Oxide-free hybrid silicon nanowires: From fundamentals to applied nanotechnology

Progress in Surface Science 88 (2013) 39–60

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Review

Oxide-free hybrid silicon nanowires: From fundamentals to applied nanotechnology Muhammad Y. Bashouti a,⇑, Kasra Sardashti a, Sebastian W. Schmitt a, Matthias Pietsch a, Jürgen Ristein b, Hossam Haick c, Silke H. Christiansen a,d a

Max-Planck Institute for the Science of Light, Günther-Scharowsky-Str.-1, Erlangen 91058, Germany Technical Physics, University of Erlangen – Nürnberg, Erlangen 91058, Germany c The Department of Chemical Engineering and Russell Berrie Nanotechnology Institute, Technion – Israel Institute of Technology, Haifa 32000, Israel d Institute of Photonic Technology, Albert-Einstein-Str. 1, 07745 Jena, Germany b

a r t i c l e

i n f o

Keywords: Silicon nanowire Hybrid functionalization Solar cells Heterojunction Field effect transistors

a b s t r a c t The ability to control physical properties of silicon nanowires (Si NWs) by designing their surface bonds is important for their applicability in devices in the areas of nano-electronics, nano-photonics, including photovoltaics and sensing. In principle a wealth of different molecules can be attached to the bare Si NW surface atoms to create e.g. Si–O, Si–C, Si–N, etc. to mention just the most prominent ones. Si–O bond formation, i.e. oxidation usually takes place automatically as soon as Si NWs are exposed to ambient conditions and this is undesired is since a defective oxide layer (i.e. native silicon dioxide – SiO2) can cause uncontrolled trap states in the band gap of silicon. Surface functionalization of Si NW surfaces with the aim to avoid oxidation can be carried out by permitting e.g. Si–C bond formation when alkyl chains are covalently attached to the Si NW surfaces by employing a versatile two-step chlorination/alkylation process that does not affect the original length and diameter of the NWs. Termination of Si NWs with alkyl molecules through covalent Si–C bonds can provide long term stability against oxidation of the Si NW surfaces. The alkyl chain length determines the molecular coverage of Si NW surfaces and thus the surface energy and next to simple Si–C bonds even bond types such as C@C and C„C can be realized. When integrating differently functionalized Si NWs in functional devices such as field effect transistors (FETs) and solar cells, the physical properties of the resultant devices vary. Ó 2013 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. E-mail addresses: [email protected], [email protected] (M.Y. Bashouti). 0079-6816/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.progsurf.2012.12.001

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Contents 1. 2.

3.

4.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Functionalization of oxide-free Si NW surfaces via the chlorination–alkylation process . . . . . . . . . . . . 2.1. Fabrication of Si NWs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Organic functionalization via chlorination/alkylation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.1. Hydrogen-terminated Si NW . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.2. Chlorination/alkylation process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Secondary functionalization: alkenyl and alkynyl as the starting monolayers . . . . . . . . . . . . . . . 2.4. Stability of molecules on the Si NW . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.1. Effect of molecular coverage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.2. Effect of molecular backbone bonds: C–C vs. CC vs. CC . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5. Effect of surface energy: Si NWs vs. planar surfaces. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hybrid Si NW electronic properties and devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Manipulation of the surface fermi level. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Controlling the work function, electron affinity, surface dipoles and band bending . . . . . . . . . . 3.3. Hybrid Si NW-based field effect transistor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. Hybrid Si NW-based solar cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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1. Introduction Much effort has been devoted to developing new nanomaterial-based technological devices with significant performance improvements at a reduced cost [1–4]. In this quest, a substantial number of nanomaterials have been developed, including but not limited to nanoparticles [5–14], nanorods [15–18], and nanowires [18–25]. Nanowires provide additional advantages compared to the other nanostructures in terms of their strong light absorbance, efficient charge separation and direct charge transport path. This makes them a promising candidate for many technological applications. In short, nanowires are important one-dimensional nanostructures that have demonstrated advantageous characteristics for applications in electronics [26–28], photovoltaics [29] and sensing [30–33]. Up till now, nanowires of many different inorganic materials have been synthesized and characterized, including silicon [34], gallium nitride [35] and lead selenide [18,36] among others. The use of silicon nanowires (Si NWs) in particular to further miniaturize devices while avoiding major process changes and their corresponding costs has been a topic of much research in the past decade. Thus, Si NWs are generally considered to be an essential class of nanodevice building blocks [37– 39]. Si NWs have shown significant potential for field effect transistors (FETs). Potential applications of Si NWs in FETs includes aligning on insulating substrate surface, selective deposition of the source and drain contacts on the Si NW edges, and the configuration of either a bottom or top gate electrode [26,40]. Another significant potential application of Si NWs is solar power generation. The indirect optical band gap of 1.12 eV, low absorption co-efficient of 104/cm and high wafer cost of bulk Si reduces the suitability of this material for photovoltaic and optoelectronic devices. Differently engineered band gaps (1.4 eV) and a high absorption co-efficient (at least 105/cm) are required to significantly improve the solar cell efficiency. Si NWs are promising alternatives to bulk Si for use in solar cell applications due to their high light harvest; convenient band gap tuneability from 1.1 to 1.4 eV, which is achievable through decreases in the diameter [41]; and the possibility of fabricating the structures with a axial and lateral junction to decrease the charge bath separation [25,42–44]. Surface treatment is one of the main obstacles to Si NW application. A large body of chemistry has been developed for chemically linking moieties to oxidized Si NW surfaces, generally through –OH

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chemistry. Many pertinent applications have been illustrated [28,45–48]. Although Si NWs offer unique opportunities as building blocks for nanoelectronic devices, the presence of native oxide (SiOx), which instantly forms on the Si NW surfaces upon air exposure, is undesirable [49,50]. This is because of the poor quality of the oxide that forms on the surface, which induces a large number of unwanted and uncontrolled interfacial states in the Si band gap [50,51]. The low quality, highly impure oxides that form in ambient conditions have a tendency to cause uncontrolled oxide/silicon interfaces and surface states. This necessitates the protection of Si NW surfaces against oxidation. Moreover, due to the larger surface to volume ratio, surface properties become more significant in smaller Si NWs and dominate the properties of the device as a whole [26]. Therefore, it is necessary to explore models and methods to predict and control the surface physical characteristics of oxide-free Si NW. Etched Si NWs with surface-bound hydrogen atoms, the simplest termination group, show low charge carrier surface recombination velocities [41,52]. However, these surfaces tend to oxidize within a few minutes of exposure to ambient air, leading to higher surface recombination velocities [40]. Therefore, it is of considerable interest to develop a sound strategy to prevent extensive Si NW surface oxidation while preserving the low surface recombination velocities. A promising method for controlling the surface properties of Si is termination through the use of organic molecules (polar and nonpolar) which form Si–C surface bonds. Functionalization by Si–C bonds causes the stability time to increase to a few hundred hours at room temperature [53–55]. Therefore, termination of the dangling surface bonds with chemical and biochemical moieties [39,56–58] can affect the device reliability over time. This is expected to have extreme significance on the final physical and chemical properties of the Si NWs. Generally speaking; the functionalization of organic molecules introduces a net electrical dipole perpendicular to the surface/interface. The dipole, in turn, modifies the work function and electron affinity, alters the band offset and band bending, and tunes the surface Fermi level [26]. Moreover, many future devices will require their building blocks to be selectively sensitive to the environment. Indeed, the main objectives of molecular functionalization can be outlined as follows: (1) to increase the oxidation resistance of Si NWs; (2) to allow a systematic tuning of the desired physical properties of an electric device by appropriate choice of the functional groups [59,60]; (3) to exhibit selectivity to different environmental species, depending on the type of molecules and their coverage [61]. In order to achieve the above objectives, several techniques that induce a high density of organic molecules attachments to the silicon surface have been suggested. Here we will describe one of these techniques while showing the disadvantages of the others. The most widely reported organic monolayers on the oxide-free Si surface are alkyl chains. As a starting step, the oxide is removed and surfaces are covered with either hydrogen or halogens. The H-terminated monolayer can be converted to alkyl monolayers through the use of alkyl-magnesium reagents [31,53–55,62–64] while the halogenated surfaces can be alkylated using alkyl-magnesium or alkyl-lithium reagents [65]. Alkylation has been successfully carried out on two dimensional surfaces through free radical initiation methods such as irradiation with ultraviolet light [66,67], chemical free-radical activation [68], thermal activation [69], Lewis acid catalyzed hydrosilylation [70,71], and visible-light-initiated modification [72]. However, these methods have as yet not been applied on Si NW. Unless extreme measures are taken, these reactions may result in an incomplete coverage of the organic monolayer (i.e. less than one monolayer of coverage) as well as significant amounts of oxygen on the surface [73,74]. Alternative reactions such as terpyridine termination have been directly demonstrated on hydrogenated Si NW [75]. However, these reactions are limited to terpyridine and cover the surface only partially. The current review describes strategies used to prepare organic monolayers on Si NWs, which achieve both electrical and chemical passivation, and allow for the introduction of productive oxide free components into practical nanodevices. A detailed report on scientific progress in the production and characterization of oxide-free high-density organic monolayers (single and multiple) on Si NW, through a two-step process involving chlorination/alkylation is described in Section 2. The kinetics of the alkylation process and the effect of organic chain length as well as second monolayer functionalization are also discussed. The stability of the hydrogen and organic molecules on the Si NW and the effect of molecular coverage and bonding type are demonstrated. Section 3 presents two examples of functional devices based on hybrid Si NWs: Field Effect Transistors (FETs) and solar cells. The electrical characteristics of molecularly-modified Si NW-based devices are presented, analyzed and compared to pristine Si NWs. Fig. 1 schematically illustrates the organization of the current review.

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Fig. 1. Scheme showing the basic steps from the synthesis to the application of hybrid Si NWs.

2. Functionalization of oxide-free Si NW surfaces via the chlorination–alkylation process 2.1. Fabrication of Si NWs The fabrication of Si NWs with a controlled diameter, length, and electronic properties is essential to technical application [76–78]. Significant progress has been made in recent years in the development of superficially controlled methods of Si NW fabrication [79]. There are two general Si NW fabrication approaches: bottom-up and top-down. The bottom-up approach is an assembly process joining Si atoms to form Si NWs and includes the vapor–liquid solid (VLS) growth technique [80]. The top-down approach prepares Si NWs by etching a bulk Si by a lithography or etching technique such as reactive ion etching (RIE) [81] or metal-catalyzed electroless etching (MCEE) [82]. Each approach has its advantages and disadvantages. For example, the top-down method has difficulty producing Si NWs with diameters below 10 nm. In contrast, the bottom-up approach can readily grow Si NWs with very small diameters <10 nm, but suffers from the presence of gold nanoparticle residues (catalyst nanoparticles) in the Si NWs [83–85]. However, both approaches can produce Si NWs with lengths on the scale of tens of micrometers. Fig. 2 illustrates a SEM picture of Si NWs fabricated by the two approaches. Fig. 2a illustrates state-of-the-art Si NWs, formed on large grained, multi-crystalline silicon thin films on glass using nanosphere lithography

Fig. 2. SEM micrograph of the Si NW fabricated by (a) RIE, top down, full scale bar of 60 lm, and (b) CVD.

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Fig. 3. Scheme illustrating the functionalization of Si NWs through the two-step chlorination/alkylation process. Reproduced with permission from Ref. [55].

(NSL) in combination with RIE [81]. The Si NWs are 800 nm in diameter and 6 lm in length. Fig. 2b presents an example of VLS grown Si NW with lengths of 6 ± 2 lm and diameters of 60 ± 10 nm. The red dots represent the gold catalyst which tends to remain at the edge of the Si NWs. 2.2. Organic functionalization via chlorination/alkylation One of the most significant methods employed for Si NW surface functionalization, is the versatile two-step chlorination/alkylation process wherein oxide-free silicon surfaces are first covered by chlorine atoms and later by silicon-alkyl surface bonds [53–55]. The chlorination/alkylation process is schematically shown in Fig. 3 and can be summarized in the following three steps: (1) Surface preparation and oxide removal: Si NW samples are washed with water and dried in N2 flow. They are then dipped sequentially in buffered HF solution (pH = 5) and NH4F. This results in hydrogen terminated surfaces. (2) Chlorine termination (Chlorination): H-terminated Si NWs are immersed into a saturated PCl5 solution (0.65 M in C6H5Cl) with C6H5OOC6H5 grains acting as radical initiators. The reaction is carried out at low temperatures (80–100 °C). The resultant Si NW surfaces are covered with chlorine atoms.

Fig. 4. Si NW with interfacial transient sub-stoichiometric oxides (SiOx, i.e., Si2O, SiO and Si2O3) and full oxide (SiO2) atop.

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(3) Alkyl termination (Alkylation): Cl-terminated Si NWs are immersed in alkyl Grignard (R-MgCl) with a tetrahydrofuran (THF) solvent. Depending on the chain length, the alkylation step might last from 5 min to up to 4320 min (72 h) at 80 °C. Upon reaction completion, the samples are removed from the reaction solution, rinsed in THF and methanol and dried under a stream of N2(g). These conditions were found to result in the maximal Si NW alkyl chain coverage.

2.2.1. Hydrogen-terminated Si NW Hydrogen is the smallest group that can currently be attached to the Si NW surface. To terminate Si NWs with hydrogen, the native oxide shell on Si NWs is etched away by hydrofluoric acid (pH = 5 for 30 s) and ammonium fluoride (for 1–2 min). Hydrogen-terminated Si NW (H–Si NW) surfaces exposed to the ambient air tend to demonstrate low stabilities. Thus, kinetic studies on H–Si NW surfaces can supply fundamental knowledge on the oxidation kinetics which is necessary to design strategies to prevent oxidation in the hybrid Si NW devices [86]. As schematically illustrated in Fig. 4, the sub-stoichiometric oxides (termed as transient oxides including: Si2O, SiO and Si2O3) is followed by the stoichiometric or full oxide (SiO2). X-ray photoelectron spectroscopy (XPS) was applied to measure the oxide level at the very early stages and with very thin oxide layers. Subsequently, recently made H–Si NWs were annealed in ambient conditions at distinct temperatures and at early on in the course of the reaction time (up to 60 min). An example of the Si2p XPS spectra of the H–Si NW surfaces annealed at 500 °C for different times is shown in Fig. 5a. Himpsel decomposition of the spectrum results in six sub-peaks; two Si2p spin splitting peaks (Si2p3/2, Si2p1/2) and four silicon oxide peaks. Sub and full-stoichiometric oxides are shown in Fig. 5b. The chemical shifts (DE) of the different oxidation states relative to the Si2p3/2 peak (at 99.62 ± 0.02 eV) are determined by the relation DE = BEoxBESi2p3/2, wherein BE is the binding energy assigned to a peak position. The individual binding energies are: (Si2O, DE = 0.97 ± 0.03 eV), (SiO, DE = 1.77 ± 0.03 eV), (Si2O3, DE = 2.50 ± 0.04 eV), and (SiO2, DE = 3.87 ± 0.34 eV) [87,88]. The oxide amount used for each oxidation state can be established by dividing the corresponding peak areas by the sum of the two Si peaks (i.e. Si2p3/2 + Si2p1/2). The resulting parameter is termed the oxide intensity and is illustrated by Iox. Assigning an intensity of 0.21 to an oxide monolayer, Iox can be expressed in terms of the number of the monolayers [89]. Fig. 6 depicts the variation of the average number of monolayers for individual sub-oxides (Iox) and the overall oxide intensity ðItot ¼ ISi2 O þ ISiO þ ISi2 O3 þ ISiO2 Þ over time. Comparison of the sub-oxide growth rates illuminates their temperature and time dependence. At higher temperatures (T P 200 °C), the time required to achieve 60 ± 10% of the final intensity (defined as Csat) is 5 ± 2 min, while at lower temperatures (T < 200 °C), Csat shifts to 38 ± 2 min. Despite the oxide growth occurring through annealing, the intensities of the sub oxides

Fig. 5. (a) An example of the Si2p XPS spectrum for the H–Si NW surfaces annealed at 500 °C for different times; the inclined dashed line represents the blue shift upon formation of the oxide layer. (b) Decomposed Si2p sample spectrum of the oxidized Si NWs with the resultant six sub-peaks. Reproduced with permission from Ref. [86].

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Fig. 6. Number of monolayers over time for the sub-oxides Si2O (green), SiO (gray), Si2O3 (blue), SiO2 (red) and their sum as total oxide (black) in the (a) high-temperature (a) and low-temperature regimes. The vertical dashed lines represent the Csat. Adopted from Ref. [86]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 7. Schematic illustration of oxidation mechanism of H-terminated Si NWs at low and high temperatures. Reproduced with permission from Ref. [86].

vary over a fairly small range (0.0–0.2) i.e. less than one monolayer apart. By contrast, the intensity of the full oxide has a magnitude distribution from 0.0 to 0.5 inducing the formation of more than one monolayer. At high temperatures, oxidation is initially dominated by growth site formation. Below Csat, no complete monolayer develops, indicating the formation of transited oxide sites. Later on, a complete oxide layer is formed. Further oxidation is controlled by the diffusion of oxidant, particularly via selflimited oxidation, through the oxide layer [86]. On the other hand, the oxidation of H–Si NW at lower temperatures is based on another mechanism, which is sensitive to the presence of surface pinholes and surface bonds. In this regime, the Si–Si backbonds start to interact with the arriving oxygen atoms, forming sub-oxides in the process. Through the further application of backbond oxidation, sub-oxides can be made to reach their saturation level and the growth of SiO2 leads the overall oxidation process [53,86]. At this stage, a disordered oxide layer forms on the Si NW surface and grows to form a

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Fig. 8. Saturation level of alkyls’ coverage versus alkyl chain length for Si NWs. Adopted from Ref. [55].

continuous oxide monolayer by gradual removal of the Si–H surface bonds. After removal of the surface bonds, a full oxide layer develops, and the oxidation rate again becomes dependent on the oxygen diffusion rate. To understand the relation between the rate-determining step and the surface bond strength, the Si–H bonds were replaced by Si–C (via methyl-termination of the Si NWs, as will be shown in next section), which naturally provides the same coverage as hydrogen functionalities with Si–C bonds. Throughout 44 days of exposure to ambient air at room temperature, SiO2 growth was strongly impeded, showing an intensity of 0.11 (which is reported for H–Si NWs for 1-h exposure) while sub-oxide intensities were more or less equivalent to those of H–Si NW [55,90]. This implies that Si–Si backbonds adjacent to the Si–C bonds are less prone to oxidation reactions in comparison with those connected to Si–H surface bonds. The resulting oxidation mechanism can be outlined as follows (schematically shown in Fig. 7): (i) At low temperatures, Si–Si backbond oxidation and later Si–H bond propagation are the rate-determining steps. (ii) At higher temperatures, the oxygen diffusion through the oxide layers is considered the rate-determining step. To further support the latter speculations, respective activation energies (EAox), based on the Arrhenius equation, have been calculated by fitting the logarithm of oxidation rate vs. the reciprocal of temperature during the early stages of oxidation. The EAox for the low and high temperature oxidation are 48.22 meV and 23.31 meV, respectively [86]. Nevertheless, most electronic devices are operated at lower temperatures (below 200 °C). Thus, it is crucial to also study the oxidation behavior at this reduced temperature. It should be noted that, in Si NWs functionalized by organic molecules, molecular shape and coverage level (packing density on the surface) in addition to surface bonds play a large role in the oxidation process through both propagation resistance and backbond reinforcement against attacking oxidants. The concentration of the molecules on the Si NW was found to be dependent upon steric effects caused by the lateral interactions between the terminating molecules on the surfaces. Those interactions affect the alkylation kinetics, resulting in very large process times being required to reach the maximum coverage level. However, the maximum coverage level decreases with increasing chain length. This is better shown in the next section, which presents the coverage saturation level from methyl (CH3) to decyl (C10H21). NOTE: the CnH2n+1 (where n = 1–10) assign by Cn. Therefore, methyl and decyl are represented by C1 and C10, respectively. 2.2.2. Chlorination/alkylation process Steric effects and respective van der Waals diameter can hinder the formation of dense packing of alkyls longer than C1 [54,55]. Increasing the length of the alkyl chains increases the van der Waals diameter from 2.5 Å, in the case of C1, to more than 4.5–5.0 Å for longer alkyl chains as in the case

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of C2-C5. These van der Waals diameters, larger than the internuclear distance of the adjacent Si atoms (3.8 Å), decrease the molecular adsorption rate and limit the coverage to a maximum value of 50–55% of the Si monolayer surface sites as illustrated in Fig. 8 [54,55,68]. However, the coverage rises for molecules longer than C6 show anomalous coverage behavior which justified by the time of flight secondary ion mass spectroscopy (ToF-SIMS) experiments. The ToF-SIMS demonstrated higher absolute coverage long chain lengths, consistent with XPS observations [55]. It is reasonable to consider surface termination of Si NWs by alkyls to be a function of two major factors: (I) molecule–molecule lateral interactions and (II) molecule–substrate vertical interactions. For short alkyl chains (C1–C6), exhibiting liquid-like behavior and thermal fluctuations [91], the determining factor is the vertical interaction. By increasing the chain length to C6–C10 and forming a solid-like phase, lateral interactions can be made to dominate in the surface functionalization process. It is worth pointing out that the lateral interactions between long alkyl chains might even occur during the early physisorption stage prior to the successive covalent bonding (or chemisorption) between the carbon and silicon atoms [92]. At the beginning of alkylation, the replacement reaction of Si–Cl bonds with Si–C bonds can be explained by the introduction of a transition state in which three reactions occurring simultaneously: nucleophilic carbons attack electron deficient electrophilic centers (Si atoms atop); carbon atoms bond to the silicon atoms; chlorine atoms are expelled from the interface vicinity. In other words, the incoming group replaces the expelled group, as illustrated in Fig. 9. The addition of the nucleophile and elimination of the expelled groups take place simultaneously and determines the overall reaction rate. This rate-determining step is termed bimolecular nucleophilic substitution (or SN2) and exhibits a second-order reaction rate since it depends on the concentration of the nucleophile (R-MgCl; see Fig. 9, path 1) and the Cl-terminated Si sites (path 2). The longer the alkyl chain, the lower the reaction constant (kp1) for short alkylation times. For example, the kp1 of the methyl (C1) group (2.64  102 min1) was 38 larger than that of the decyl (C10) group (7.0  104 min1). When the inter-steric effects become more relevant than the nucleophilic attack (chains longer than C7) the reaction becomes ‘‘zero-order’’ and is characterized by a constant reaction rate (kp2). 2.3. Secondary functionalization: alkenyl and alkynyl as the starting monolayers Secondary functionalization of Si NW with a variety of moieties is expected to open up a wide range of opportunities for the production of stable molecule-based (bio)sensing and (opto)electronic

Fig. 9. At the transition state, an alkyl molecule is added to the chlorinated silicon surface. Three reactions occur simultaneously. First, the anionic carbon extracts (i.e., nucleophilic carbon attack) the silicon atom to generate the Si–C bond. Second, the Si–Cl bond breaks. Finally Cl atoms are transported away from the surface. Adopted from Ref. [55].

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Fig. 10. Higher order functionalization of propenyl-terminated Si NW by hydroxyl, amidogen, paraphenylenediamine and gold nanoparticle (AuNPs). AuNPs are displayed as solid red circles. Reproduced with permission from Ref. [74]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 11. Observed oxidation intensity (SiO2/Si2p peak ratio) of alkyl-terminated Si NWs at different exposure times to ambient air. Reproduced with permission from Ref. [54,55].

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devices [93]. The versatility of this approach allows further functionalization steps (tertiary), not only by organic species, but also by inorganic species (i.e. metallic nanoparticles). Through secondary functionalization, the density of reactive cross-linkers can be controlled without affecting the stability of the Si NWs or forming metallic (or catalytic) residues on the surfaces. (See Fig. 10) Propenyl (CH3–CH@CH–) is a promising molecule for the modification of silicon surfaces and enablement of secondary functionalization, in which it gives nearly full coverage as in methyl case [62–64]. Propenyl-terminated Si NWs exhibit high stability against oxidations (700 h) and are selectively reactive in consequent chemical modifications. The double carbon–carbon bond (C@C) of propenyl provides Si NW surfaces with high oxidation resistance and highly flexible functional reactive sites, which endow further functionalities of more intricate chemical structures [62–64]. Secondary and tertiary functionalizations of the propenyl-terminated Si surfaces were achieved with the mediation of N-bromosuccinimide (NBS), as illustrated in Fig. 10. This functionality has an active leaving group for nucleophilic reactions (Br), thus allowing easy insertion into the C–H bonds of the terminal methyl (CH3) groups [94]. Fig. 11 also illustrates hydroxyl (OH) and amidogen (NH2) groups anchored to the primary functionalities by conversion of Br to OH (by hydrolysis) and NH2 (by aminolysis). Amidogen permits attachment of paraphenylenediamine (PPD) and gold nanoparticles (AuNPs) to the Si NW’s surface via tertiary and quaternary functionalities.

2.4. Stability of molecules on the Si NW 2.4.1. Effect of molecular coverage The Si–C bond is chemically more stable than the Si–O bond on oxidized Si surfaces and thus less susceptible to nucleophilic substitution reactions. Whatever the maximum number of Si–C bonds, it is clear that a significant number of unreacted Si–H and/or Si–Cl sites still remain on the surface. The type of molecule, number of Si–C bonds and the atomic sites not connected to the alkyl chains, which are also known as atomic pinholes, determine the susceptibility of the surfaces to interaction with oxidants and the subsequent oxide growth. Consequently, the surfaces are vulnerable to oxidation by water and oxygen, which can penetrate through the monolayer. This inherent instability is inconsistent with the development of stable, robust devices – the application cited by most of the published works in this area [95,96]. As shown previously, the oxidation of silicon can result in the formation of electrically active surface states that change the electrical properties of the underlying silicon [97,98]. Any strategy used to develop hybrid silicon-organic devices should ideally limit the number of oxide-based surface states and, at a minimum, stabilize them to ensure they do not change over time. To investigate the stability of the alkylated Si NWs, freshly prepared Si NWs with the maximum alkyl functionality coverage (mostly obtained after an alkylation time of 24 h) were exposed to ambient air for several hundreds of hours, as shown in Fig. 11. For the alkyl molecule, the oxide grew monotonically with exposure time. SiO2 growth on all of the alkylated Si NWs was negligible for the first 2–3 days of the experiment, but became considerable after 8 days exposure to air. For example, Si NWs functionalized by methyl and ethyl remain free from oxide up to 48 h. After 336 h of exposure, oxide intensity reached 0.03 for C1-Si NWs and 0.05 for C2-Si NWs. In comparison, after 24 h of exposure, the oxide intensity in Si NWs terminated with C3–C6 rose to 0.01, 0.02, and 0.02, respectively. This implies that Si NWs terminated with C3–C6 alkyl molecules exhibit less stability against oxidation compared to C1- and C2-Si NWs. C1 & C2-Si NWs showed a 2-fold higher oxidation resistance than that of C3–C6-Si NWs. Nevertheless, after 336 h, the oxide intensity of C3- to C6-terminated Si NWs all has approximately the same magnitude (0.13). From Fig. 11, it can be deduced that, the shorter the alkyl chain, the lower the SiOx/Si2p ratio and the less liable the terminated Si NWs are to oxidation. When the molecules have the same coverage, as in case of C3–C6, the observations can be explained by the fact that shorter molecular chains have a higher probability of interaction between oxidizing agents (O2 and H2O) and molecule-free sites (or pinholes). When the alkyl chain become longer (more than C6), the oxidation stability decreases by one fold relative to C3–C6 molecules and 3–4-fold relative to C1 and C2.

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Fig. 12. Ratio of the SiO2 to Si2p peak areas for the different surface modifications of Si NWs, exposed to air over extended time periods. Reproduced with permission from Ref. [62].

2.4.2. Effect of molecular backbone bonds: C–C vs. C@C vs. C„C The stability of the Si surfaces has also been found to vary depending on the type of chemical bonds present. Si surfaces embedded with organic molecules of similar backbone structures but different C-C bonds (i.e., C–C vs. C@C vs. C„C bonds) demonstrate different levels of stability. Unsaturated bonds, such as those in alkenyls (C@C) and alkynyls (C„C), may react with the oxidants, preventing them from reaching the underlying silicon atoms. For instance, propenyl (CH3–CH@CH–) and propynyl (CH3–C„C–) have been shown to have approximately the same molecular coverage levels as that of methyl groups with fully packed molecules on the surfaces [62,63]. However, as shown in Fig. 12, the oxidation of CH3–CH@CH–Si and CH3–Si NWs began after only 100 h of exposure and the process was more stable than that with propynyl. Upon completion of this exposure period, the SiO2/Si2p ratio of CH3–CH@CH–Si NWs increased by 0.027 ± 0.005 (0.12 monolayer of oxide) within 50 h of exposure, after which it stabilized to a level that was comparable (0.036 ± 0.012 SiO2/Si2p; 0.15 monolayer of oxide) to the value at 720 h (0.04 ± 0.01). In contrast, the oxidation of CH3–Si NWs continuously increased after 100 h of exposure, reaching 0.115 ± 0.017 SiO2/Si2p after 720 h, without showing any indication of stopping. The oxidation of CH3–C„C–Si NWs continuously increased at higher rates than that of the other two samples, reaching 0.154 ± 0.014 after 720 h of exposure. The high stability of the CH3–CH@CH–Si NW can be attributed to the p–p interactions between the adjacent molecules. In the case of CH3–C„C–Si NW, p–p interactions occur between the adjacent molecules but leave one pair of electrons free. This pair of free electrons can easily be transferred to the surface Si site (beneath the molecule) and interact with oxidizing agents (water, O2, etc.). It is of note that modification of Si NWs with (simple) C2–C6 alkyl chains showed C–Si/Si2p peak ratios of (49–68)±5% relative to that of CH3–Si NW surfaces and a 3–5-fold higher oxidation rate than that of CH3–Si NWs [62,63].

2.5. Effect of surface energy: Si NWs vs. planar surfaces The stability of the alkyl molecules is dependent on their surface energy. Bashouti and co-workers showed that the stability of alkyl molecules on Si NW is higher than on planar Si of either the (1 0 0) or (1 1 1) configuration [53]. In order to realize this effect, Si NW and planar surfaces were terminated with CH3 and exposed to large amounts of ambient air. C1-Si NWs exposed to air over a period of 336 h illustrated a ca. 3-fold lower oxidation intensity than the equivalent planar Si(1 0 0)surfaces, despite similar initial coverage levels (see Fig. 13a). These observations could be attributed to the stronger Si–C bonds on Si NW surfaces and is supported by the shift in the Si–C bond binding energy from 284.22 ± 0.02 eV for Si NW to 284.11 ± 0.02 eV for planar 2D Si. To attain a wider perspective, we made a comparison between 2D CH3–Si(1 1 1) surfaces (naturally with 15–20% coverage) and 2D CH3–Si(1 0 0). (See Fig 13a).

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Fig. 13. (a) Ratio of the oxidized Si2p peak area to the bulk Si2p peak area for the methyl modification of Si NWs, 2D Si(1 0 0), and 2D Si(1 1 1), exposed to air over extended time periods. (b) Si–C decay as a function of exposure time for Si NW and 2D Si(1 0 0).

Fig. 14. Si2p window of XPS survey for SiO2–Si NW, H–Si NW and CH3–Si NW. The vertical solid green line indicates the position of the chemically unshifted Si2p3/2 component in each spectrum. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Interestingly, 2D CH3–Si(1 1 1) surfaces exhibit a higher stability against oxidation than equivalent 2D Si(1 0 0) surfaces. They showed ca. 2-fold higher oxide intensity than corresponding CH3–SiNWs. This can also be referred to as the surface bond energy. The 0.11 ± 0.02 eV higher binding energy observed for CH3–Si NW, compared to equivalent planar CH3–Si(1 0 0) and (1 1 1) surfaces, could further be ascribed to the higher reactivity of atop sites. Indeed, the CH3–Si NW C1 spectra showed a 22% decrease in the Si–C bond signal throughout 20 days. The 2D CH3–Si(1 0 0) specimen showed a 34% decrease of the same signal over this time frame (Fig. 13b). Surface bond strength dependency on structural geometry can be backed by the reports of Lee and co-workers on the strengthened H-termination of Si NWs relative to 2D Si surfaces. They suggest that the robustness of the Si NW hydride is a consequence of bending stresses, where the re-bonding of dangling bonds at the edge of two adjoining facets of a Si NW takes place. The differences in surface energy effect the chemical reaction on planar surfaces as well. For example, 2D Si(1 0 0) surfaces were alkylated following the Si NW procedure, but with two new conditions: (I) the samples were chlorinated (i.e., immersed in the hot solution of PCl5) for 60 min and (II) the alkylation time for all samples was 24 h. These conditions were found to give the maximal coverage of alkyl chain on 2D Si(1 0 0). Alkylation of 2D Si(1 0 0) for 48 and 72 h gave similar coverage to those processed for 24 h.

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Fig. 15. Energy band diagrams of CH3–Si NW, H–Si NW and SiO2–Si NW surfaces including their vacuum energy (Evac), conduction band (EC), valence band (EV), Fermi level (EF) and surface dipole (dss). All values are in eV.

3. Hybrid Si NW electronic properties and devices Systematic alteration of the Si NW physical surface properties via the formation of hybrid surfaces permits diverse custom-tailoring possibilities for optimization for application in a variety of functional devices. Molecular functionalization can be used to modify the surface properties of Si NWs, which affect performance of the Si NW-based devices such as the surface dipole, surface Fermi level, surface band bending, concentration of surface states, etc. In this section, two different devices, FETs and solar cells, which employ hybrid Si NWs as their main building blocks are presented and discussed. Merits and drawbacks are also considered. 3.1. Manipulation of the surface fermi level The observed binding energy of the Si2p3/2 signal allows the difference between the Fermi level and the energy of the valance-band maximum (VBM) at the surface to be determined, as shown in Fig. 14. The binding energy of the Si2p signal with respect to the VBM is derived from the comparison between the Si2p core level and the valance band spectra of the three specimens. The obtained value is EvESi2p3/2 = 98.72 ± 0.02 eV, which similar to previous studies done by Himpsel et al. and Hunger et al. i.e. 98.74 eV [87,88]. Correspondingly, calculation of the difference between the Fermi level and VBM (i.e. EFEVBM) led to the following values: 0.83 eV, 0.98 eV and 1.05 eV for CH3–Si NW, H– Si NW and SiO2–Si NW, respectively. This yielded a slight downward surface band bending (B.B) for the H–Si NW and SiO2–Si NW and an upward surface band bending (B.B) for the CH3–Si NW. 3.2. Controlling the work function, electron affinity, surface dipoles and band bending The work functions (U) of CH3–Si NW, H–Si NW and SiO2–Si NW measured individually by the Kelvin probe method were USiO2  USiNW = 4.32 eV, UH–SiNW = 4.26 eV and UCH3 SiNW = 4.22 eV. By combining these data with the Fermi level position relative to the band edges (EFEV), the electron affinity (v)

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Fig. 16. Schematic illustration of the FET based hybrid Si NW with thin SiO2 = 100 nm and lightly doped (n-type, 10– 20 V X cm). On the Right side, is the top-view SEM image of the device with the PMMA protective layer on top. Adopted from Ref. [26].

can be obtained according to v = UEg + (EFEV) as vSiO2 SiNW = 4.29 eV, vH–SiNW = 4.12 eV and vCH3 SiNW 3.93 eV. Fig. 15 displays the band diagrams obtained for the three samples. The bulk Fermi level position obtained from the resistivity of the specific resistance of the n-type samples (1–5 O cm) was equal to EFEV = 0.88 ± 0.02 eV. The CH3–Si NW upward band bending of 0.05 eV was deduced to be within our error margins (±0.01). Due to electron accumulation at the interfaces, downward band bending was observed in the H–Si NW and SiO2–Si NW. Such an uncommon aspect clearly indicates that an unspecified surface doping mechanism is responsible for the formation of a stronger n-type character. This may be tentatively explained by the theory of Dittrich et al., which assumes that specific water-based functional groups are responsible for this effect [99]. In the SiO2–Si NW, the band edges of the SiO2 top layer displayed an almost symmetric heterojunction with identical valance and conduction band discontinuities. With an Eg = 1.12 eV and an estimated Si bulk electron affinity of 4.05 eV, the surface dipole (dss) induced by SiO2, hydrogen and CH3 single monolayers were found to be +0.24 eV, +0.07 eV and 0.12 eV, respectively. The surface dipoles are attributed to the different bond charge distributions between the three samples.

3.3. Hybrid Si NW-based field effect transistor As shown in previous sections, hybrid Si NWs modified with covalent Si–C bonds showed excellent atmospheric stability. This unique character allows the formation of technically feasible air-stable Si NW field-effect transistors (FETs) and other devices applied to photovoltaics and sensing applications [26,40]. Haick and coworkers demonstrated the electrical characteristics of such Si NW-based devices that were molecularly modified by the two-step chlorination/alkylation method. Molecules chemically anchored to the Si NW surface introduce extrinsic effects such as surface states and surface charge and can significantly affect the performance of relevant devices [26]. Fig. 16 illustrates the scheme and SEM micrograph of the Si NW-FET, which is coated with a PMMA protective layer. The PMMA was essential to protect the electrodes from the chemical that used for chlorination/alkylation during the two step process. The current–voltage characteristics of SiO2-covered Si NW-FETs and butyl-terminated Si NW FETs (as a representative example of the molecule-terminated Si NW-FETs) at different gate voltages are depicted in Fig. 17. The source–drain current (Isd) of both samples is increased with increasing gate voltage, indicating that the transport through the semiconducting Si NW is dominated by negative carriers (electrons), i.e. n-type Si NW behavior. For butyl-Si NW FET, the density and the energy dispersion of the surface states may change over time [100–102]. The density of surface states describes how tightly the local electrochemical potential/Fermi level (ECP) is pinned near the CNL and depends on the molecular

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Fig. 17. I–Vds curves of (a) SiO2–Si NW FET (b) butyl-Si NW at different back-gate voltages (Vg). Adopted from Ref. [26].

layer coverage, which in this case is about 50 ± 10% [103,104]. Since the SOI channel is thin (100 nm thick) and lightly doped (n-type 10–20 V cm), the surface electrochemical potential position has a dominant effect on the conduction characteristics throughout the channel cross section. In the absence of a strong applied electric field, surface states with a low CNL act as p-type dopants, whereas surface states with a CNL positioned high in the Si band gap act as n-type dopants [105]. Thus, surface states have an effect similar to that of dopants. The butyl-Si NWs-FET thus behave as if the channel has a higher doping level than the SiO2–Si NW-FET. The enhanced tansconductance at large positive biases can be explained by the enhanced carrier density and, probably more importantly, the much reduced contact resistance due to surface leakage. However, the surface-state mechanism differs from that of real dopants in its bi-directional nature. That is, surface states are able to drive the ECP much more quickly toward the CNL from either side of the gap. A second important effect of surface states is the screening of the electric field from either side of the surface. That is, the electric field originating from either the vacuum or the silicon substrate (gate) side will be reduced across the molecular layer [106,107]. A third consequence of surface states is the pinning effect, which prevents the local ECP and, therefore, the quasi Fermi level of the carriers, drifting far from the CNL. Under severe bias conditions, the electronic transport in such a device is significantly modified from the SiO2-passivated device because the local carrier densities are more closely tied to local specific conditions. In addition, the presence of surface states tends to make the metal–Si contacts ‘‘leaky’’ [108–110]. The off/on ratio observed in Fig. 17b represent the importance of the surface states of molecule-covered Si channels, which may partially shield the electric field, allowing the ECP to approach the CNL in the vicinity of the metal pad regions. Without surface pinning, the SiO2–Si NW FET bands more easily respond to the gate voltage [37,111]. The electric field readily penetrates the gap between the source and the drain when surface states are absent, leading to more effective gate action near the Si NW [111]. The molecule induced ‘‘doping effect’’ has also been shown for diamond films [112–116], carbon nanotubes [117,118], porous silicon [119], Si NW based FET [40,120], or other semiconductors [116], for which tunable electrical properties have been obtained by the chemi-adsorption of molecules. This effect has also been observed in solar cell, which will be discussed in the following section. 3.4. Hybrid Si NW-based solar cells The degree of surface band bending provides a measure for the quality of the electronic passivation on a semiconductor surface. Flat band conditions can usually only be achieved when surface defect states such as dangling bonds are passivated, making them electronically inactive. The improved electronic properties of the interface also directly influence the photoemission of electrons as well as the solar cell performance. These photoemission processes can be studied by the photoelectron yield spectroscopy (PYS). The photoelectron yield spectra of the hybrid Si NWs (SiO2–Si NWs, H–Si NWs and CH3–Si NWs) samples are shown in Fig. 18a. The top horizontal axis in the plots can be related to

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Fig. 18. (a) PYS of the different hybrid Si NW. (b) A SEM of the hybrid Si NW with PEDOT:PSS on top. (c) J–V characteristic under AM1.5 illuminations of the radial hybrid Si NW solar cells from CH3–Si NW and SiO2–Si NW. Inset: schematic view of solar cell device structure.

the VBM instead of the vacuum level by using the electron affinities of different Si NW and the band gap energy of silicon. Each PY spectrum showed two thresholds near 5.0 ± 0.2 eV and 4.2 ± 0.2 eV (plotted as dashed lines). The higher energy band corresponds to the valence band density of states of silicon while the lower energy band corresponds to the band gap defects and essentially represents the occupied state density. The Fermi level position was determined by the kelvin probe. Obviously the ratio between defect states and valence band states in the three samples is significantly different. The highest defect density is seen for the SiO2–Si NWs. In contrast, CH3 show low defect emission and interestingly, a visible third band between 4.7 eV and 5.3 eV, which can be distinguished by the spectrum’s significantly smaller slope [121]. To evaluate the impact of the distinct photoemission behavior in molecularly-modified Si NWs, PEDOT:PSS solar cells based on CH3–Si NW and Si NW was fabricated and compared with SiO2–Si NW. Fig. 18b illustrates a SEM micrograph of a hybrid solar cell made by vertically aligned etched Si NWs with a PEDOT:PSS top layer. The characteristic current–voltage (I–V) curves for CH3–Si NW/ PEDOT:PSS and SiO2–Si NW/PEDOT:PSS solar cells under AM1.5 illumination are shown in Fig. 18c. The SiO2–Si NW/PEDOT:PSS samples exhibit a short circuit current (Jsc) of 1.6 mA/cm2, an open circuit voltage (Voc) of 320 mV, a fill factor (FF) of 0.53 and a conversion efficiency (l) of 0.28%. CH3–Si NW/ PEDOT:PSS devices exhibit an improved performance with Jsc, Voc, FF and l magnitudes of 7.0 mA/cm2, 399 mV, 0.44 and 1.2%, respectively. Low FF and Jsc values occur in both devices due to the high contact resistance levels (Rs = 300 X). In spite of notably lower efficiencies compared to the values reported for efficient solar cells, the nearly fourfold relative efficiency boost offers methylation (alkylation in general) as a promising prospective approach to improve photovoltaic devices. The improved performance of the CH3-Si NWs may be ascribed to the removal of the tunnelling oxide and reduction of defect states, demonstrated by PYS, caused by reduced defect densities at the heterojunction interface and the resultant improvement in Voc. Voc can be defined according to the Shockley diode equation as Voc = kBT/qln(Jsc/J0), wherein J0 is the saturation current. Since increasing Jsc influences Voc as well the observed gain in Voc, one cannot solely rely on J0 increase for Voc. Assuming a similar J0, corresponding to the removal of the tunneling oxide without changing the interface properties, the increase of Jsc would lead to a Voc gain of DVoc = kBT/qln(Jsc,methyl/Jsc,oxide) = kBT/qln(7.0/1.6) = 0.037 V. Regarding the observed gain of DVoc = 0.079 V, only a reduced surface recombination (as measured by PY) and/or a favorable barrier formation (surface dipole) can establish consistency with the experimental data [121]. 4. Concluding remarks In the current review, the fundamental physical surface properties and applications of hybrid systems composed of Si NWs functionalized by different organic and inorganic molecules were discussed. A two-step chlorination/alkylation process has been applied to connect organic molecular (C–C, C@C and C„C) functionalities with Si NWs. Among the alkyl functionalities, methyl provided the highest

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coverage. Increasing the chain length from methyl to pentyl decreased the coverage to 50–70%. However, increasing the chain length from hexyl to decyl increased the coverage by 15%. The nonmonotonic correlation between molecule coverage and chain length demonstrates the importance of the vertical and lateral interactions. Molecules with double and triple bonds in their backbone also yield 100% coverage. Propenyl (– CH@CH–CH3) surfaces showed excellent surface passivation, with a very small amount of silicon oxide forming after more than two months of exposure to ambient air, in contrast to molecules containing single and triple bonds. The additional stability in the case of the double bond is explained by the p–p interaction. Propenyl provides the possibility of Si NW termination with subsequent second and third monolayers through termination of the edge group of propenyls with amino-reactive N-succinimidyl groups. This approach produced better-defined base monolayers of reactive cross-linkers with controlled density and an enhanced ability to covalently bind to the moieties building the monolayer atop. This approach has the potential to increase the stability of molecularly modified surfaces/junctions and prevent (or reduce) the inter-diffusion of contacting metals into the monolayer and/or the Si substrate. Finally, the ability to control the average distance between termination groups added by the subsequent functionalization of, for example, Si–CH@CH–CH3 is expected to have a significant impact on molecular electronic, sensing, and biochip technology. Control of the distance between functional groups is of great interest, since this control offers a possibility to study the electrostatic effects of ideal and non-ideal polar organic monolayers. The thermal stabilities of the organically-tailored Si NWs are dependent on the molecular coverage of the functionalities, Si NW surface energy, and the intermolecular surface bonding. Studies on the Hterminated Si NW oxidation kinetics revealed that their thermal stability relies strongly on the temperature and length of exposure to oxidants. At lower temperatures, Si–Si backbond oxidation and later Si–H bond propagation are the rate-determining steps. At higher temperatures, the oxygen diffusion is considered to be the initial rate-determining step, as it controls the growth site concentration. In methyl-terminated Si NWs, the oxidation resistance of the Si NW surfaces decreased with increasing alkyl chain length during exposure at ambient conditions, regardless of the Cmax-alkyl values of the various alkyl-Si NWs. This observation can be explained by the fact that longer alkyl chains correspond to increased heterogeneous molecular adsorption rates in the Si NWs, i.e., an increased probability for the formation of molecular domains separated by nanometer-sized molecule-free pinholes. In other words, the higher the concentration of molecule-free pinholes, the easier the oxidation process. A mechanism explaining the stability of the Si–C bonds on Si NWs has been presented. The oxide free surfaces are important and necessary in many technological aspects, e.g. radial epitaxy on NWs to realize vertical P–N junctions in photovoltaics or radial Si super-lattices in optoelectronics. Hybrid Si NWs have been were integrated into FETs and solar cells in order to utilize their advantages. The hybrid Si NW-FET characteristics have been found to be depend strongly on the properties of the molecules absorbed on the conduction channel. When passivated by SiO2, Si NW-FET source–drain current could not be turned off by negative gate voltages below 20 V. However, butyl termination of the Si NWs increases the ‘‘on’’ current at large positive gate voltages and reduces the ‘‘off’’ current at large negative gate voltages by several orders of magnitude. The possible creation of surface states along with molecular attachment to the surface can provide a qualitative explanation of those electrical measurements. Several fundamental surface properties relevant to solar cells have been found to be strongly dependent on the type of functionalization used. Thus, the right choice of surface modification is crucial to solar cell performance. For instance, Si NW attached to CH3, H or oxide groups produces a corelevel shift and band bending in the Si2p emission derived from the Si surface atoms of 50 meV, 100 meV and 170 meV, respectively. The good passivation of CH3 on the Si NW reduces the density of states and create a negative dipole (0.12 eV), which lowers the vacuum level and thereby increases the yield. Hydrogen- and SiO2-terminated SiNW demonstrate enhanced vacuum levels and an increased surface density. As a result, solar cells with CH3-based radial heterojunctions demonstrated better performance and efficiencies up to four times better than that of conventional cells: SiO2-SiNW/PEDOT:PSS cells obey a short circuit current (Jsc) of 1.6 mA/cm2, an open circuit voltage (Voc) of 320 mV, a fill factor (FF) of 0.53 and a conversion efficiency (l) of 0.28%. CH3-Si NW/PEDOT:PSS devices exhibit improved performance with values for Jsc, Voc, FF and l of 7.0 mA/cm2, 399 mV, 0.44

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and 1.2%, respectively. The examples provided are meant to illuminate the significant role of functionalities in the performance of nano-devices built upon hybrid Si NWs. Through the proper design of hybrid Si NW systems, a large range of combinations of physical characteristics can be achieved to suit many different applications.

Acknowledgments M.Y.B gratefully acknowledges the LCAOS and Max-Planck Society for the Post-Doctoral fellowship. K.S wishes to thank University of Erlangen-Nürnberg and the Elite Advanced Materials and Processes (MAP) graduate program for the MS thesis scholarship. S.H.C. and H.H. acknowledge the financial support by the FP7 EU project LCAOS (nr. 258868, HEALTH priority). P. M and S.C acknowledge the NAWION project funded by German Ministry of research and education (BMBF). Ms. Heidemarie Embrechts’s help with editing is greatfully acknowledged.

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