupling and ruthenium-catalyzed olefin cross-metathesis, yielded final surface coverages of ∼13 and ∼37%, respectively, for the immobilized aryl molecules.35,36 The downside to the initial allyl termination is that complete coverage is not possible because the van der Waals diameter of −CH2− is larger than the spacing between Si atoms. X-ray photoelectron spectroscopy (XPS) measurements indicate that ∼75% of the Si atop atoms were passivated by Si−C bonds for the allyl monolayer, resulting in a weaker resistance against oxidation and higher electronic defect densities than for the Si(111)−CH3 surfaces.36 To preserve the reactivity of the monolayer while retaining the beneficial properties of the well-passivated CH3-terminated surface, a mixture of two Grignard species was introduced into the reaction (Figure 2).37,38 With this technique, the allyl Grignard enables secondary surface modification, and the methyl Grignard fills in the remaining H sites. After 4 weeks of exposure to air, the oxidation of the substrate with a low fraction of allyl groups (2%) matches that of a neat methylterminated surface.38 For larger fractions of allyl groups,

2. PREPARATION OF HIGH-DENSITY SI−C BOUND MONOLAYERS ON SI SURFACES 2.1. High-Density Monolayers Prepared by the Alkylation of Halide-Terminated Surfaces and Their Functionalization. One synthesis technique that has proven to be a useful strategy for preparing well-defined, alkylfunctionalized silicon substrates is the two-step chlorination/ alkylation that was first described by Lewis et al.26 In this process, the Si−H surface is first chlorinated by PCl5 to make a metastable chloride-terminated surface, which then reacts with alkyl Grignard or organolithium reagents to form the desired Si−C bound monolayer. CH3-terminated surfaces were prepared by this two-step halogenation/alkylation process (Figure 1A), yielding the only proven (at least on the scale of scanning tunneling microscopy (STM) measurements; see below) 100% coverage of all Si(111) atop sites.18,26 A recent investigation of the structural and electronic properties of the methyl-terminated Si(111) surface using first-principles calculations found that the most stable geometry corresponds to a (1 × 1) symmetric pattern, 9921


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immobilized on the surface (e.g., through the conversion of Br to OH groups via a hydrolysis process or converting Br to NH2 by aminolysis, both of which are accomplished without the use of any catalyst). This approach may be useful for the immobilization of inorganic nanomaterials on top of the Si surface to develop devices such as sensors.4,41 Si−CCH and Si−CC−CH3 linkages can also be prepared with sodium acetylide reagents.42,43 Despite the relatively weak resistance against oxidation, the −CCH group (Figure 4) provides a much more useful chemical handle

Figure 2. Formation of methyl, mixed methyl/allyl, and allyl monolayers on Si(111) and the investigation of their open-circuit potentials in aqueous solutions at various pH values. Reproduced from ref 37. Copyright 2011 American Chemical Society.

however, XPS spectra show increasing amounts of oxidation. There is therefore a give-and-take relationship between increasing the reactive functionality and protecting against oxidation. This relationship is also reflected by the degree of electronic stabilization in aqueous solutions. The Si atop sites that are not organically terminated generate oxide patches that can induce a pH-dependent dipole. As shown in Figure 2, the mixed CH3−/CH2CHCH2− monolayers and CH3-terminated surfaces displayed a weaker pH dependence on the band edges than substrates with CH2CHCH2− groups alone. This is also the first indication that the sensitivity of the band-edge position to the pH value can be used to estimate the Si−C surface coverage.37 Propenyl (−CHCH−CH3) and alkynyl (−CCH or −CC−CH3) moieties with footprints as small as the −CH3 group are other possibilities for passivating the unreconstructed Si(111) surface. Using the two-step chlorination/Grignard process, Haick et al. functionalized chlorinated Si(111) surfaces using propenyl magnesium bromide (CH3−CHCH−MgBr) and propynyl magnesium bromide (CH3−CC−MgBr).39 They deduced from their data nearly complete coverage of Si atop sites, similar to that of CH3-terminated Si surfaces, and found the Si−CHCH−CH3 interface to be less susceptible to oxidation than the other surfaces.39 This effect was attributed to the combination of the full surface coverage and the π−π interaction between molecules.39 Subsequent functionalization of the propenyl-terminated surfaces was achieved via bromination at the allylic position using N-bromosuccinimide (NBS),40 as illustrated in Figure 3. After bromination, a variety of functionalities can be

Figure 4. Formation of an alkyne-terminated Si substrate by the alkynylation of a halide-terminated surface and further functionalization via the CuAAC reaction.42

for additional functionalization than do the −CHCH−CH3 and −CH3 moieties. Inspired by the versatility of click chemistry, Heath and co-workers reported the first Cu(I)catalyzed alkyne−azide cycloaddition (CuAAC) on this fully acetylenylated Si(111) surface.42 In this reaction, benzoquinone with an N3-terminated spacer is bound to the acetylenylated Si surface, and the subsequent electrochemical reduction results in quantitative cleavage of the protecting groups, generating an amino-terminated surface that the authors then used for the attachment of ferrocene carboxylic acid. Compared to the direct introduction of hydroquinone on Si−H surfaces,44 the attachment via a CuAAC reaction substantially reduced the oxidation of the silicon substrate because of the protection by the densely packed ethynyl groups. However, the reaction is hypothesized to occur only at the step edges and defect sites, leading to a low overall yield (7%). In addition, Cu is a wellknown electronic poisoning agent for Si, and as a result, click chemistry has not yet been successful in producing the type of high (electronic)-quality functionalized Si surfaces that are needed for electronic transport applications. Copper-free click reactions that link functionalities to surfaces, such as the thiol− ene and SPAAC reactions, are therefore likely to be very useful.45 Although the aforementioned two-step chlorination/alkylation route generally affords a high coverage of hydrocarbon chains and chemically stable surfaces, there are also some limitations. This route requires strict anaerobic conditions for both chlorination and alkylation reactions, discouraging routine use in many laboratories.12 In addition, few functional groups are compatible with the highly reactive organometallic reagents, and the resulting densely packed monolayers lead to low reactivity toward secondary functionalization, whereas the mixed monolayers retain their electronic properties only at low percentages (2%) of reactive groups.37 Future work in this direction would therefore need to consider either the use of higher fractions of reactive mixed-monolayer components (such as Si−CH2−CHCH2) without compromising the electronic properties or the involvement of dendrimeric or polymer components to combine high-quality electronics with

Figure 3. Functionalization of propenyl-terminated surfaces with the mediation of NBS.40 9922


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reactive or sensing groups that are compatible with the sensitivity requirements of high-end electronics. 2.2. High-Density Monolayers Prepared by Hydrosilyation. 2.2.1. High-Density Monolayers Derived from 1Alkynes. Chidsey and Linford were the first to demonstrate the successful formation of organic monolayers on oxide-free silicon via hydrosilylation reactions.46 Initially relatively harsh conditions were required for the grafting (e.g., temperatures >100 °C with pure compounds,27 in dilute solution,47 or with short-wavelength UV irradiation48−50). However, in subsequent years, milder reaction conditions relying on sonochemical activation51 and visible-light-induced monolayer formation11,52 were reported by some of us and by other groups. It is commonly accepted that thermally induced or visiblelight-induced hydrosilylation on a Si−H substrate proceeds via a radical chain mechanism (Figure 5).23,53,54 In this process, the

Despite the presence of many unreacted Si−H bonds, acceptable surface passivation has been achieved for alkenederived monolayers, yielding a surface recombination velocity of 120 cm s−1 on Si(100).58 Although this result shows the potential application of monolayers formed via hydrosilylation reactions, this value is still 5 times higher than that obtained for methyl-terminated Si surfaces and 25−30 times higher than that obtained with the stable MeOH treatment.59 Therefore, it was very desirable to increase the coverage and quality of the monolayers with respect to what can be obtained with 1alkenes. As an improvement to 1-alkene-derived monolayers, 1alkynes can also react with H−Si(111) substrates, yielding an interface with a Si−CHCH− linkage as depicted in Figure 7a.60 Although the structural difference between the two

Figure 5. Proposed mechanism for grafting 1-alkenes on a Si−H substrate under thermal activation or visible-light irradiation.

nucleophilic attack of positively charged surface Si sites by alkenes leads to the formation of a Si−C bond. The subsequent hydride transfer from an adjacent site of the Si backbone generates a new silyl radical, which allows the propagation of the reaction at the Si−H surface. The self-limiting nature of the radical chain mechanism has been examined with STM images.53 As the modification progresses, the H−Si(111) surface displays the formation of many irregular islands (Figure 6) as modified areas gradually form on the bare Si−H surface.

Figure 7. (a) Different linkages of organic monolayers on Si(111): an alkyl monolayer (left) and an alkenyl monolayer (right). (b) Surface coverage obtained by ATR dichroism (○) and XPS (◊) of 1-alkene (black curves) and 1-alkyne (red curves) monolayers on Si(111). Reproduced from ref 61. Copyright 2010 American Chemical Society.

linkages seems small, the influence on the properties of the resulting monolayers is significant with respect to the density of packing, the strength of the Si−C bonds, the rate of reaction, and the resistance against oxidation. Prior studies have indicated that 1-alkynes are more reactive than 1-alkenes toward Si−H54,60 and that the resulting Si−CHCH− linkage affords better resistance against oxidation (as discussed in section 2.1).39 Additionally, the structure at the organic/Si interface (e.g., the hybridization change and the van der Waals radii) also influences the overall monolayer packing density. This finding has prompted us to investigate the differences affecting the molecular coverage of these two kinds of monolayers. It was found that alkyne-derived monolayers possessed higher C/Si atomic ratios and increased film thicknesses than the alkene-derived monolayers under identical preparation conditions. By comparison with well-defined alkanethiol monolayers on Au, the monolayer thickness can be used to estimate the surface coverage.57 As shown in Figure 7b, all 1alkene monolayers have a surface coverage of 50−55%, whereas

Figure 6. STM images of hexadecyl monolayers on a Si(111) surface taken at different time intervals: (a) 15 min, (b) 2 h, and (c) 15 h of irradiation at 447 nm. Adopted from refs 53 and 55.

Upon completion of the monolayer, the areas between the islands become smaller and smaller until they nearly disappear after 2 h and fully disappear after 15 h.53,55 Hydrosilylation reactions are generally used to prepare monolayers with relatively long alkyl chains (Cn, n ≥ 8). Alkyl chains with more than one carbon atom cannot yield a Si−C-type termination for every atop site on Si(111) because long alkyl chains exhibit larger van der Waals diameters (0.4 to 0.5 nm) than the spacing between Si atoms (0.38 nm).12,56,57 9923


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packing density is expected to be higher than that of the theoretically most negative attachment energy per chain (60%). This finding is in good agreement with the experimentally observed surface coverage of 65% for alkenyl monolayers on Si(111).62 2.2.3. Experimental and Theoretical Studies of HighDensity Monolayers Prepared with Other Unsaturated Hydrocarbons. Both experimental and molecular modeling studies have indicated that the smaller van der Waals radius of the Si−CHCH− linkage and the greater exothermicity of the reaction of 1-alkynes with the Si−H surface make surface coverages above 60% sterically and thermodynamically feasible. Although these results presented a significant advance toward higher-density monolayers, emerging studies are focused on finding adsorbents that are even more reactive toward the Si−H surface or more sterically favorable than terminal alkynes. To design new precursors and predict their reactivity, a model Si compound with four Si atoms (1 in Figure 9A) was

a higher coverage of up to 65% was observed for 1-alkynederived monolayers.61 Because the hydrosilylation of 1-alkynes is faster than that of 1-alkenes, the surface oxidation during the formation of the monolayer is also reduced. 2.2.2. Molecular Modeling of the 1-Alkyne-Derived Organic Monolayers on Si(111). In the previous section, it was demonstrated that a minor structural difference in the linkage to the Si surface (Si−CH2−CH2− vs Si−CHCH−) has a substantial effect on the monolayer coverage and the electronic properties of the resulting organic/Si interface. Computational studies have also been devoted to understanding the structure and reaction mechanism involved in the hydrosilylation process. The coverage of Si−H substrates with an organic monolayer is essentially restricted by two factors: the packing energy and the binding energy. The packing energy (Figure 8, dotted lines)

Figure 8. Energy of monolayer formation for 1-octadecene and 1octadecyne on H−Si(111) for several coverages determined by the summation of the packing and binding energies. Reproduced from ref 62. Copyright 2011 American Chemical Society. Figure 9. (A) Formation of silyl radicals and subsequent reactions with alkene and alkyne derivatives. (B) Some unsaturated species used to study the hydrosilylation reaction with silyl radicals.64

reflects the bond strain close to the organic/silicon interface during the formation of the monolayer, and the binding energy indicates the energy change associated with the chemisorption of a chain on the Si substrate (i.e., Si−C bond formation). As a representative example, the packing energies of octadecyl and octadecenyl chains with 67% coverage on H−Si(111) surfaces are compared. It was found that the van der Waals radius of the Si−CHCH− group is smaller than that of the Si−CH2− CH2− group, resulting in less overlap and fewer unfavorable conformations of the molecular chains, in particular, those at the monolayer/Si interface. The dependence of the steric energy on the packing density is also considerably smaller than for the saturated analog, which is likely due to the lower steric constraints of Si−CHCH− bonds. Meanwhile, the Si−C binding energy was investigated by ab initio G3 calculations. The formation of Si−CHCH− is ∼42 kJ/mol more exothermic than that of Si−CH2−CH2−, which likely also contributes to the improved packing density compared to the reaction with 1-alkenes.62 A general picture for comparing the reactivity of 1-alkenes and 1-alkynes is therefore depicted in Figure 8, in which the total energy (ETot, solid lines) of monolayer formation per chain was estimated by adding the average packing energy and the binding energy. Because the enthalpy of attachment can compensate for the increasing steric repulsion, the maximum

prepared to mimic the Si−H surface. The reactions between silyl radical cations (2 in Figure 9A) with various unsaturated species (a−e in Figure 9B) were studied experimentally and theoretically to compare their reactivity with Si−H surfaces and gain insight into the mechanisms of Si−C bond formation.63 The alkyne functionality was found to react 8 times faster than alkenes, and a further extension of the conjugation length by additional CC or CC units leads to a significant increase in reactivity that is due to the better stabilization of the formed β-carbon radical. For example, HCC−CHCR and HCC−CCHR (R = alkyl) react 200 and 1400 times faster than a 1-alkene with the Si model compound, respectively.64 The above-mentioned study suggests that such novel precursors are potential candidates for the preparation of high-density monolayers.64 In practice, however, some of these conjugated reactants have complications not encountered with the nonconjugated analogues. For example, during the hydrosilylation reaction with hexadeca-1,3-diyne, a color change in the reactant was observed, which is attributed to lightinduced polymerization. Indeed, diacetylene derivatives with two conjugated CC bonds have a unique property of 9924


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inversion.68 Thus, the current density is independent of both the length of the molecule and the type of binding (alkyl vs alkenyl). However, in the case of highly doped Si, the internal barrier was smaller and as a result the charge transport properties of the junction were strongly affected by changing from an alkyl to an alkenyl monolayer. This effect is explained using DFT calculations, from which the local densities of states of the two systems are shown as a function of position (in the direction perpendicular to the surface) and energy (Figure 11).72,73 The calculation exposes

topochemical photopolymerization upon the absorption of UV light, which has been widely used in many optical and electronic applications.65 In contrast, the 3-en-1-yne group, which is easy to synthesize, was shown to yield excellent monolayers on oxide-free Si surfaces (Figure 10).64 Both quantitative XPS studies and

Figure 10. Thermally induced monolayer formation on Si(111) using hexadec-3-en-1-yne.64

infrared reflection absorption spectroscopy (IRRAS) measurements indicate the formation of a high-quality monolayer with a coverage of 63% on Si(111), which is even higher than for the corresponding hexadecyne-derived monolayer (59%).

3. ELECTRONIC PROPERTIES OF SI−C BOUND MONOLAYERS ON SI SURFACES As mentioned in the Introduction, the insertion of judiciously chosen organic monolayers at selected metal/semiconductor or semiconductor/semiconductor interfaces can affect the properties of electronic devices based on these interfaces. Efforts to understand and ultimately control the mechanisms through which monolayers on oxide-free Si-adsorbed monolayers determine electrical device properties15 are experimentally typically composed of two parts. The first part is the study of the Si/molecule system prior to the deposition of a top contact (usually metallic). This is often achieved by a combination of experimental methods (i.e., ultraviolet photoemission (UPS) and inverse photoemission (IPES) experiments are aimed at investigating level alignment,66 whereas contact potential difference (CPD) and surface photovoltage (SPV) measurements67 are used to investigate band bending and work function effects68). The second part is the study of electronic properties after the top contact is deposited on the monolayer, and complete junction information is primarily extracted from the electrical behavior of the complete device (e.g., current− voltage (J−V) and impedance spectroscopy measurements), which provides information on barriers for charge transport across the Si/metal interface.69 It is to be noted that such electronic measurements are usually more sensitive to defects than direct chemical methods. As a special case of this general observation, J−V characteristics are known to be extremely sensitive to the presence of SiOx,70 even more so than methods such as XPS. Thus, device studies can also serve as a stringent test for monolayer quality. Lastly, sufficiently ordered monolayers also offer the opportunity to compare experimental results to theoretical predictions, typically using density functional theory (DFT), thus gaining further insight from the interplay between theory and experiment. As an example, consider the electrical device properties of large area Hg/organic monolayer−Si junctions, with both alkyl and alkenyl monolayers used on both moderately and highly doped n-Si surfaces. For moderately doped Si, the internal semiconductor barrier was found to control transport completely, with the attached molecules influencing the transport of such junctions only in that they drive the Si into

Figure 11. Contour maps of the local density of states for (a) alkyl and (b) alkenyl monolayers on Si(111) The interfacial transition region is emphasized in red (from ref 71).

significant penetration of the periodic Bloch states from the Si substrate into the alkyl/alkenyl chain region. This behavior, referred to as an induced density of interface states (IDIS), is well known in inorganic heterostructures. It is strongly related to the concept of metal-induced gap states (MIGS), which similarly describes evanescent electron waves from a metal into a semiconductor, but here the decay proceeds separately for filled valence states and for empty conduction states, both of which “tail” from the lower gap material (here, Si) to the higher gap material (here, alkyl/alkenyl chains). IDIS-related phenomena leave clear fingerprints in PES and IPES data,72,74 and their understanding is essential to deducing the level of alignment from such experiments and to using such alignment to understand transport across the Si/organic interface.15,71,72,74 Compared to the Si−CH2−CH2− interface, Figure 11 shows that Si-related states are found to extend further into the molecular region in the Si−CHCH− case, where they additionally hybridize with π states originating from the double bond. This results in a shorter effective tunneling length and stronger electrode−molecule coupling. Hence, because of molecule−semiconductor coupling even just one CC bond in a long hydrocarbon chain can affect the properties of electronic devices.71

4. FUNCTIONALIZATION OF HIGH-DENSITY MONOLAYERS ON SI(111) PREPARED VIA A HYDROSILYLATION REACTION In principle, two basic strategies exist for the preparation of functionalized molecular assemblies on surfaces. One approach is to attach the reactant possessing both the desired functionalities and a chemical anchoring group directly. Alternatively, the desired functionality can be immobilized on the substrate through a secondary reaction with a preexisting reactive monolayer.75 The direct attachment of large, active 9925


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spatial confinement of Au nanoparticles and reactive printing with complex biomolecules, such as oligo-DNA.79 The modification of this reactive platform was also carried out with electroactive components, and the resulting redoxactive monolayer was characterized by scanning electrochemical microscopy (SECM) to probe the 2D charge propagation rate between covalently coupled ferrocene redox centers.80 To tune the surface coverage of the ferrocene moieties, the precursor acid fluoride monolayer was diluted with electroinactive dodecenyl chains as shown in Figure 13. From these

components to oxide-free Si via hydrosilylation reactions is generally not favored except in some rare cases.76 Besides the potential incompatibility between many functional groups and the grafting conditions, the formed monolayer will be loosely packed because of the large footprint of the entire molecule, resulting in the oxidation of the underlying Si surface. In this respect, the preparation of densely packed monolayers with the capability of secondary modifications is desirable. For example, carboxylic acid moieties (−COOH) can be used directly in the hydrosilylation reaction with Si−H substrates, typically resulting in a negligible amount of upside-down attachment.77 Nevertheless, H bonding causes acid bilayer formation, which makes these monolayers harder to clean,78 whereas for further functionalization an additional activation step is required via carboxylic anhydrides or Nhydroxysuccinimide (NHS) chemistry. To circumvent these issues, we recently reported the fabrication of a reactive, highdensity monolayer that was prepared with 10-undecynoyl fluoride on oxide-free Si.79 The terminal alkyne functionality on one end can react with Si−H under mild conditions to yield a high-density monolayer that effectively protects the underlying Si, whereas the resulting terminal acid fluoride group is very reactive and selective toward amines. These features make the resulting platform superior to the traditional carboxylic acidterminated monolayers because bilayer formation from H bonding does not occur and no additional activation step is required. Acid fluoride-terminated monolayers allow local functionalization on the passivated Si substrate via microcontact printing (μCP, Figure 12). After a primary amine was stamped on the

Figure 13. Preparation of the ferrocene-terminated monolayers on Si(111) surfaces with 10-undecynoyl fluoride.80

experiments, it was found that the kinetics of lateral charge propagation between the surface-bound electroactive components can be easily tuned via the coverage of Fc moieties. This observation provides novel access to the design of electrochemical sensors and other electrochemically switchable systems based on insulating substrates.

5. SUMMARY AND OUTLOOK We have attempted to connect the development of new synthesis routes for high-density monolayers on oxide-free Si to the study of the electronic properties of such hybrid systems, the charge transport of solid-state device structures that use such molecularly modified Si, and the secondary functionalization of the monolayers because all of these present steps toward the realization of molecular electronics. Two major strategies for fabricating high-density monolayers on oxide-free Si were summarized. One is the alkylation of the Si surface using a twostep halogenation/Grignard route that generally results in a monolayer with high coverage but is practically limited to short hydrocarbon chains. Another method is based on the hydrosilylation reaction, which can be employed for the attachment of various long unsaturated molecular chains on a Si−H substrate under relatively mild conditions. We hope that methods that can combine the best of both of these approaches can yield a basis for even further improvements. With the guidance of electronic structure computational studies and time-resolved spectroscopy, we demonstrated how to improve the density of the resulting monolayers through the proper choice of the attaching functionalities. For example, 1alkyne and ene−yne derivatives can speed up the hydrosilylation process, yielding an impressively high coverage. These densely packed monolayers with the Si−CHCH− linkage can create a structurally and electronically robust Si/organic interface that provides better resistance against oxidation than conventional alkene-derived monolayers. Furthermore, with the continuing exploration of suitable distal functionalities, the generated monolayers are very stable in aqueous solution and can also allow a high reactivity toward secondary modification, which are both crucial in the fabrication of Si-based devices for bioanalytical chemistry.

Figure 12. Schematic representation of the procedure used for μCP on oxide-free silicon via highly reactive acid fluoride-functionalized monolayers. Reproduced from ref 79.

acid fluoride surface, AFM measurements show amide formation from μCP in only 20 s. The high density of the alkyne-derived monolayer also fully preserves the wellpassivated Si interface, and no traces of SiOx were observed even after immersion in water for 16 h. This acid fluorideterminated monolayer can also be utilized as a platform for the 9926


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We briefly touched on the critical (for most electronic devices) electronic transport behavior of the modified surfaces because this topic has recently been extensively reviewed.15 The type of binding between the molecule and the Si is shown to be important for allowing the molecules to exert their effect on the transport behavior of the system. The connection to the preparation of the monolayers was stressed because the optimal use of molecules will always require as good as possible electronic passivation of the surface, a surface that should also be stably chemically passivated so as to make such electronic passivation last. Although this review is focused on the research of highdensity organic monolayers on flat silicon substrates, these techniques can certainly be adapted to other forms of unoxidized Si, such as Si-based nanowires, nanoparticles, and porous silicon, because effective approaches to passivating highly reactive Si−H surfaces while producing a functional surface are vital for all forms of Si. Through improving the preparation methods, obtaining further increased surface coverage, and amassing a library of functional groups compatible with the grafting process, oxide-free silicon surfaces will likely find great use in a wide variety of applications.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +31 −317-482361. Author Contributions ∥

The manuscript was written through the contributions of all authors. These two authors contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Sidhu Pujari (Wageningen University) and Dr. Luc Scheres (Surfix) for stimulating discussions and help with the artwork.

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REFERENCES

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