Dynamics of Substituted Alkyl Monolayers Covalently Bonded to

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Dynamics of Substituted Alkyl Monolayers Covalently Bonded to Silicon: A Broadband Admittance Spectroscopy Study Christian Godet,*,† Alain-Bruno Fadjie-Djomkam,† Soraya Ababou-Girard,† Sylvain Tricot,† Pascal Turban,† Yan Li,‡ Sidharam P. Pujari,‡ Luc Scheres,§ Han Zuilhof,‡,∥ and Bruno Fabre⊥ †

Institut de Physique de Rennes, UMR 6251 CNRS-Université de Rennes 1, 35042 Rennes-Cedex, France Laboratory of Organic Chemistry, Wageningen University, Dreijenplein 8, 6703 HB Wageningen, The Netherlands § Surfix BV, Dreijenplein 8, 6703 HB Wageningen, The Netherlands ∥ Department of Chemical and Materials Engineering, King Abdulaziz University, Jeddah, Saudi Arabia ⊥ Institut des Sciences Chimiques de Rennes, UMR 6226 CNRS-Université de Rennes 1, Matière Condensée et Systèmes Electroactifs (MaCSE), 35042 Rennes-Cedex, France ‡

ABSTRACT: The relaxation dynamics of surface-bound n-alkyl chains was studied by broadband admittance spectroscopy (10 mHz−10 MHz) measured at low temperature (130−300 K) in the reverse bias regime of rectifying Hg// organic monolayer (OML)−n-doped Si tunnel junctions. To obtain molecularlevel information on the structure and dynamics of grafted monolayers, carboxyl or amide dipolar moieties were located either at the top free surface with variable acid concentration (0%, 5%, and 100%) or at the inner position in the alkyl backbone (100% amide units). Two classes of dipolar relaxation mechanisms are found with different thermally activated behavior. At low T, only peak A is observed (f ≈ 102−105 Hz) with very small activation energy (EA = 20−40 meV) and pre-exponential factor (f 0A ≈ 103−106 Hz). With increasing T, peak B also appears, with higher values of activation energy, EB = 0.25−0.40 eV, and pre-exponential factor ( f 0B ≈ 108−1010 Hz). The biasindependent relaxation mechanism A, with very low activation energy typical of dipole−dipole interaction, is attributed to extrinsic relaxation of adventitious H2O molecules in hydrogen-bond clusters. Mechanism B is attributed to intrinsic relaxation of the alkyl chain assembly. In the acid series, the relative intensity of peak B is consistent with the acid group coverage given by XPS, in contrast with peak A, and its activation energy reveals increased motional constraints in the acid-substituted OML. The shape of dipolar relaxation peaks, discussed in the framework of DissadoHill/Jonscher theories for many-body interactions, is useful to discriminate near-substrate and molecular tail relaxations through order/disorder effects. sion)19−21 or electronic10,22,23 properties. However, it is still not well understood how intermolecular (electrostatic, dipolar, and van der Waals) interactions affect the OML organization, homogeneity, and ordering of alkyl chains.1,24 However, steric hindrance of the covalently bound chains during the grafting process can produce substantial fluctuations in packing density or domains with variable orientation.1,25,26 Pinholes, areas with lower molecular coverage, and domain boundaries may in turn lead to enhanced oxidation kinetics of OML−Si assemblies exposed to ambient conditions. Covalent binding of linear saturated (n-alkyl) chains to hydrogenated Si(111):H surfaces forms model molecular layers with a high coverage, which play the role of a nanometer-thick tunnel barrier.1−11,16,27−36 Because of steric constraints, the density of surface Si−H sites that are not grafted with alkyl

1. INTRODUCTION Miniaturization of microelectronic devices and control of electrical barrier heights via interface dipole engineering bring increasing interest in molecular modifications of silicon substrates.1−11 A wide variety of chemical processes have been developed to directly bind organic monolayers (OML) to oxide-free silicon surfaces via a chemically stable nonpolar Si−C bond.1,12−16 Direct electronic coupling is therefore possible between the organic functionality and the semiconductor, making OML−Si hybrid devices highly interesting for biosensor, molecular electronics, and solar cell applications. Annealing studies have pointed out the importance of interface covalent bonding in thermal stability of self-assembled monolayers.17,18 Organic monolayers offer a wide choice of functionalities for molecular-level control of layer structure and surface chemistry. Intense experimental work has been performed to understand how the orientation and conformal order in OML coatings influence their mechanical (friction, wetting, and adhe© 2014 American Chemical Society

Received: December 5, 2013 Revised: March 7, 2014 Published: March 20, 2014 6773

dx.doi.org/10.1021/jp411937t | J. Phys. Chem. C 2014, 118, 6773−6787

The Journal of Physical Chemistry C

Article

Scheme 1. Molecular Assemblies Covalently Bound to n-Type Si(111) Surfaces

molecules is larger than 4 × 1014 cm−2 (≈ 50% of surface Si atom density). Although a very low density of electrically active states at the OML−Si interface has been reported for the asgrafted devices (in the 1 × 10 10−3 × 10 11 eV−1·cm−2 range34,37−41), surface oxidation results in an increase in interface states density and enhancement of the effective barrier height inhomogeneity.36 Admittance spectroscopy measures the modulation of a localized density of electrical charge and the ability of dipolar objects to reorient in response to a time-dependent applied electric field. This technique is also called dielectric spectroscopy when applied to insulating materials. Different representations of the frequency response have been used in the literature, including the electrical modulus M* and the dielectric permittivity ε*, to investigate dissipation (energy loss) mechanisms. As far as dipole reorientation is concerned, the system must overcome energy barriers, whose magnitude may be related either to local or more collective mechanisms. Admittance spectroscopy has been widely applied to a large number of polymers, lipids, liquids, and composites to understand phase transition and phase composition effects,42,43 and to a lesser extent to nanophase separation44 and very thin organic monolayers.45,46 Recently, it has been developed to study the dynamics and the electronic response of monolayer assemblies of surface-bound molecules immobilized on a variety of substrates including gold,47 porous glass,48 glass plates,49 and crystalline silicon.25,36,41,50 The response of Metal−OML−Si junctions to a small-signal electrical stimulus has been investigated by a few laboratories, showing the sensitivity of admittance spectroscopy to both electronically active defects localized at the silicon/molecule interface and structural defects in the molecular monolayer organization detected through their dipolar relaxation signature.25,36,41 Modeling of the admittance of ultrathin tunnel barriers is helpful to discriminate the characteristic response times corresponding to the modulation of the space-charge layer width and to the tunneling mechanism.50 In contrast with the bias-dependent admittance response related to the space-charge layer and the electrically active defect states, dipolar relaxation is expected to be essentially bias-independent. Indeed, a low-temperature investigation of Hg//C12H25−n-Si junctions has revealed a dipolar relaxation mechanism at a frequency near 2 kHz, independent of the dc bias and very weakly dependent on temperature (activation energy EACT ≈ 40 meV).36 It was tentatively attributed either to dipoles in the alkyl chain induced by the strong permanent

dipoles of interfacial silicon oxide or to the relaxation of water molecules trapped at the OML/silicon interface. The present work elucidates this alternative by comparing junctions with and without silicon oxide at the OML-Si interface. It shows that this bias-independent relaxation (defined as mechanism A) is present independently of silicon oxidation and should rather be attributed to adventitious physisorbed water. However, in Al− C18H37−n-Si devices, a dipolar relaxation mechanism with a bias-dependent frequency (20−200 kHz at room temperature) has been attributed to the relaxation of molecular segments preferentially located at some structural defects that interact with the rest of the OML through van der Waals forces.41 In this work, a bias-dependent relaxation (defined as mechanism B) is also observed. In order to get new insights into the origin of relaxation mechanisms A and B, this broadband admittance spectroscopy study compares metal−OML−Si assemblies with dipoles embedded at different locations of the molecule backbone. In organic monolayers covalently immobilized to various substrates, characteristic dipolar relaxation frequencies have been compared to the relaxation behavior in polyethylene.48,49 However, a comparison of the activation energies found in different reports is not straightforward because different interface chemistry reactions have been used for grafting on glass48,49 and silicon25,36,41,50 substrates. Since larger disorder and low coating density are expected in the curved internal surfaces of the porous glass matrix, the role of molecular coverage should also be considered.49 Finally, the modulated electric field has an in-plane orientation for interdigitated electrodes on glass,49 while it is perpendicular to the substrate for Si-based devices.25,36,41,50 Although alkyl-based polymers (e.g., polyethylene) are characterized by a very low dipolar activity, they can be rendered dielectrically active by oxidation or chemical substitution of the precursors.51 Furthermore, different OML dipolar relaxation frequencies were found upon introducing polar groups at targeted positions in the molecular monolayer, e.g., at the substrate interface or end group positions, and a clear dependence of dipolar relaxation strength on molecular coverage has been evidenced.48 In this work, this approach is developed to characterize monolayer assemblies of surface-bound n-alkyl chains substituted by polar groups (Scheme 1), using the broadband admittance spectroscopy technique in a well-defined metal− organic monolayer−semiconductor sandwich configuration, relevant for molecular electronics devices. Polar groups were 6774



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2. EXPERIMENTAL SECTION 2.1. Monolayer Formation. Covalent grafting was performed on hydrogenated Si(111):H surfaces using either linear alkene molecules (pure alkyl or acid and mixed alkyl-acid OML) with a UV-assisted liquid phase process, or linear alkyne molecules (acid fluoride end-groups) with a low-temperature thermally assisted liquid phase process. A low-doped n-type Si (phosphorus doped, 1−10 Ω·cm resistivity, from Siltronix) has been chosen to obtain rectifying junctions.29,30,34,36 2.1.1. Hydrogenation of Si(111) Surfaces. The chemicals used for cleaning and etching silicon wafer pieces (30% H2O2, 96−97% H2SO4, and 40% NH4F solutions) were of VLSI semiconductor grade (Riedel-de-Haën). Before grafting, a single side polished n-type Si(111) substrate was sonicated for 10 min successively in acetone (MOS semiconductor grade, Carlo Erba), ethanol (99.8%, VLSI semiconductor grade), and ultrapure 18.2 MΩ cm water (Elga Purelab Classic UV, Veolia Water STI). Organic decontamination was performed in 3:1 v/ v concentrated H2SO4/30% H2O2 at 100 °C for 30 min, followed by copious rinsing with ultrapure water. The surface was etched with argon-deaerated ppb grade 40% aqueous NH4F for 15−20 min at room temperature,57 dipped in argondeaerated water for a few seconds, and blown dry with argon. This fully hydrogen-passivated Si(111):H surface is hydrophobic (water contact angle of 84°) and free of C and O contamination, as shown by XPS. 2.1.2. Single-Component Alkyl, Acid, and Mixed Alkyl/Acid Monolayers Prepared from Alkene Precursors. After etching, the hydrogen-terminated Si(111):H was used immediately for the covalent attachment of either single-component monolayers (n-dodecyl Si−C12, n-hexadecyl Si−C16, or undecanoic acid SiAcid 100) or the mixed n-dodecyl/acid-terminated monolayer. Such monolayers were prepared from the photochemical reaction at 300 nm for 3 h of Si(111):H with neat 1-dodecene, 1-hexadecene (Sigma-Aldrich, >99%, previously passed through a neutral, activated alumina column to remove residual water and peroxides, then distilled over sodium and stored under argon in the fridge), or undecylenic acid (Acros, 99%, previously passed through a neutral, activated alumina column), or a mixture of undecylenic acid/1-dodecene with a 5/95 molar ratio.58−60 It has been demonstrated that this direct hydrosilylation route does not lead to appreciable reaction between the carboxyl groups and the surface provided that short UV irradiation times are used (typically, less than 4 h).61 The covalently alkyl monolayer-modified surfaces were rinsed copiously with trichloroethylene and acetone, and dried under an argon stream. The single-component and mixed acid-modified surfaces were rinsed copiously with dichloromethane, then dipped in hot acetic acid at 65 °C (2 × 20 min), and dried under an argon stream.62 The mixed acid-modified surface is denoted as Si-Acid 5. 2.1.3. Butylamide Monolayer Prepared from Alkyne Precursor. A two-step grafting procedure was adopted to perform the reaction of butylamine molecules with an acid fluoride-terminated OML immobilized on Si(111):H using a low-temperature grafting process.63 Pieces of an n-Si(111) wafer were rinsed several times with acetone (Sigma/Honeywell, semiconductor grade) and sonicated in acetone for 10 min. Then, the samples were cleaned using oxygen plasma (Harrick PDC-002 setup) for 3 min. Subsequently, the Si(111) substrates were etched in an argon-saturated 40% NH4F solution (Sigma/Honeywell, semiconductor grade) for 15 min

introduced at specific positions in the n-alkyl chains to selectively enhance the local dielectric response, providing molecular level information on the dynamics of grafted monolayers. This article shows that a clear identification of dipolar mechanisms requires a study of the modulation of the charge density as a function of dc bias and temperature, in particular when multiple dipolar signatures are observed. In addition to the OML response, dipolar relaxation spectra may be affected by the presence of physisorbed water molecules, which are expected to form a complex hydrogen-bond network interacting with such polar moieties. XPS characterization of a Si−C12H25 assembly, after aging in air, has shown that the density of adsorbed polar H2O molecules may be larger than the density of grafted molecules;36 hence, adventitious physisorbed water should also be considered in the discussion of dipolar relaxation at the molecular scale. The role of water on electrical transport and device stability is also of paramount importance. Section 2 gives some details of the immobilization processes for covalent grafting of alkene or alkyne linear molecules on hydrogenated n-doped Si(111) surfaces (Scheme 1). In this work, carboxyl or amide dipolar moieties were introduced either at the top free surface (acid end-group) or at the inner position (amide) in the alkyl backbone. Since it is not clear how polar groups affect molecular interactions between neighboring chains, a series of devices with variable concentrations of acid end-groups (0%, 5%, and 100%) has been studied. In addition, a Si−C12 assembly has been aged at ambient conditions to study the influence of Si substrate postoxidation. The OML−Si assemblies were characterized by scanning tunneling microscopy (STM), atomic force microscopy (AFM), spectroscopic ellipsometry (SE), and X-ray photoelectron spectroscopy (XPS). For electrical measurements, a mercury top contact was chosen to avoid metal shorts through the nanometer-thick OML. Since Hg//OML−n Si tunnel junctions are strongly rectifying for n-type doping, broadband admittance measurements (10 mHz−10 MHz) were performed at low temperature (130−300 K) in the reverse bias regime to minimize the dc conductance. Section 3 summarizes the topography results (STM and AFM), OML thickness (SE), and packing densities derived from XPS analysis. The most significant admittance Y(V, T, ω) characteristics are reported as a function of applied dc bias and temperature, in particular by selecting a temperature range where peaks A and B do not overlap. The relaxation behavior of acid-terminated and butylamide molecular assemblies is compared with that of pure alkyl junctions, either as grafted (hexadecyl-C16H33) or after aging at ambient conditions (dodecyl-C12H25). Section 4 summarizes the main results and details the analysis of admittance Y(V, T, ω) measurements as a function of temperature (at selected dc voltage) in order to extract activation energies and pre-exponential factors of the characteristic relaxation frequencies. The results obtained for various end-group or inner-group substitutions are discussed in terms of intrinsic relaxation (peak B) of the alkyl chain assembly and extrinsic relaxation (peak A) due to some adsorption of ambient water molecules in specific configurations. Finally, an interpretation of the shapes of the electrical modulus, M″(ω), loss peaks in the framework of Dissado-Hill, and Jonscher theories for many-body interactions52−56 will be proposed in order to relate the prepeak and postpeak slopes to some order/ disorder effects in the collective relaxation mechanisms. 6775



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oxide thickness, dOX, was derived from the relative intensities of the Si2p bands at 103 eV (SiO2) and 99.6 eV (Si) binding energies, assuming a homogeneous coverage of the Si substrate and using (dOX/LSiO2 cosα) = ln[1+ R0−1([Si2p]SiO2/[Si2p]Si)] with R0 = 0.76 ± 0.2 and an attenuation length of LSiO2 = 2.923 nm in silicon oxide for a Mg Kα source.67 2.2.4. Electrical Transport and Admittance Measurements. A mercury top electrode (contact area S = 5 × 10−3 cm2) was used to avoid electrical shorts through possible pinholes in the OML. The top contact to the OML was taken through a Pt wire and a fresh Hg drop (99.999% Fluka); an ohmic back contact was obtained by applying a silver paste electrode on the back of scratched Si. A homemade parallel plate Teflon cell, compatible with Hg, was used for current density J(V, T) and admittance Y(V, T, ω) measurements, as described previously.34 The cell was placed in a cryostat under dry nitrogen flow to avoid extensive water condensation and to minimize surface oxidation during electrical measurements. In this work, a solid Hg electrode is obtained in the low temperature range (T < 233 K) useful to investigate dipolar relaxation. Admittance measurements were carried out with a frequency response analyzer (Alpha-A High Resolution measurement system, Novocontrol Technologies) in two runs: a voltage sweep (step 25 mV) and a frequency sweep (range 1 × 10−2 Hz to 1 × 107 Hz), to study the measured complex admittance, Y*(V, T, ω) = Gm + jω Cm (for the equivalent parallel R-C circuit) against voltage, V, and frequency, ω /2π. The ac modulation amplitude VAC was set at 20 mV. The capacitance (4.5 pF) of the empty Teflon cell in parallel with the molecular junction was subtracted to obtain Cm. Some J(V) characteristics obtained using the Novocontrol system at low frequencies (1− 100 Hz) were compared to the J(V) data acquisition using a Keithley 6487 picoammeter. At high frequencies, useful information on dipolar mechanisms is limited by the presence of a series resistance RS due to bulk Si and back contact resistance; note that RS is given by the high frequency value of the real part of the impedance, RS = Gm/(Gm2 + ω2Cm2).68 The effect of RS (typically above 100 kHz) has not been corrected in this study; however, it has been included in previous modeling50 of ultrathin tunnel junctions. Complex admittance Y* = Gm + jω Cm may also be analyzed using the complex capacitance C* = (Y*/jω) or the complex electrical modulus M* = (ε*)−1 = jω C0/Y* (here C0 = C*/ε* is arbitrarily set at 100 pF). Loss peaks in the imaginary modulus, M″(ω), or in the imaginary permittivity, ε″(ω), appear at characteristic frequencies corresponding to a delay between electric field and local charge modulation or dipole rotation.34,36,43,50,69 In a previous admittance spectroscopy study of Si−C12//Hg junctions, it has been shown that modulation of the semiconductor space charge region dominates the forward bias response with a bias dependent peak ( f 1 = G/2π C) as expected from a kinetic modeling of the MIS tunnel diode.50 In contrast, the reverse bias regime with small dc conductance is explored at low temperatures (130−300 K) to observe relaxation mechanisms in the tethered molecular layer. In this work, dipolar relaxation mechanisms are discriminated using temperature and bias dependence of their respective intensities (M″MAX), peak shape exponents, and characteristic frequencies. The peak shape exponents (mDH, nDH-1) are the low and high frequency slopes in ln[M″(ω)] vs ln(ω) plots, obtained from experimental data analysis; a physical interpretation in the context of Dissado-Hill theory for many-body dipolar

under an argon atmosphere. After being etched, the samples were thoroughly rinsed with water and finally blown dry with a stream of nitrogen. A small three-necked flask, equipped with a capillary as the argon inlet and a reflux-condenser connected to a vacuum pump, was charged with freshly distilled 10-undecynoyl fluoride, obtained by reacting 10-undecynoic acid and cyanuric fluoride.63 The freshly etched Si(111):H substrates were transferred into the reaction flask after deoxygenation with argon (for at least 30 min at 100 °C), and a continuous argon flow was maintained. After 16 h of reaction at 80 °C, the samples were extensively rinsed with CH2Cl2 and sonicated in CH2Cl2 for 5 min to remove physisorbed molecules and finally blown dry with a stream of nitrogen. The condensation reaction of acid fluoride-terminated monolayers with primary amines (25 mM solution of n-butylamine in 1-methyl-2-pyrrolidinone) was performed for 1 h. The resulting Si−C14-butylamide assemblies were finally rinsed and sonicated with distilled CH2Cl2. 2.2. Monolayer Characterization. The OML−Si(111) assemblies were cut in several pieces for AFM, STM, and XPS experiments. After XPS measurement, the sample was stored in ultrahigh vacuum (UHV) for several days to several weeks. After removal from UHV, spectroscopic ellipsometry measurements were immediately performed, and the same sample was then mounted and inserted in the cryostat for dc and ac transport measurements of the Hg//OML−Si junctions. 2.2.1. Spectroscopic Ellipsometry. SE experiments were performed in the range from 1.0 to 4.7 eV, at an incidence angle of 70°, using a Horiba (UVISEL) ellipsometer, and analyzed with a planar three-layer model (ambient, organic monolayer, and substrate). To describe the dielectric function of the OML, an energy independent refractive index was chosen because no significant improvement in the fitting result was found with a dispersion formula. The optical thickness, dSE, values were derived using a refractive index nSE = 1.50. 2.2.2. Scanning Tunneling Microscopy. STM experiments were performed in a ultrahigh vacuum (UHV) chamber with a base pressure lower than 10−10 mbar using an Omicron VT STM operating at room temperature in the constant current mode. Electrochemically etched W tips were cleaned in situ by thermal heating before the STM experiments. An ohmic contact was taken at the back of the silicon substrate to ensure sample grounding (Vsample = 0). The applied tunnel voltage Ugap is defined by Ugap = Vsample − Vtip = −Vtip, and the tunnel current is noted as IT. Freshly grafted samples were promptly transferred from the ambient to UHV and degassed for 2 h at 400 K to remove traces of water contamination; the improvement in the tip−sample tunnel contact shows that water is indeed physisorbed at the OML surface. STM images were processed by a simple background plane correction and were not corrected for sample thermal drift, which can be neglected on the large scale images presented in this study.60 2.2.3. X-ray Photoelectron Spectroscopy. The molecular coverage (ΣOML) and unwanted silicon oxidation were characterized by XPS after a few minutes of exposure to the ambient conditions, using a Mg Kα (1253.6 eV) anode source and Omicron HA100 electron energy analyzer (1.0 eV resolution). The thickness, dOML, of the molecular layer immobilized on the Si surface was derived from the attenuation [Si 2p]grafted/[Si 2p]bare = exp(−dOML/λOML cosα), with an inelastic mean free path value λOML = 3.5 nm (for Si2p photoelectrons) typical of a dense molecular layer.64−66 The 6776



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For the Si−C12 surface, the optical thickness (dSE = 1.45 ± 0.1 nm) fitted to SE data (Table 1) is consistent with previous reports for alkyl monolayers1,15 and with the geometrical length of C12 linear molecules, including a tilt angle of 25−30°. A larger average tilt angle (40° to 50°) is found for the Si-Acid 5 (dSE = 1.1 ± 0.1 nm), Si-Acid 100 (dSE = 1.1 ± 0.1 nm), Si− C16 alkyl (dSE = 1.6 ± 0.1 nm), and Si−C14-butylamide (dSE = 1.8 ± 0.1 nm) monolayers. 3.2. STM Data. STM imaging before and after molecular immobilization is important to assess the conformal coverage of the OML−Si assembly. Typical large scale STM images (1100 × 1100 nm2) of a hydrogenated Si(111):H surface and a mixed alkyl/acid-terminated monolayer-modified Si(111) surface (SiAcid 5) are shown in Figure 2a and b. In both images, large flat

interactions is proposed in section 4.3, in line with previous reports.25,41

3. EXPERIMENTAL RESULTS In order to understand the behavior of the characteristic parameters (frequency, intensity, shape exponents, and their temperature dependence) of dipolar relaxation in monolayer assemblies of surface-bound n-alkyl chains, complementary information is gathered to estimate their conformal coverage (STM, AFM), OML thickness (SE, XPS), molecular packing density (XPS), and possible interface oxidation of the Si substrate (XPS). The average dipole density was also obtained by XPS for an OML series with variable acid end-group concentration (0%, 5%, and 100%) and for an OML with inner amide functionality. 3.1. Spectroscopic Ellipsometry. Figure 1 shows the pseudodielectric functions of the Si-Acid 5, Si-Acid 100, Si−

Figure 2. STM image of Si(111):H before and after covalent grafting of the Si-Acid 5 monolayer: (a) Si(111):H (1000 × 1000 nm2, Ugap =2.051 V, IT = 0.154 nA) and (b) Si-Acid 5 (1100 × 1100 nm2, Ugap =3.9229 V, IT = 31.62 pA).

terraces (typical width 200 nm) are observed and separated by well-defined parallel steps. The heights-histogram calculated from Figure 2b displays distinct peaks, separated by a constant value of 3.0 ± 0.1 Å, which matches the Si(111) inter-reticular distance. The observed steps are thus monatomic steps of the underlying Si(111) substrate, indicating that the alkyl/acidterminated monolayer grafting has preserved the substrate surface morphology. The corresponding root-mean square roughness on this image is ca. 0.3 nm. These observations are basically confirmed by AFM experiments. The Si(111) monatomic terraces present a flat molecular layer with a tiny and randomly distributed height modulation of typically 0.25 nm in amplitude. This subnanometer-scale topographic modulation, that is much smaller than the grafted

Figure 1. Pseudodielectric functions derived from SE data for OML− n-Si covalent assemblies, as compared with the crystalline Si reference (ref 70).

C12, Si−C16, and Si−C14-butylamide assemblies, as compared with a crystalline Si reference.70 Similar spectra are obtained for Si-Acid 5 and Si-Acid 100 samples. As shown previously (Figure 3a, in ref 60), the optical properties of the mixed alkyl/ acid-terminated monolayers (0%, 5%, 33%, and 100% acid molar fraction) grafted to n-Si(111) are independent of the acid fraction over the whole range of concentrations. Hence, the formation of acid bilayers which may result from hydrogen bond formation (acid/acid stacking)62,71 can be discarded.

Table 1. Analysis of Broadband Admittance Spectroscopy Data Measured in n-Si−OML//Hg Junctions in the Reverse Bias Regime (−0.2 V Applied to the Metal) at Variable Temperaturea sample (treatment)

thickness (nm)

coverage (1014 cm−2)

Si−C12 aged in air

1.45

2.3

Si−C16 as-grafted

1.55

2.5

Si Acid 5 end dipole

1.06

2.6 (total) 0.4 (acid)

Si Acid 100 end dipole

1.12

2.8

Si−C14-butylamide inner dipole

1.76

3.3

A B A B A B A B A B

EACT (eV)

f 0 (Hz)

M″MAX peak value

slope m

slope (1− n)

0.033 0.30 0.020 0.33 0.026 0.40 0.026 0.43 0.040 0.25

104 109 106 109 104 1010 104 1010 103 108

0.063 0.080 0.020 0.051 0.019 0.083 0.012 0.230 0.007 0.027

0.75 0.0 0.87 0.75 0.86 0.72 (0.86) 0.62 (0.7) (0.8)

0.59 0.40 0.57 0.70 0.61 0.48 (0.61) 0.53

a

The dipolar relaxation characteristics in organic monolayers, covalently bonded to Si(111), include the activation energy and pre-exponential factor of the relaxation frequency, and the imaginary electrical modulus (M″) peak height value for the high-frequency (peak A) and low-frequency (peak B) relaxations. Peak shape exponents mDH and (1-nDH) were obtained in the low temperature range where peak overlap is minimized; they are analyzed according to Dissado-Hill theory where nDH and mDH are the intra- and inter-cluster correlation parameters (see text), respectively. 6777



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molecular chains length, attests to the absence of pinholes in the molecular layer and is most likely related to the difference between the acid and alkyl chain lengths. Similar surface morphology was obtained for a diluted acid layer prepared in the same conditions (Figure 4a, in ref 60). From these STM observations, we deduce that the mixed acid-alkyl grafting leads to the formation of a dense and continuous molecular monolayer. The acid-terminated and alkyl chains are randomly distributed in the molecular monolayer, as expected for this direct grafting reaction involving two alkene precursors. 3.3. X-ray Photoelectron Spectroscopy. 3.3.1. Molecular Coverage. Besides the Si substrate signal, only C1s and O1s signals appear in the survey spectra measured for the OML− Si(111) assemblies, along with N1s in the butylamide assembly. On n-type Si(111), the Si2p3/2 maximum appears at 99.6 eV, and there is no evidence of oxidation of underlying silicon surface immediately after grafting. For the single-component and mixed acid monolayers, resolved C1s spectra clearly show the main peak at 285.3 eV due to C−C bonds and a COOH component at 290.1 eV. The intensity of this latter component increases with the molar fraction of undecylenic acid. The coverage of acid-terminated chains (Table 1) has been calculated using the C1s (COOH) signal. The total organic layer (acid + alkyl) coverage has been estimated using the total area under the C1s signal.60 3.3.2. OML Thickness. The thickness of the grafted organic layer (dXPS) has been estimated from the attenuation of the angular silicon signal (averaged to account for diffraction effects).65 The value of the organic layer thickness found by this method is slightly smaller than that deduced from spectroscopic ellipsometry data (dSE), probably because the value chosen for the inelastic mean free path underestimates the actual value when the packing density is less than optimal. As an illustration, for butylamide monolayer with a molecular coverage of ΣOML = 3.3 × 1014 cm−2, the optical thickness of dSE = 1.8 nm (using n = 1.50) is larger than the XPS attenuation thickness dXPS = 1.5 nm. However, both values are consistent with grafting of a rather dense single molecular layer. 3.3.3. Interface Oxidation. The oxidation rate of Si substrates grafted with densely packed alkyl monolayers is very slow6,12,36,72 with a 2 month typical time scale, in contrast with a few hours time scale for the oxidation of hydrogenated Si(111):H surfaces under ambient conditions.16,73 However, an accurate quantification is required for a careful interpretation of molecular dipolar relaxation. For the as-received Si−C14-butylamide assembly (i.e., after a few days at the ambient conditions), the SiO2 component observed near 103 eV in Si2p spectra (Figure 3) represents an equivalent surface density, ΣSiO2 = 0.9 × 1014 cm−2, i.e., about one tenth of a monolayer, as derived from the average SiO2 thickness of 0.04 nm using a density of 2.3 g·cm−3. Note that this estimate is very rough since inhomogeneous oxidation requires that the interface oxide thickness has locally larger values. Using infrared measurements, Faucheux et al. have shown that some inhomogeneous distribution of silicon oxide can in fact correspond to the local presence of 1 nm-thick SiO2 clusters.62 In this study, the junctions with the lowest residual silicon oxide have been selected. Their silicon oxide coverage is typically one decade smaller than that obtained after intentional aging of a Si−C12 assembly at ambient conditions for 18 months (denoted here as “Si−C12 aged”); this postoxidation results in a coverage of ΣSiO2 = 4 × 1014 cm−2, i.e., about half of

Figure 3. XPS spectrum (Si 2p1/2−2p3/2 core level, α = 45°) of the Si−C14-butylamide assembly; the inset shows the weak Si4+ peak used to obtain the effective SiO2 coverage (see text).

the monolayer of Si4+ (0.17 nm of SiO2), while the alkyl molecular coverage remains unchanged.36 3.4. Current Density Analysis. In this investigation of dipolar relaxation mechanisms, the available frequency window is limited by the dc conductance at low frequency and by the series resistance at high frequency. Hence, minimizing the dc conductance is crucial in order to decrease the frequency of the space charge layer response at f 1 = G/2π C. In this study, the reverse bias regime is explored with a typical current density J(−1 V) smaller than 1 × 10−6 A·cm−2 at 300 K and 1 × 10−10 A·cm−2 at 200 K, for all the studied junctions. This behavior is illustrated in Figure 4a for the Si−C14-butylamide//Hg junction. It has also been checked that the quasi-dc current

Figure 4. (a) Temperature dependence of J(V) for the Si−C14butylamide//Hg junction showing a decrease of reverse bias conductance by more than five decades; some oscillations in the forward bias regime are attributed to some inhomogeneous barrier height distribution. (b) Effective barrier height ΦEFF vs VDC in the reverse bias regime, showing a decrease ΔΦEFF in the low bias range. 6778



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junctions. Dipolar mechanisms are identified as weak asymmetric peaks in the imaginary electrical modulus M″(ω) or in the imaginary permittivity ε″(ω) = (C″/C0) representations of broadband admittance data.43 Figure 5a shows that some differences appear in the low frequency range, in particular a better resolution of peak shape appears in the M″(ω) representation.

density measured in the frequency range 1−100 Hz is similar to the J(V) data, within a factor of 2. A typical value of the effective barrier height ΦEFF ≈ 0.85 eV, at room temperature, is derived from J(V) in the forward bias range, typically 0.15−0.45 V. The effective barrier height is qΦEFF(T ) = qΦB + (kT )β 0d T

(1)

and the current for a thermionic emission (TE) mechanism, above a barrier ΦB with an ideality factor, n, is given by I(V ) = A * ST 2exp( − β 0d T)exp( − qΦB /kT )exp(qV /nkT )[1 − exp( −qV /kT )]

(2) −2

−2

where A* is the Richardson constant (A* = 112 A·cm ·K for n-type Si(111)), q the electron charge, k the Boltzmann constant, T the temperature, and S the Hg drop contact area (5 × 10−3 cm2); here, β0 = 2(2m*ΦT/ℏ2)(1)/(2) is the inverse attenuation length of the tunnel barrier at zero applied bias with tunnel barrier height ΦT, ℏ the reduced Planck’s constant, and m* the effective mass of electrical carriers in the OML. The ideality factor for the studied junctions is in the range n = 1.5 to 2.0. The characteristics of Si-Acid 5 and Si-Acid 100, with a rectification ratio R = 106 at ±1 V, are comparable to state-ofthe-art alkyl monolayer junctions on n-type Si and very similar to our reference C12 junction, reported and analyzed previously to obtain the temperature dependence of the effective barrier height ΦEFF (T).34,36 As expected for OML junctions with a thicker tunnel barrier (Si−C14-butylamide and Si−C16), the tunnel current density near +1 V is smaller; hence, a smaller rectification is observed, e.g., R (±1 V) = 104 in the Si−C14butylamide junction. As compared with the TE equation (eq 2) for a homogeneous tunnel junction, some excess current is observed at low temperature and low dc bias (Figure 4a); this behavior is typical of lateral inhomogeneities of the barrier height.36 3.5. ac Electrical Transport. The capacitance hysteresis in a C(V) plot is important to detect slow charge emission or trapping at some localized states.68 As illustrated previously (Figure 4a in ref 34) immediately after grafting, no capacitance hysteresis is observed near room temperature in a cyclic voltage scan, from 0 V to −2 V (depletion) to +0.8 V (accumulation near flat-band conditions) to 0 V, indicating the electronic quality of the device. The capacitance at reverse bias (V < +0.2 V) is low, CREV ≈ 10−20 pF (2−4 nF·cm−2), at all frequencies showing the formation of a depletion layer in the semiconductor. The highfrequency capacitance (1 MHz to 100 kHz) is useful for a Mott−Schottky analysis of the flat-band voltage in order to obtain the device’s barrier height; the flat-band voltage of the junction is obtained as the V-axis intercept in a C−2 vs V plot of the junction capacitance. It is basically identical for all junctions (0.70 ± 0.05 V); this result indicates that the different chemical substitutions used in this study do not have a strong effect on the band bending. The barrier height value, ΦCV EFF = (0.95 ± 0.05 V), derived from a Mott−Schottky analysis, is systematically larger than the value, ΦJV EFF, derived from the saturation current of J(V, T) in the forward bias regime.34 This difference has been attributed to a distribution of interface barrier heights.36 3.6. Dipolar Relaxation at the Molecular Scale. Valuable information on the OML structure can be derived from the dynamic properties of the Hg//OML−Si(111)

Figure 5. Imaginary electrical modulus M″(ω) for the Si-Acid 100// Hg junction: (a) comparison of electrical modulus M″(ω) and capacitance (−CP″(ω)/C0) representations showing a better definition of low frequency peaks in M″(ω) plots (the increase in M″(ω) above 1 MHz is due to the series resistance); (b) M″(ω) as a function of temperature (123 K−293 K) showing (i) a bias-independent peak A ( fA ≈ 2 kHz) with weak T dependence (mA ≈ 0.86; (1-nA) ≈ 0.61); (ii) a bias-dependent peak B with large intensity and strong T dependence (mB ≈ 0.62 ; (1-nB) ≈ 0.53).

Dipolar relaxation is investigated at low temperature and reverse bias to minimize the dc conductance response, which would otherwise mask the dipolar response; typically at 153 K, reverse biasing of the junction is necessary to observe a dipolar relaxation peak centered at 1 Hz, as illustrated in Figures 5−9. In this work, a specific reverse dc bias, VDC = −0.2 V, has been chosen for comparison of the substituted OML behavior. In the following, to rationalize the dynamic properties of Hg//OML− Si(111) junctions, we will show that two classes of relaxation mechanisms A and B, respectively, can be defined according to the criterion of weak ( fA) and strong ( f B) temperature dependence of relaxation frequencies. 3.6.1. Dipolar Relaxation Frequency. At low T, only peak A is observed (f ≈ 102−105 Hz) with very small activation energy (EA = 20−40 meV). With increasing T, peak B also appears with higher values of activation energy, EB = 0.25−0.40 eV. As a result, peak overlap occurs in the high temperature range, typically above 180 K (amide) or 270 K (acid). In the following, data analysis includes peak frequency, peak intensity, and (m, n) values derived from linear fits of log−log plots, as illustrated in Figure 6b. Each relaxation mechanism is 6779



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Figure 6. Imaginary electrical modulus of the Si-Acid 5//Hg junction: (a) bias dependence of M″(ω) at T = 263 K; and (b) temperature dependence (123−293 K) under small reverse bias (−0.2 V). The data show (i) a bias-independent relaxation peak A ( fA ≈ 2 kHz) with very weak T dependence (mA ≈ 0.86; (1-nA) ≈ 0.61 at 123 K); and (ii) two bias-dependent peaks (B1 and B2) at lower frequency with strong T dependence (mB ≈ 0.72; (1-nB) ≈ 0.48) at 153 K).

Figure 7. Imaginary electrical modulus of the Si−C14-butylamide// Hg junction showing at least two dipolar relaxation peaks: (a) weak bias dependence of M″(ω) at T = 153 K and (b) T dependence in the reverse bias regime (VDC = −0.2 V). A strong increase in the relaxation frequency of peak B is found with increasing temperature. In the forward bias regime or at high temperature, the weak dipolar response is masked by the semiconductor space-charge layer response (dashed lines).

thus defined by five characteristic parameters: activation energy, preexponential factor, amplitude, and two characteristic slope exponents for an asymmetric relaxation peak (Table 1). Because of this large number of unknown parameters, in this work we do not attempt to perform a decomposition of broadband admittance spectra, and the analysis is limited to a temperature range where the different peaks do not overlap. In this general picture, differences appear between the different OML assemblies. The main peak B intensity is much stronger in the Si-Acid 100 junction (Figure 5a and b) than in the Si-Acid 5 junction (Figure 6a and b). Evidence of two main dipolar relaxation mechanisms, A and B, is very clear in Hg// OML−Si(111) junctions incorporating inner amide (Si−C14butylamide, Figure 7a) or pure alkyl molecules (Si−C12 (aged) in Figure 8, and Si−C16 in Figure 9). The situation is more complex in junctions with carboxyl end group dipoles (in particular Si-Acid 5) where, in addition to peak A, two peaks B1 and B2 clearly appear; a larger intensity is found for the lower frequency peak B1 (Figure 6); note that in this junction, the main peak (B1) frequency has been considered in Figure 10. Peak A is observed at intermediate frequencies ( f ≈ 103−104 Hz) for Si-Acid 5, Si-Acid 100, and Si−C12 (aged) junctions, whereas a high frequency response ( f ≈ 105 Hz) is observed for the Si−C16 junction. In the Si−C14-butylamide junction, mechanism A is found at 60 Hz, although a weak shoulder appears near 2 kHz, possibly related to some residual unreacted acid moieties at the free surface (Figure 7a). The dc bias dependence also allows for discriminating both mechanisms. The characteristic frequency of peak A is clearly

Figure 8. Imaginary electrical modulus M″(ω) of the Si−C12//Hg junction after aging, as a function of temperature (133 K−293 K). Besides the main peak related to the semiconductor barrier at frequency f1 = G/2πC (visible above 233 K), two relaxation peaks are observed: (i) peak A ( fA ≈ 1 kHz) is bias-independent and has very weak T dependence (mA ≈ 0.77; (1-nA) ≈ 0.52); and (ii) peak B at lower frequency shows weak bias dependence [ref 36] and stronger T dependence (mB ≈ 0; (1-nB) ≈ 0.40).

bias-independent (Si-Acid 5, Figure 6a; Si−C14-butylamide, Figure 7a; and Si−C12 (aged)36). In contrast, peak B generally moves to lower frequencies as the junction is reversely biased, as illustrated for Si-Acid 5 (Figure 6a) and Si−C12 (aged)36) junctions; however, a weaker shift is observed for the Si−C14butylamide junction (Figure 7a). 6780



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measured by XPS show a reasonable agreement if some dipolar screening effect is taken into account. The temperature dependence of peak intensity, M″MAX, and shape parameters nDH and mDH is not easy to assess because they are strongly affected by any peak overlap. Surprisingly, in some junctions (Si-Acid 100 and Si−C16), the intensity of peak A is strongly depressed with increasing T (range 123-213 K). Hence, reliable results for peak A were obtained at low temperature (133 K) as reported in Table 1. In contrast, peak B intensity is very stable in this temperature range, as shown by Si-Acid 100, Si-Acid 5, and Si−C16 junctions; some of the weak increase (respectively decrease) detected in Si−C12 (aged) (respectively Si−C14-butylamide) might be due to some peak overlap. 3.6.4. Dipolar Relaxation Peak Shape. Characteristic exponents derived from linear fits of log(M″) vs log(ω) plots are reported in Table 1. In the studied assemblies, the shape of peak A depends weakly on the polar group substitution, with typical values of the low frequency slope (mDH ≈ 0.8 ± 0.07) and high frequency slope (1-nDH ≈ 0.6 ± 0.05) revealing a small asymmetry, (1-nDH)/mDH ≈ 0.8. A weak decrease of mDH(A) with increasing temperature is detected (see, e.g., Si Acid 100 in Figure 5b and Si Acid 5 in Figure 6b). In contrast, the shape of peak B is more sensitive to OML chemistry, with a broad range of high frequency slopes (1-nDH ≈ 0.4−0.7) and low frequency slopes (mDH ≈ 0−0.8). The main trends observed in this set of data are the decrease of parameter mDH(B) from 0.75 to 0.62 with increasing acid concentration and the value of mDH(B) ≈ 0 observed for the Si−C12 (aged) junction. As far as temperature effects are concerned, accurate results for mDH(B) are obtained for Si Acid 100, Si Acid 5, Si−C12 (aged), and Si−C16 junctions, showing no detectable T dependence; the best estimates of (1-nDH)(B) are obtained with Si−C16, Si−C12 (aged) and Si-Acid 100 junctions, again showing no detectable T dependence. Experimental prepeak and postpeak slopes, mDH and (1-nDH), will be discussed in section 4.3 according to the Dissado-Hill theory, where nDH and mDH are the intra- and intercluster correlation parameters, also related to short-time and long-time relaxation functions.52−56

Figure 9. Temperature dependence of M″(ω) for the Si−C16//Hg junction, showing two relaxation peaks: (i) peak A (fA ≈ 2 kHz) is bias-independent and shows some attenuation with increasing T (mA ≈ 0.87; (1-nA) ≈ 0.57 at 153 K); and (ii) the bias-dependent peak B at lower frequency shows strong T dependence (mB = 0.75; (1-nB) ≈ 0.70).

Figure 10. Summary of the temperature dependence of relaxation peak frequencies fA and f B in n-Si−OML//Hg junctions (−0.2 V reverse bias) including Si-Acid 100 (up triangle), Si-Acid 5 (square), Si−C14-butylamide (diamond), Si−C12 aged (circles), and Si−C16 (down triangles): (i) peak A (open symbols); and (ii) peak B (filled symbols). The lines are Arrhenius fits to the data (Table 1).

3.6.2. Activation Energy of Relaxation Frequency. The temperature dependence of the relaxation frequencies ( fA, f B) derived from Figures 5−9 is summarized in the Arrhenius plot shown in Figure 10. The activation energy and pre-exponential factor values are reported in Table 1. Peak A is characterized by very small values of activation energy (EA = 30 ± 10 meV) and pre-exponential factor ( f 0A ≈ 103−106 Hz). Peak B is characterized by higher values of activation energy (EB = 0.25−0.40 eV) and pre-exponential factor (f 0B ≈ 108−1010 Hz). The larger EB values correspond to the junctions with terminal acid moieties (Si-Acid 5 and SiAcid 100), whereas the larger relaxation frequency and smaller activation energy EB are found for the Si−C14-butylamide and Si−C12 (aged) junctions. 3.6.3. Dipolar Relaxation Strength. Peak intensities, M″MAX, reported in Table 1, were obtained without baseline subtraction nor peak decomposition because analysis is limited to a temperature range where the different peaks do not overlap. Peak A has a relatively small intensity, M″MAX, in the range 0.01−0.06, which is neither correlated with the carboxylic acid density nor with the total molecular coverage, measured by XPS. Peak B has a larger intensity, M″MAX, in the range 0.02− 0.23. In the acid dipole assemblies (Si-Acid 5 and Si-Acid 100), the peak intensity ratio (≈3) and the dipole density ratio (≈7)

4. DISCUSSION As discussed in polymer physics for many years, dipole reorientations need to overcome energy barriers, which are signatures of the motional constraints in the solid; they can be related to either local or highly collective chain reorientation movements. For the identification of relaxation mechanisms, additional information is derived from the relaxation peak shape which can be influenced by many-body interactions (see section 4.3). In the following, the main results are summarized and discussed in terms of the intrinsic relaxation of tethered alkyl chain assemblies (mechanism B) and the extrinsic relaxation of adventitious H2O molecules physisorbed on the OML surface (mechanism A). The behavior of relaxation frequency and intensity is addressed first; peak shape exponents will be further elucidated in the context of the Dissado-Hill theory. In summary, this set of broadband admittance data, as a whole, points toward a different origin for relaxation mechanisms A and B in Hg//OML−Si(111) junctions, revealing a different sensitivity to the OML chain dynamics (Table 2). In summary, besides their very different activation energies (EA ≪ EB) and preexponential factors (f 0A ≪ f 0B), 6781



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has been attributed to reversible water adsorption at the bottom of the OML, with formation of a water-induced energy level at the monolayer/Si interface, enabling an electrochemical charge transfer reaction between water-related species and electrons coming from the silicon electrode. In this work, the very small activation energy of peak A is consistent with such resonant electron transfer. However, the strong variations of peak A frequency over more than three decades (102−105 Hz) and its dependence on the OML chemistry (Figure 10) indicate a different origin, possibly related to some steric effects. Minimal constraints are expected when water molecules adsorb upon a hydrophobic surface, e.g., CH3 alkyl chain terminations of Si− C16, which may qualitatively explain the observation of peak A at higher frequencies for the latter molecular junctions. At SiAcid 5 and Si-Acid 100 surfaces, water−acid dipolar interactions produce a modified hydrogen-bonded network. This additional constraint may slow down the dipolar relaxation of water molecules (and potentially also that of the surface acid groups). Stronger steric constraints are expected when the dipolar moiety is embedded within the OML; if water molecules are able to penetrate inside the Si−C14-butylamide layer, water−amide dipolar interactions will be affected by the close chain packing promoted by van der Waals interactions between aliphatic chains. SE data show that the assemblies do not consist of molecular bilayers; however, some formation of acid−acid pairs is expected at the Si-Acid 100 surface; the acid−acid interaction energy is about 15 kcal/mol (0.65 eV) which is up to two H bonds.79 The conformation of the water layer may also be affected by the relative strength of water−water vs water−acid dipolar interactions; the latter probably dominates, as inferred from the coverage dependence of temperature desorption data.80 The fact that relaxation peak A is bias-independent is consistent with dipole−dipole interactions being stronger than bias-induced effects on the water network. The attenuation of peak A with increasing T, observed in the range 123−213 K, could result from some reorientation of water; this effect has been reported for water molecules adsorbed on graphite at cryogenic temperatures, as a result of van der Waals (dispersive) interactions with the hydrophobic graphite substrate.81 Interestingly, mechanism A is observed in all studied junctions, independently of the presence of silicon oxide at the OML−Si interface. Hence, this work elucidates some pending questions about dipolar relaxation features reported previously36 for a Hg//C12−n-Si junction after aging at ambient conditions; it shows that the peak located near 2 kHz, independent of dc bias, must be attributed to water (class A mechanism) rather than to dipoles in the alkyl chain induced by the strong permanent dipoles of interfacial silicon oxide. Since the frequency fA of the Si−C12 (aged) assembly coincides with that of the acid-modified surfaces (Figure 10), this could indicate some potential influence of silanol dipoles at the interfacial SiOx regions in the adsorbed water layer organization. 4.2. Intrinsic Relaxation. Larger relaxation frequency and smaller activation energy EB are found for the Si−C14butylamide and Si−C12 (aged) junctions, as compared with the Si-Acid 5 and Si-Acid 100 junctions with terminal acid moieties (Table 1). These activation energies (0.25−0.40 eV) are rather similar to previous results for molecular grafting on porous glass48 and consistent with the γ local relaxation in polyethylene.82 Assuming that the intrinsic peak B reflects the

Table 2. Characteristic Features of Class A and Class B Relaxation Mechanisms in Hg//OML−Si Junctions Observed in the Reverse Bias Regime by Broadband Admittance Spectroscopy relaxation mechanism

class A

class B

f MAX vs VDC EACT (meV) M″MAX vs T low-frequency slope m vs T high-frequency slope (1-n) vs T

weak 20−40 may decrease decrease constant

medium to strong 250−400 constant constant constant

the characteristic frequency of peak A is bias-independent, whereas peak B generally moves to lower frequencies as the device is reversely biased. Peak B has a rather large intensity (M″MAX = 0.02−0.23) showing some positive correlation with the dipole density derived from XPS, in contrast with peak A that has smaller intensity (0.01−0.06). In the studied junctions, the shape of peak A depends weakly on the polar group substitution, while the shape of peak B is more sensitive with a broad range of nDH and mDH parameters. Some decrease of mDH(A) with increasing temperature is observed, whereas mDH(B) and (1-nDH)(B) do not show any detectable T dependence. Finally, the intensity of peak A is strongly depressed with increasing T (range 123-213 K) in some junctions (Si-Acid 100 and Si−C16), while peak B intensity is very stable in this temperature range. 4.1. Extrinsic Relaxation. The fact that polar group substitution has little influence on peak A indicates an extrinsic origin for relaxation mechanism A. In the following, we argue that it results from adventitious water physisorbed on the free surface or trapped inside the monolayer; as a consequence, extrinsic peak A may reveal useful information on the OML junction exposed to ambient conditions, relevant to real device characteristics. Alternative hypotheses such as modulation of the charge distribution across the interface Si−C bond are very unlikely because peak A intensity decreases by about one decade from the Si−C12 aged junction (M″MAX = 0.063) to the butylamide junction (M″MAX = 0.007), while the coverage increases by about 40%. The bias-independent relaxation peak A has very low activation energy (EA = 30 ± 10 meV), which is typical of dipole−dipole interactions and indicates weak motional constraints. Water molecules have a large dipole moment which favors the association of several water molecules through strong directional hydrogen bonds. Some H-bond network can be readily formed at the OML free surface without strong steric constraints, and this H-bonded network is assumed to remain present when the OML−Si assembly is capped with the Hg electrode. Molecular dynamics simulations have shown that the adsorbed water layer conformation may be strongly influenced by the chemical nature of chain ends, varying from hydrophilic acid moieties (with the formation of hydrogen bonds between water and acid moieties) to hydrophobic CH3 alkyl chain terminations.24,74−77 In the presence of strongly hydrophilic functionalities, the adsorption of water may also disrupt the packing of the OML.24,62 In this context, recent investigations have revealed some influence of hydration on capacitance measurements and dc electrical transport (forward bias regime at room-temperature) through molecular junctions on silicon substrates, with a reversible effect upon annealing at 150 °C.78 This observation 6782



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overall chain dynamics, the measured activation energy, EB, can be compared with the magnitude of van der Waals interactions between n-alkyl chains, typically 6 kJ per mole of CH2 (0.06 eV per CH2 unit);24,80,83,84 In the context of an individual dipole interacting with the harmonic modes of the molecular layer, the observed values of EB = 0.25−0.40 eV would thus correspond to the simultaneous response of 4−7 CH2 units. For molecular grafting on porous glass substrates, as coverage decreases, the loss peak becomes less intense and, at fixed temperatures, moves to higher frequencies.48 To our best knowledge, a similar experiment has not been reported in a parallel-plate configuration. In this study, increasing concentration of acid dipole markers (while maintaining a nearly constant total chain density) shows a strong increase in peak B intensity (Figures 5b and 6b). In addition, in ref 48, the dipolar strength (peak amplitude) increases with temperature, in contrast with the constant amplitude of the main peak B1 of acid-substituted end groups (see, e.g., Figures 5b and 6b) performed in a parallel-plate configuration. The dependence of the characteristic frequency of mechanism B on applied reverse bias (particularly important in Si−C12 (aged), Si-Acid 5, and Si-Acid 100 junctions) indicates some influence of internal electric field on OML conformation, which increases motional constraints. Interestingly, the latter dynamic effect may be related to the analysis of dc transport according to the TE mechanism in the MIS tunnel junction (eq 2). As illustrated in Figure 4b for the Si−C14butylamide//Hg junction, a decreasing effective barrier height (ΦEFF = ΦB + kT βd) is observed with increasing reverse bias; in this junction, the fast decrease found in the low bias range (ΔΦEFF ≈14 meV at −0.2 V) is followed by a slower decrease at larger bias. This behavior was reported previously in Si−C12 junctions (Figure 6 in ref 34). This observation may be related to some electrostriction of the OML−Si assembly, i.e., electrostatic pressure could decrease the tunnel barrier thickness; the calculated pressure obtained at 1 V for a 1.5 nm-thick capacitor being rather low (upper limit of 5 MPa), the bending of the chain terminal groups is more likely than tilting the whole OML assembly.27,85 Assuming that ΦB and β are weakly affected by reverse biasing, the observed ΔΦEFF value gives a physically acceptable decrease in OML thickness of 0.08 nm, using β = 8 nm−1 for the attenuation factor in alkyl OML.86 4.3. Dissado-Hill Theory. The shape of dipolar relaxation peaks is discussed in the framework of Dissado-Hill and Jonscher theories for many-body interactions in complex relaxing systems, proposed to explain commonly observed deviations from the ideal Debye relaxation. In this model, the dipoles are interrelated through the structure, and the material is described by an ideal structure with perturbations occurring at different scales: (i) at the microscopic level, a random number of active dipoles, those that follow changes of the external field, is selected; their individual relaxation rates βiN are determined by the interactions of an active dipole with inactive neighbors forming around it a cluster of size Ni. (ii) At the mesoscopic level, correlated-cluster regions of sizes Mj depending on the strength of dipolar screening52−54 appear; the collective rate of relaxation of the active dipoles in such mesoscopic cooperative region becomes correlated with the number of the active dipoles in the region and with the distribution of their individual relaxation rates. (iii) At the macroscopic level, averaging over the number of effective contributions and their rates (of all cooperative mesoscopic regions) leads to the

universal relaxation for the entire system, given by the dielectric susceptibility:55,56,87 ε(ω) − ε∞(ω) = (εS(ω) − ε∞(ω))[1 − jω/ωDH]−1 + nDH × 2 F1[1 − nDH , 1 − mDH ; 2 − nDH ; (1 − jω/ωDH)−1] /2 F1[1 − nDH , 1 − mDH ; 2 − nDH ; 1]

(3)

where 2F1 (, , ; ;) is the Gauss hypergeometric function with 0 ≤ m ≤ 1 and 0 ≤ n ≤ 1, ωDH is the peak frequency, and ω is the frequency. Asymptotic limits, namely, ωm for ω ≪ ωDH and ωn‑1 for ω ≫ ωDH, are consistent with Jonscher universal behavior52−54 and Havriliak−Negami88,89 expressions. In the Dissado-Hill (DH) model,55,56 the high frequency slope (1-nDH) and the low-frequency slope (mDH) are, respectively, related to intra- and intercluster correlations. The degree of structural ordering of the average cluster is characterized by the nDH parameter. At nDH ≈ 0, there is no correlation between molecule reorientations (system where dipoles relax independently, which leads to the Debye classical model). At n ≈ 1, clusters have a crystalline structure, in which short-time molecular reorientations are fully correlated. According to the DH model, the collective behavior of active dipoles results in the appearance of mesoscopic cooperative regions forming intercluster structures; the degree of their ordering determines the mDH parameter. The m ≈ 0 and m ≈ 1 limiting values characterize the structures of an ideal crystal lattice without fluctuations (identical clusters) and a liquid with ideal hydrodynamic motion. Experimentally, both parameters fulfill the conditions 0 < nDH < 1 (high frequency) and 0 < mDH < 1 (low frequency). In the framework of DH theory, intra- and intercluster correlation parameters (nDH and mDH) provide structural information at different length scales. For intrinsic relaxation (peak B), nB values are found between 0.3 (Si−C16) and 0.6 (Si−C12 (aged)). For the dipole (acid, amide) substituted assemblies, experimental values are close to nB ≈ 0.5, indicating some degree of collective behavior, intermediate between isolated (Debye) relaxation (nB = 0) and highly collective relaxation (nB = 1). A wide variation is found for the disorder parameter at the cluster scale (mB). The very low value of mB ≈ 0 in the Si−C12 (aged) junction indicates a very strong similarity between clusters; this result is attributed to interface dipole ordering imposed by interface Si−C binding to crystalline Si(111). In contrast, a larger disorder is observed for Si-Acid 100 (mB = 0.62) and Si-Acid 5 (mB = 0.72) assemblies, where carboxyl dipoles are located at the chain ends, i.e., in a more disordered region with a high concentration of gauche defects.90 For the Si−C16 assembly, a value (mB = 0.75) similar to that of the acid response is observed, indicating that the relevant dipoles are likely located at the chain ends. In the Si−C14butylamide junction (Figure 7b), the small intensity of peak B and the low frequency of peak A lead to a larger error on mB; hence, more work is needed to understand the behavior of inner dipoles in a grafted monolayer. For extrinsic relaxation (peak A) attributed to physisorbed water, the value of nA ≈ 0.4 ± 0.05 is nearly constant for all junctions. The larger apparent value (nA = 0.65) found for the Si−C14-butylamide assembly (Figure 7a) may result from superposition of another peak at higher frequency (2 kHz). At inter-cluster scale, a disordered (liquid-like) behavior of relaxation peak A occurs for Si−C16, Si-Acid 100, and Si6783



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Acid 5 (mA = 0.86 ± 0.02); in contrast, slightly less disorder is found for the junctions with either inner dipoles (Si−C14butylamide, mA = 0.7 ± 0.1) or oxidized OML−Si interface (Si−C12 (aged), mA = 0.75). This small difference could arise from a variety of hydrogen-bonding configurations of physisorbed water. 4.4. Discrimination of Relaxation Mechanisms. Guidelines for the discrimination of relaxation mechanisms A and B in OML−Si assemblies are given in Table 2. The extrinsic peak A brings information on the hydrogen-bonding configurations of physisorbed water molecules. In the OML−Si assemblies, the disorder parameter at the cluster scale (mB) for intrinsic peak B can be used to discriminate near-substrate and molecule tail relaxation mechanisms, for example, in the case of pure alkyl chains, which have two small permanent dipoles (between 0.5 and 1 D)41 localized at both molecular ends (e.g., in the Si−C16 junction). Peak intensity analysis is useful for comparison between similar dipolar moieties, e.g., acid dipoles (1.74 D) in Si-Acid junctions with different dilutions. However, a precise knowledge of the respective CO and C−OH dipole orientations is still missing,83 and additional complexity in Si-Acid 100 could arise from acid−acid pairing through hydrogen-bond interactions.62,79,83 Note that, as a consequence of DH theory, several peaks with slightly different frequencies but similar shapes are expected for the CO and C−OH dipoles of the carboxylic acid, as observed in Figure 6. Since the peaks B1 and B2 are better resolved in the Si-Acid 5 junction, the role of methyl end groups and gauche defects, which bear a significant dipole, cannot be excluded. However, if a comparison is made between OML assemblies with either terminal carboxyl groups or inner amide moieties, it should be taken into account that different orientations of carbonyl groups with respect to the applied ac field direction may be present in the different monolayers. This orientation effect is a possible explanation for the weak dipolar peak B observed in the Si−C14-Butylamide case, along with the enhanced rigidity expected from stronger van der Waals interaction between longer (C20) alkyl chains. Changes in the dynamic properties (frequency and shape of peak B) as a function of applied dc bias are attributed to some influence of internal electric field on the OML conformation, consistent with some electrostriction effects observed in the dc current (section 4.2). Qualitatively similar effects were observed previously at room-temperature in Al−octadecyl−n Si junctions (with low mB ≈ 0.2−0.4 and high nB ≈ 0.7−0.8 values), where a decrease of slope parameter mB with increasing electric field was attributed to some enhanced OML rigidity.25,41 In this picture, the relaxation of a dipole (mechanism B) is influenced by the applied dc bias through a change in the OML conformation (determined by the mechanical reaction to electrostriction) and by the dipole interaction with the harmonic modes of the molecular layer.

criteria are established to experimentally discriminate both mechanisms (Table 2): (a) magnitude of the activation energy (EA = 20−40 meV and EB = 0.25−0.40 eV) and (b) stronger dc bias dependence of the relaxation frequency for mechanism B. The activation energy for peak A, typical of dipole−dipole interactions (to, e.g., polar groups within the monolayer or between adjacent water molecules) is weakly sensitive to molecular chemistry, bias, and temperature. It is observed at intermediate frequencies, i.e., similar to glassy water condensed on various surfaces; however, its broad frequency range (f ≈ 102−105 Hz) indicates variations in the H-bond environment of physisorbed water. Mechanism A is observed in all studied junctions, in particular with and without silicon oxide at the OML−Si interface. In the acid series, the relative intensity of peak B is consistent with the acid coverage given by XPS, in contrast with peak A. Some dependence of peak B activation frequency on OML chemistry and applied dc bias has been observed, showing increased motional constraints in the acid-substituted OML. The dynamic behavior qualitatively explains the dependence of effective barrier height on dc reverse bias derived from I−V characteristics. The shape of peak B relaxation is sensitive to the dipole location (molecular head vs tail region) because the near-substrate dipoles are in a well-ordered region influenced by covalent binding to a crystalline substrate, whereas the molecular tail region is more disordered (e.g., acid-functionalized OML). More generally, this study shows that broadband admittance spectroscopy is useful to investigate molecular electronics devices, such as metal−OML−semiconductor or metal− OML−metal in sandwich configuration, in ambient conditions (including water adsorption) relevant to real-world device operation. Substitution of polar groups in OML tunnel barriers is a powerful method to selectively enhance the local dielectric response. The sensitivity of the technique was shown using a low modulation bias (20 mV), small device area (5 × 10−3 cm2), and diluted acid end-groups corresponding to 2 × 1011 carboxyl dipoles (0.3 pmol). It is important to maintain a low junction conductance to observe low-frequency dipolar relaxation and decrease the series resistance to improve the high-frequency limitation of the technique. Finally, this study shows that a clear identification of the processes behind the dipolar response requires investigations as a function of both dc bias and temperature, in particular when multiple dipolar signatures are likely to be observed or when a hydrogen-bond network of physisorbed water molecules is expected. In this article, we have considered activation energies at small reverse dc bias value in order to minimize the disturbance of the OML organization by dc bias. Future work will address in more detail the changes in the OML properties induced by stronger dc bias.

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5. CONCLUSIONS Broadband admittance spectroscopy has been used over a broad temperature range to provide dynamic information on organic monolayers tethered to Si substrates, using a series of Hg//OML−n Si junctions with a variety of dipoles embedded at different locations of the molecule backbone. The response to changes in the electric field reflects motional constraints with different activation barriers in the OML. Two classes of dipolar relaxation mechanisms (extrinsic A/intrinsic B) are found, and

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*EPSI−IPR (Bât. 11E - Beaulieu), Université Rennes 1, 35042 Rennes, France. Tel: +33 2 23 23 57 06. Fax: +33 2 23 23 61 98. E-mail: [email protected]. Notes

The authors declare no competing financial interest. 6784



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ACKNOWLEDGMENTS We wish to thank Roopa Hiremath for some admittance measurements, Cristelle Mériadec for some XPS characterizations, and Arnaud Le Pottier for his technical support. This work was partly funded by Région Bretagne, Rennes Métropole, Agence Nationale de la Recherche (ANR-07-BLAN-017002), and FEDER (Nanosoft project). Région Bretagne is acknowledged for a Ph.D. grant (to A.-B.F.-D.).

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Dynamics of Substituted Alkyl Monolayers Covalently Bonded to

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