Molecular Length, Monolayer Density, and Charge Transport: Lessons

ARTICLE pubs.acs.org/Langmuir

Molecular Length, Monolayer Density, and Charge Transport: Lessons from Al
AlOx/Alkyl
Phosphonate/Hg Junctions Igal Levine, Stephanie M. Weber, Yishay Feldman, Tatyana Bendikov, Hagai Cohen, David Cahen,* and Ayelet Vilan* Weizmann Institute of Science, Rehovot 76100, Israel

bS Supporting Information ABSTRACT: A combined electronic transport
structure characterization of selfassembled monolayers (MLs) of alkyl
phosphonate (AP) chains on Al
AlOx substrates indicates a strong molecular structural effect on charge transport. On the basis of X-ray reflectivity, XPS, and FTIR data, we conclude that “long” APs (C14 and C16) form much denser MLs than do “short” APs (C8, C10, C12). While current through all junctions showed a tunneling-like exponential length-attenuation, junctions with sparsely packed “short” AP MLs attenuate the current relatively more efficiently than those with densely packed, “long” ones. Furthermore, “long” AP ML junctions showed strong bias variation of the length decay coefficient, β, while for “short” AP ML junctions β is nearly independent of bias. Therefore, even for these simple molecular systems made up of what are considered to be inert molecules, the tunneling distance cannot be varied independently of other electrical properties, as is commonly assumed.

’ INTRODUCTION Alkyl chains are a type of “fruit-fly” in molecular electronics, mainly because of their low chemical reactivity (see, though, ref 1 for radiation damage). Mostly, they can indeed serve as “passive elements” in molecular electronics, with their large energy gap and poor electronic conductivity. It is commonly assumed that their energy levels and the resulting transport properties are rather insensitive to surface binding, to packing, or to alkyl-chain length,2 as shown, for example, by the similarity between the UPS spectrum of an 8 carbon-long alkyl ML and that of polyethylene.3 Therefore, the alkyl chain length is generally considered to be a parameter that influences only the transport distance, while other contributions to transport probability are kept identical.2 Here we show, based on extensive surface characterization (by FT-IR, XPS, ellipsometry, and X-ray reflectivity), that the structure of alkyl phosphonate (AP) MLs on AlOx changes markedly with alkyl chain-length and that this difference is directly expressed in the transport characteristics across the ML. Structural effects on molecular transport are not commonly considered, although evidence is accumulating for strong structural effects on transport. Slowinski et al. extracted different length-decay coefficients for “interchain” and “intrachain” transport across an alkyl-thiol ML.4 They later compared the lengthinduced variation in capacitance and transmission and concluded that creation of gauche defects in alkyl
thiol bilayers on liquid Hg, upon aging, increases the tunneling length-decay coefficient as compared to an all-trans alkyls.5 Selzer et al. observed a clear change in transport mechanism between a single molecule and that of a small ensemble of molecules.6 We reported previously on remarkable temperature-attenuated transport, which we r 2011 American Chemical Society

attributed to structural changes in alkyl MLs on oxide-free Si.7 For alkyl thiols on Au, it was concluded that as the chains tilt more (away from the surface normal), “interchain” tunneling becomes more efficient than “intrachain” tunneling.8
10 Here, we report on a structural effect on the transport across alkyl phosphonate (AP) ML. We trace the effect to a change in density of the molecular ML. While it is known that up to some 18 carbons, longer chain alkyls yield higher quality MLs than do shorter ones,11
13 charge transport is commonly measured across fairly short alkyl chains, mostly alkyl thiols. Using short chains is necessary if only one or a few molecules are measured, because of practical limits on the lowest measurable current.2,14 Conformation and orientation of alkyl chains in selfassembled MLs are expected to depend strongly on the length of the chain, as indicated theoretically15 and observed experimentally, for example, for thiols on gold16 and GaAs,13 for silanes on SiOx,17 for APs on TiO2,18 GaAs,19 and HfO2,20 and for carboxylic acids on Al surfaces.12 Here, we identify for junctions with AP MLs a clear change in the length-decay coefficient (β) around the C12 AP and find that, although the short AP MLs on Al
AlOx (e12 carbons) are considerably less dense, they still appear to be pinhole-free. The ability of AP to form loosely packed, yet defect-less MLs, as reported here, is rather unique and might help in understanding their recent appearance in various applications, such as in friction reduction,21 implant corrosion protection,22 and Received: September 12, 2011 Revised: November 4, 2011 Published: November 15, 2011 404

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adhesion to biomaterials.23 P
O links are widely used for binding molecules to metallic surfaces.24 P
O
P bond formation is very unlikely, and, in contrast to molecular binding via silanol groups, APs do not form multilayers or laterally linked polymeric films.25 The strong P
O
metal bond resists hydrolysis, so that PO-based MLs are much more stable than similar thiol-based ones.22,24 For electrical applications, a variety of phosphonic acids were adsorbed onto ITO to modify its workfunction26 toward improving the injection efficiency into organic PL diodes.27 The same effect was also demonstrated by AP-MLs on Al,28 which also serve as excellent electronic insulators for microcapacitors,29 organic FETs,30 and solar cells.31 The electrical insulation capability of ultrathin AP films, a capability that we trace to their ability to form defect-free nanometer-thick films, is much superior to that of inorganic oxides of comparable thickness and also better than that of alkyl
silane SAMs.30 Previous studies on electronic conduction across AP MLs on Al
AlOx included the effect of phosphonic acid chain length and morphology on OFET performance.32
34 Such deviceoriented studies29,30,32
34 reflect the increasing interest in this system. Our goal here is to use the AP-ML/AlOx junction as a model system for molecular electronics, in addition to the wellstudied alkyl thiols on Au2,8
10,35
37 and on other metallic substrates.4,14,38
40 To ensure minimal electrical artifacts, we used the reliable, nondestructive, liquid Hg top contact and made the omnipresent Al oxide that forms on the Al, and to which the APs bind, as electrically transparent as possible. To achieve high reproducibility, we used ultrasmooth Al films41 and characterized the AP MLs by static contact angle, ellipsometry, IR spectroscopy, X-ray reflectivity, and XPS. After applying the top contact, we analyzed the charge transport as a function of both alkyl chainlength (five different lengths) and applied bias (
1 to +1 V). We then analyzed the large data set in terms of exponential current attenuation and its variation with applied bias. A companion paper will present a detailed analysis of the variation of current with applied bias.42 A serious disagreement appears in the literature on whether β is bias-dependent35,36,39,43 or not.14,37 We observe both bias dependence and independence for long and short APs, respectively, thus suggesting that two leading transport mechanisms compete and that one can follow the transition between them. The β2
V relations were generally nonlinear, in contrast to the WKB prediction for tunneling, a result that can be explained by a rather narrow forbidden HOMO
LUMO energy gap, with both HOMO- and LUMO-assisted tunneling.44,45

Freshly evaporated Al films were washed with DI water and etched using 99% sulphuric acid, diluted to 10% v/v in DI water for 2 min at 0 C. Etching increases the density of hydroxyl groups (binding sites) on the surface of the alumina (as evident from XPS, see Figure S3 of the Supporting Information), with no effect on surface roughness (as verified by AFM). After etching, samples were thoroughly washed with DI water and then with copious amounts of isopropanol, to remove any residuals. Samples were then introduced immediately into the adsorption solution (5 mM of each of the phosphonic acids in isopropanol). Replacing isopropanol by DMF or THF yielded lower quality MLs. The solution was heated for 5 min to 50 C and placed under N2 flow for 3 min before closing the vial. The vials were kept in a desiccator for an over night immersion time of about 20 h. After the samples were taken out of the adsorption solution, they were dried with N2 and heated to 70 C under a flow of N2 for 10 min, to remove residual water molecules and complete the condensation reaction.46,47 Finally, the samples were cleaned in an ultrasonic bath of isopropanol for 3 min, to remove physisorbed phosphonic acids, and blown dry with a stream of N2. Electrical characterization under inert atmosphere followed immediately. Ellipsometry. This was measured using a multiple wavelength ellipsometer (M 2000 V from J. A. Woollam Co. Inc.) at a constant incidence angle of 70 under ambient conditions and was analyzed using commercial software (WVASE32). The dielectric properties of AlOx and alkyl ML are too similar for ellipsometry to separate the net dielectric thickness into ML and AlOx contributions. Therefore, for each sample, the thickness of the AlOx after etching (prior to immersion in the ML solution) was measured using a one-layer model for the AlOx. This thickness value was used as a fixed input parameter for a two-layer model, fixed AlOx and a layer, described by the Cauchy relation (that relates refractive index to wavelength),48 yielding the thickness of the organic layer, assuming that during adsorption the AlOx thickness remained constant. Infrared (IR). Infrared spectroscopy was done in the PM-IRRAS mode, using a Nicolet 6700 FTIR, at an 80 incidence angle, equipped with PEM-90 photoelastic modulator (Hinds Instruments, Hillsboro, OR) with modulation wavelengths of 2900 and 1600 cm
1 for the C
H stretching region and C
C
phosphonates region, respectively. Raw spectra were smoothed and baseline-corrected by a spline algorithm. Static Contact Angle (CA). These measurements were performed with an automated goniometer (Ram
e-Hart, model-100) and microsyringe droplets (advancing drop method) of approximately 4 μL of deionized water (Millipore Inc.). CA data were recorded immediately after ML deposition. X-ray Reflectivity (XRR). Measurements were performed at ambient on a TTRAX III (Rigaku, Japan) θ
θ diffractometer equipped with a scintillation detector, and a rotating Cu anode, operating at 50 kV and 200 mA. Using a nearly parallel X-ray beam (divergence angle 0.05) formed by a multilayered mirror (CBO, Rigaku), 2θ/θ specular scans were carried out in fixed-count mode (minimum 500 counts) with a step size of 0.01. Studied samples were at least 3 cm long to make them larger than the X-ray footprint at angles below the critical X-ray angle (2θ ≈ 0.5). Using GlobalFit Reflectivity analysis software (Rigaku), the density, thickness, and roughness of the surface layers were evaluated by the Parratt algorithm with R-factor better than 1.5%. X-ray Photoelectron Spectroscopy (XPS). This was performed in a Kratos AXIS ULTRA spectrometer, using a monochromatic Al Kα X-ray source (hν = 1486.6 eV) at 75 W at a normal takeoff angle signal collection and detection pass energies ranging between 10 and 80 eV. Composition and thickness analysis were deduced from the Al2p, O1s, C1s, and P2s lines. Curve fitting with Gaussian
Lorentzian line shapes was applied. Concentration ratios were translated to layer thickness,

’ EXPERIMENTAL SECTION Al Films. Thirty nanometer thick Al films, used as the substrates, were thermally evaporated from Al pellets, 99.999% pure (Kurt J. Lesker), onto single side polished n-Si with Æ111æ orientation and a nominal resistivity of 0.5
0.8 Ω cm (ITME). The resulting Al films are ultrasmooth with rms roughness of 0.6 nm. Exact film preparation and characterization is detailed elsewhere.41 Monolayer Preparation. All monomers, n-octylphosphonic acid (>98%), n-decylphosphonic acid (>98%), n-dodecylphosphonic acid (>99%), n-tetradecylphosphonic acid (>97%), n-hexadecylphosphonic acid (>98%), and n-octadecylphosphonic acid (>97%), were purchased from Poly Carbon Industries Inc. and used without further purification; all other chemicals and solvents were analytical grade and purchased from Sigma-Aldrich. Deionized water (DI) was passed through a Milli-Q ultra pure water filtration system to yield 1018 MΩ cm resistivity. 405

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Figure 1. Experimental setup used for measuring J
V characteristics. The contact line between the drop and the substrate is ∼0.5 mm. The second contact to the sample is on the left. assuming a uniform organic layer on top of an oxide layer, and exponential attenuation of photoelectrons, as previously described.19,49 Current
Voltage (I
V). These measurements were performed in a N2 environment (10% relative humidity), using 99.9999% pure Hg droplets (controlled growth Hg electrode, Polish Academy of Sciences), as shown in Figure 1. To be able to extract the current density (J, in A/cm2), the Hg contact area was determined from the optically measured diameter (∼0.5 ( 0.05 mm). For each molecular length, 12
18 J
V curves were recorded, each on a fresh spot, on 3
4 different samples. To contact the Al, part of the sample was scratched to expose the Al, and InGa eutectic was rubbed into it, thus sacrificing a part of the sample. The J
V measurements were collected in a voltage scan mode, using a Keithley 6430 sub-fA remote source meter. After bringing the Hg drop into contact with the sample, we allowed the system to relax for 30
60 s at 0 V, before starting the bias scan. The applied voltage was scanned between
1 and +1 V, back and forth (0 V f
1 V f 0 V f 1 V f 0 V), in steps of 0.01 V. For the longer chains, C14, C16, the raw data were very noisy, especially near 0 V, and included a finite charging current at 0 V.

Figure 2. Ellipsometry results for different PA MLs on Al/AlOx, showing the effective thickness (d) (symbols) and the nominal thickness (L0) (dashed line). Nominal thickness values were computed by Chemdraw for “all-trans”, vertically aligned alkyl chains. Effective ML thickness was deduced from ellipsometry data. Error bars are the standard deviations between ellipsometry-derived layer widths for 3
4 repeated preparations.

known as the Lotus-leaf effect.54 Here, we used only Al films of rms surface roughness of 0.6 nm or less, as measured individually by AFM (on a micrometer scale) and confirmed independently by XRR (on a millimeter scale).41 Film Thickness by Ellipsometry. We used ellipsometry to get a measure of the amount of adsorbed material and an idea of whether a multilayer or a partial ML was formed. Figure 2 plots the ellipsometry-deduced effective thickness (d) of the organic layer versus the number of carbons in the alkyl chain. The measured effective values are smaller than the nominal lengths of the stretched molecules (L0, shown as a blue dashed line), where, notably, the effective thickness (d) is barely one-half the nominal molecular length (L0) for short AP MLs, but it approaches the theoretical value for a C16 ML, yielding d ≈ 0.9 L0. The ellipsometry-deduced thickness is an effective value, because for such ultrathin films we cannot reliably extract the thickness and refractive index simultaneously. Therefore, the effective thickness values of Figure 2 were extracted, assuming that the refractive index of the AP ML is similar to that of SiOx (the standard parameters for the Cauchy equation). Furthermore, ellipsometry cannot distinguish between an organic ML and the underlying oxide, because their refractive indices are too close. Therefore, the ML thickness was deduced by assuming that the thickness of the underlying oxide did not change during the 20 h of AP adsorption (see Experimental Section). As a partial check of this assumption, we left a bare Al sample in ambient for 5 h and found that its ellipsometry-derived thickness had not changed within experimental error. Such aging of films, though, makes them much less conductive, probably due to improved oxide stoichiometry and filling of vacancy defects (see Figure S1 of the Supporting Information). The observation of less than nominal ML thickness can be explained, in principle, by a strong tilting of the molecular long axis from the surface normal. However, the combined characterization tools that we use here suggest that the shorter are the APs, the less compact is the arrangement in which they organize. This can be understood in terms of wider spaced binding of the molecules to the substrate, rather than a stronger tilt. The effective thickness that we deduce and show in Figure 2 reflects this reduction in material density and should not be misinterpreted as

’ RESULTS The results section is divided into (1) structural characterization of the AP ML by surface analyses and (2) electrical transport measurements and extraction of length-decay coefficients (β). 1. Structural Characterization of Alkyl-Phosphonates ML. Using oxidized Al substrates goes back to the pioneering work on charge transport across alkyl MLs by Polymeropoulos and Sagiv50 and by Mann and Kuhn.51 Use of such substrates was discontinued for almost two decades, because of doubts about the possible contribution of pinholes to the net transport. Thus, to carry out reliable molecular transport measurements on such substrates, we need to show that such pinholes are absent and that the molecular ML does not introduce defects that dominate the measured transport characteristics. Our characterization shows AP MLs on Al
AlOx to be uniform and continuous for all tested alkyl chain lengths (C8
C16). The ML morphology, though, varies considerably with alkyl chain length, as described next. ML Uniformity. A simple but sensitive probe for uniformity of hydrophobic surfaces, such as the methyl-terminated alkyl chains considered here, is the static water contact angle (CA). For alkyl MLs on Si, we found that a minute density of defects in the ML reduces the CA below 107, a decrease that correlates well with the transport characteristics of these MLs.52 In the present study, all samples had a CA between 108 and 110 for all chain lengths (C8
C16), without a clear trend between the chain length and the CA. While the CA for alkyl
PO3 was reported21,53 to be as high as 130, we assume that such very high CA values are actually due to high surface roughness of the underlying Al, 406

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Figure 4. PM-IRRAS spectra of the low frequency region of the adsorbed MLs after baseline correction. Inset: Methylene wagging modes in the range 1170
1300 cm
1. To be able to accurately assign the different modes, the C8 and C16 spectra in the inset were divided by the reference bare Al spectrum, and the intensity ratio is shown.

A similar shift in IR absorption peaks of up to 6 cm
1 with increased alkyl chain length (C6 to C18) was reported for alkyl phosphonic acids, adsorbed onto other substrates such as TiO2,18 GaAs,19 and HfO2,20 indicating that AP MLs undergo morphological changes with increasing chain length not only on Al/AlOx surfaces. Yet, there are cases were dense AP MLs are formed already at C2.21 The mean tilt from the surface normal can be estimated using the ratio between peak intensities of the methylene and methyl asymmetric stretches, according to:56 IðCH2 Þ 2 cos2 ð90
αÞ ¼ m 2 IðCH3 Þ 3 cos ð35
αÞ Figure 3. PM-IRRAS characterization of C
H stretching region, showing (a) spectra of shortest (C8, black) and longest (C16, green) AP MLs after baseline correction (for clarity, curves are vertically shifted). Dotted gray lines mark the stretching frequency for the C16 curve. (b) Position of the C
H antisymmetric stretch for varying molecular lengths. (c) Tilt angle with respect to the surface normal for varying molecular lengths, calculated with eq 1.

ð1Þ

where I is the peak area, m is the number of methylene units in the chain, and α is the tilt angle in degrees. The angles of 90 and 35 are derived from the direction of the relevant stretches with respect to the long molecular axis.56 The resulting tilt angles decrease from 33 for C8 to 24
25 for C14 and C16 (Figure 3c). The magnitude of FTIR-derived tilt angles is much smaller than the large differences between ellipsometry-derived effective thickness and nominal molecular length of Figure 2 (symbols and dashed line, respectively). If the effective thickness of Figure 2 was the actual ML thickness, it would imply a variation in tilt angle of 70 to 30, for C8 to C16, respectively. Further information on the binding of the phosphonate group to the Al/AlOx surface and the conformational order of the chains can be gained from the low frequency IR region (800
1400 cm
1), as shown in Figure 4. This region includes several phosphonate stretching modes, including the P
O symmetric and antisymmetric ones, which appear at 1080 and 1130 cm
1, respectively,57 and the PdO stretching mode, expected in the 1140
1320 cm
1 region.58 Figure 4 shows no trace of this PdO stretching for both the C8 and the C16 spectra, which was very clear in the PM-IRRAS of a thin, casted film of AP on Au (i.e., in the absence of Al
O features, see Figure S2 of the Supporting Information). We conclude that our adsorption procedure yields tridentate binding of APs (i.e., all 3 O atoms bind to Al).25,59,60 We do not see, though, clear IR evidence for either P
O
Al stretching (∼1127 cm
1) or P
OH stretching (946 cm
1), because these features overlap with the Al
OH stretching (1125 cm
1) and Al
O
Al vibrations,58 respectively (expected in the 800
1000 cm
1 region), as observed for a

the actual z-projection of the molecules (for the actual ML thickness, see also the XPS and XRR sections). Alkyl-Chain Packing and Conformation from FTIR. IR absorption provides direct evidence for the presence of the expected chemical groups on the surface. Figure 3 plots the PM-IRRAS spectra for the two extreme lengths of alkylphosphonate MLs used here (C8 and C16), showing the C
H stretching in CH2 (∼2920, 2850 cm
1) and in CH3 (∼2960, 2880 cm
1) groups. These peaks are visible for all molecular lengths, further confirming (in addition to the CA data) the presence of alkyls on the surface. The position of the asymmetric methylene (CH2) stretch can be used as a general indicator of the intermolecular environment of the alkyl chains in the ML.55 Figure 3 shows a clear red shift of the C16 methylene symmetric (2855 f 2850 cm
1) and antisymmetric (2926 f2920 cm
1) stretching modes, with respect to that of the C8 film. This trend, which was consistently observed for all molecular lengths, as shown in Figure 3b, indicates a gradual transition from a disordered, liquid-like assembly of the short chains (νas(CH2) > 2920 cm
1) toward a denser, closer-packed structure of the longer chains (νas(CH2) e 2920 cm
1). 407

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to the density of adsorbed molecules. We therefore obtain (with IMFP = 33 Å19,49) an effective thickness that reflects the amount of material on the surface, not necessarily its thickness. Thus, the variation in XPS-extracted relative organic layer thickness in Table 1 indicates, similar to the ellipsometry results, marked differences in layer density (i.e., in IMFP) for the three molecular lengths. We also examined the measured [C]/[P] ratio, because it is much less sensitive to variations in IMFP (see Table S1 of the Supporting Information). The XPS-derived [C]/[P] was 80
100% of the theoretically predicted [C]/[P] for all three AP MLs (see Table S1 of the Supporting Information). This finding suggests that the relatively small thickness, deduced for the short molecules by both XPS and ellipsometry, actually reflects an increased spacing between neighboring molecules, while the molecular orientation is similar, as indicated by the small span of IR-deduced tilt angles (Figure 3c). Strong variations in ML density are also directly evident from XRR study (see next section). The measured [O]/[Alox] ratios are larger than the value for stoichiometric Al2O3 (1.5), indicating that the oxide contains suboxides and hydroxides such as AlOOH and Al(OH)3, with nominal [O]/[Alox] ratios of 2 and 3, respectively.69 Suboxides and hydroxides are important, because they are expected to increase the conductivity of the AlOx and, thus, reduce its relative effect on the net transport characteristics. In addition, because phosphonic acids bind via surface hydroxyls, increased presence of hydroxyls at the surface will facilitate AP binding. We found that treating an as-evaporated Al film with H2SO4 increases the concentration of hydroxyls and, accordingly, the OH component in the O1s peak (see Figure S3 of the Supporting Information),69 which explains why we added this treatment before AP ML adsorption. In addition, an aqueous solution of APs at 100 C was reported to considerably dissolve an Al2O3 powder.25 The oxide thickness as determined by XPS (and XRR below) suggests that such dissolution is negligible here, probably due to the different adsorption conditions. The binding energies of P 2p and P 2s were at 134.8 and 192.8 eV, respectively, while the C1s peak was at 286.4 eV. No clear trend in these values as a function of alkyl chain length was observed. The P 2p and P 2s binding energies (which are best referenced to the corresponding C1s peaks, due to possible charging of the underlying oxide) are very similar to those reported for phenyl phosphonic acid, vacuum deposited on Al,70 or to C18 AP deposited by TBAG on oxidized Si(100).71 In both of these cases, the results were interpreted as evidence for tridentate binding, relying on deconvolution of the O1s peak. Such deconvolution is not possible here, because of the large amount of oxidized Al. No direct conclusions can be drawn from the position of the P binding energies, which are fairly insensitive to the chemical environment.72 Surface Profile from X-ray Reflectivity (XRR). Measured and fitted X-ray reflectivity spectra are shown in Figure 5, and Table 2 summarizes the obtained fitting parameters. Because of the large number of variables required to calculate the reflectivity curves, the uniqueness of the calculated reflectivity was checked by varying the initial parameter values several times. We find the curves to converge to the same values within the error, shown in brackets in Table 2. As shown in Table 2, the thickness deduced for the C8 ML (1 nm) is higher than the values derived from ellipsometry and XPS data (∼0.5
0.6 nm). However, the C16 ML thickness agrees with the ellipsometric value (∼2 nm), within experimental error. Table 2 suggests a notable decrease

Table 1. Summary of XPS Results for the C8, C12, and C16 AP-MLs ML thicknessa

ratio to nominal

O/Alox

AlOx thickness

sample

[nm] ( 15%

length

ratio ( 20%

[nm] ( 10%

C8 C12

0.66 (1.2) 1.30 (1.7)

0.55 0.76

1.8 2.0

3.6 3.2

C16

1.80 (2.1)

0.86

1.7

3.0

b

Values in brackets are the nominal lengths of “all-trans” calculated using Chemdraw. These values refer to the nominal thickness of the AP ML for vertically aligned alkyl chains. b This is the ratio between the effective and nominal thickness values of the first column. a

reference sample with freshly etched hydroxylated Al surface (see Figure S2 of the Supporting Information). Figure 4 also includes contributions from the alkyl chains. The methylene wagging and twisting coupled modes, often referred to as progression bands,16,61 are clearly seen in the 1170
1300 cm
1 range (see also inset to Figure 4). Such methylene progression bands serve as an indication for all-trans zigzag conformation of the entire alkyl chain.16,61 They were observed on several types of alkyl MLs, such as phosphonic acids on Al,59 carboxylic acids on Al,12 silanes on Si,62 and thiols on Ag63,64 and Cu.65 The two peaks above 1300 cm
1 are outside the wagging pattern; the peak at 1381 cm
1 is assigned to the methyl symmetric bending, or umbrella mode,66,67 while the peak at ∼1330 cm
1 is unidentified and may result from absorption of CO2(g) on the surface.65 The spacing and position of the progression bands depend on the number of coupled trans CH2 units, m, according to:68 Δυ = ((326)/(m + 1)). For the C16 ML, m = 15, and, therefore, a spacing of 20.4 cm
1 is expected, which is in excellent agreement with the observed spacing of 20.3 ( 1.6 cm
1 (see inset of Figure 4), suggesting that in this ML all methylene units in the C16 chain are in the trans conformation with no gauche defects. The absence of gauche defects and kinks is also supported by the symmetric shape of the 1381 cm
1 peak, without the bumpy shoulder toward 1341 cm
1, which is typical for gauche conformers.66,67 In principle, the number of observable equi-spaced wagging modes for an even numbered alkyl chain should roughly be (m + 1)/2.61 Thus, eight peaks are expected for the C16 ML, while Figure 4 detects only six peaks for the C16. Still, most papers report on only six peaks that are practically observed (seven were observed using surface-plasmon-enhanced IR65). As shown in the inset to Figure 4, for the C8 ML, two peaks are observed at 1205 and 1244 cm
1. These may also originate from wagging of the methylene units, because the expected Δν for a C8 ML is 40.7 cm
1, close to the observed value of 39 cm
1. However, the intensities of these modes are lower (S/N ratio ∼4) than those observed for the C16 ML (S/N ratio ∼6), and only half (2) of the expected65 number of peaks (4) is seen. On the basis of this result, we suggest that for the C8 ML, only part of the alkyl chains are in the “all-trans” conformation, in contrast to what we deduced above to be the case for the C16 ML. Chemical Composition by XPS. Table 1 shows the XPSderived thickness values and atomic ratios of the AP-ML and oxidized Al for C8, C12, and C16 samples. The XPS-extracted values are in good agreement with the ellipsometry-deduced effective thickness values. Similar to ellipsometry, we use here a parameter (the inelastic mean free path, IMFP) that is sensitive 408

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Table 2. Summary of XRR Fitting Resultsa for AP-MLs on Al
AlOx thickness [nm] ( 10%

roughness [nm] ( 10%

layer

C8

C16

C8

C16

C8

C16

ML

1.0

2.15

0.55

0.85

0.75

0.4

AlOx

2.3

1.85

3.8

3.4

1.0

0.65

2.7

2.7

0.45

0.4

Al a

density [g/cm3] ( 10%

29.5

32.6

All numbers were rounded to reflect the uncertainty in the fitting procedure.

is not accompanied by a decrease in uniformity. FTIR even suggests that the widely spaced C8 ML contains a large fraction of all-trans chains. In the next section, we show that, despite their loose packing (or, more likely, because of that), AP-MLs are excellent insulators. 2. Transport Measurements. Electrical transport across alkyl-phosphonates/AlOx MLs was measured using a Hg top contact (see Figure 1). To gain insight into the charge transport process, we measured the current through the MLs as a function of molecular length (five different lengths) and of applied bias. We did not extend the molecular length beyond C16, because results from attempts to measure current across C18 AP MLs were too noisy to provide reliable characterization at the relatively low bias voltages we were interested in. The reason to focus on low biases is that large fields are expected to drive the transport away from simple (near) equilibrium models. Hg is known as a nondestructive, nonpenetrating contact, compatible with the AP layer. A major disadvantage of Hg is that it reacts strongly with many metals including Au, Ag, and Al (amalgamation). The interaction is so favorable that Hg vapor diffusing through a near-perfect ML suffices to induce it. This is the reason why a Hg contact is often covered by a second ML,4,38,39 or used over substrates that do not interact with Hg, such as Si43,49 or, to a lesser extent, GaAs.19 In our case, the thin AlOx slows much of the amalgamation, but amalgamation does occur eventually. We found that with longer AP chains, amalgamation upon contact with Hg was more likely than with short APs, and, therefore, we had to measure more samples with C14 and C16 APs than with the shorter ones. Transport characteristics of Hg/alkyl
PO3
AlOx
Al junctions are plotted in Figure 6. Varying the length of the alkyl chain from 8 to 16 carbons yields the expected exponential current decay. Data reproducibility was excellent; the log-normal standard deviation of at least 12 different junctions, prepared from 3
4 different samples for each molecule, was 25
75%, indicating the high quality and uniformity of the molecular junctions.39 The junctions showed negligible hysteresis (Figure 6), although we did not attempt to test their stability over prolonged cycling (hours to days). In contrast to previous work, which focused on improving the insulating properties of the ML + oxide,30,33,34 our goal here is to investigate the molecular transport characteristics. Therefore, we tried to minimize as much as possible blocking of current by the oxide. We did so by the earlier mentioned sulfuric pre-etching of the native Al oxide. While XPS, XRR, and ellipsometry indicate a rather thick (2
3 nm) remaining AlOx film, the J
V measurements show this film to be fairly conductive, probably related to the nonstoichiometric ratio between Al and O in the oxide. For example, even the thinnest ML (C8, ∼1 nm thick, as compared to a 3 nm oxide) reduced the current by 3 orders of magnitude, as compared to the freshly etched sample (see Figure S1 of the

Figure 5. X-ray reflectivity results for C8 and C16 MLs: Observed absolute reflectivity of the C8 ML (red 4) and C16 ML (green O). Solid black lines are reflectivity fits, calculated with the parameters from Table 2.

Figure 6. Log(J) versus V plots for Al/AlOx/alkylphosphonate ML/Hg junctions with alkyl phosphonates of different lengths, as indicated on the figure. Data are log-averaged over 12
18 traces recorded on at least three different preparations. Error bars represent the log-standard deviations, and they are doubled because of the overlap of dual scan.

in ML density from C16 to C8 films (from 0.85 to 0.55 g/cm3). These values can be compared to those of polyethylene (0.92 g/cm3) and alkyl-silane (OTS) MLs on Si (0.87 g/cm3).73 Using the XRR-derived ML thickness (d) and density (F), the area per molecule, Amol, can be calculated as Amol = MW/(dFNA), where MW is the molecular weight and NA is Avogadro’s number. The resulting molecular footprint areas are ∼0.57 and ∼0.28 nm2 for C8 and C16 MLs, respectively. This demonstrates quantitatively the marked difference between long and short chains, where the C16 ML is denser and more closely packed than the C8 one. Still, both the high quality of the XRR spectra and the high insulating properties (see below) indicate that this lower density 409

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Figure 7. Plot of ln(GE) versus number of carbons. Dashed lines show linear fits to the data for short (C8
C12, green) and long (C14
C16, including C12 for the linear fit, red) chains. The equilibrium conductance was extracted by numerically differentiating the data of Figure 6, and averaging between 0 and
50 mV. The logarithmic average of GE was calculated for each chain length (C8 until C16), and the error was taken as the standard deviation of ln(GE). The errors in β0 values are those of the linear fits in Origin.

Figure 8. Variation of the attenuation factor, β2, with applied bias, for short (C8
C12, black line) and long (C12
C16, red line) APs. β is extracted at each bias from the slope of ln|J| versus the number of carbons by linear regression over all recorded data points, using the same J
V
L data set as used for Figure 6, but without averaging. Error bars represent the 95% confidence interval in the slope of the linear regression. β values refer to only one direction of scanning the bias, from 0 V to high bias, and they slightly differ from those derived for the opposite scan direction (see Figure S4 of the Supporting Information).

Supporting Information). Our fairly conductive AlOx also explains the ∼2 orders of magnitude higher current densities in Figure 6 than previously reported values for alkyl
PO3 on Al.29,33,34 Practically, reproducing a “leaky” AlOx is also easier than a stoichiometric composition for such thin oxide films. Equilibrium Length Attenuation. The simplest quantification of molecular transport is from the current
length dependence, which allows extracting the exponential attenuation factor of current with tunneling distance (L), that is, J ≈ exp(
βLL), where βL (in 1/Å) is the length-decay coefficient. However, defining the transport distance relates directly to the transport path, that is, whether charge flows along the shortest way (“through-space”) or along the molecular skeleton (“throughbond”).4 In molecular electronics, both because the exact tunneling distance (L) is difficult to determine experimentally and because of the common assumption that the molecules facilitate the transport, the decay parameter is often expressed per CH2 unit, that is, “per Carbon”,2,14 with J ≈ exp(
βNN). For alkyl chains, reported βN values2,39 are in the range of 0.5
1.15/CH2, depending on the measurement technique, the contact size (number of contacted molecules), binding group, and type of electrodes.2 In principle, the value of β should decrease with bias, because the applied bias alters the potential profile across the molecular layer.37,43,44 Therefore, it is best to compare β values, extracted near 0 V, using the length attenuation of the equilibrium conductance,74 GE, defined as: dJ GE ¼ ð2aÞ dV

regression coefficient to >0.99. For C8
C12 junctions, the slope yields βN,0 = 1.34 ( 0.004, whereas the long chain data, C12
C16, yield a much smaller decay coefficient, βN,0 = 0.77 ( 0.05, values that are clearly different. These values quantitate the qualitative results in Figure 6, which show that the “short” AP-MLs attenuate charge transport more efficiently (per unit length) than do the longer ones. At the same time, both values are within or close to the (rather wide) range of decay parameters for saturated alkyl chains reported in the literature.2 To further study this difference, we will now look at the response of β to the applied bias for the two subgroups. Bias Variation of the Length-Decay Coefficient, β(V). We deliberately avoid here numerical curve fitting of J
V traces to accepted transport models (e.g., the Simmons model76), because we74,77 and others14,78 have found that different sets of parameters can be fit equally well to a given experimental data set. We address the issue of extracting physical parameters such as the tunneling barrier height in a companion paper.42 Instead, we focus here on the more commonly accepted “β
V analysis” method,37,43 where the current decay coefficient β is extracted for each (fixed) bias point, and then the variation of β with bias is examined (Figure 8). The β(V) values were extracted by a linear regression, for each bias point, between raw ln(J) data (excluding ∼10% of outlier J
V sets) and the number of carbons, where β is the absolute value of the slope in the linear regression. Similar to Figure 7, ln|J| shows different trends for long and short alkyl chains. Therefore, the extraction of β was repeated twice: for short chains (C8
C12) and for long chains (C12
C16). The resultant β2
V curves are shown in Figure 8 for short and long APMLs, by black and red lines, respectively. The considerable spread in raw data yields a rather wide confidence interval for the extracted β values, marked as single error bars in Figures 8. We also observed slightly different β
V curves, depending on whether the scan was from 0 to (1 V (as shown in Figure 8) or the return scan (see Figure S4 of the Supporting Information for comparison). The major finding of Figure 8 is the weak bias dependence of β for short APs, as compared to the strong bias effect on β of long APs. The β
V dependence reported in the literature is rather

V f0

GE ¼ GC expð
βN, 0 NÞ

ð2bÞ

where GC is the contact conductance (for N f 0), which, ideally, should equal the quantum conductance (77 μS/molecule),75 and βN,0 is the equilibrium (“unbiased”, @ 0 V) decay coefficient per carbon. Figure 7 shows a plot of ln(GE) as a function of the number of carbons in the AP. The slope of this plot (
βN,0, see eq 2b) shows a clear change in trend around C12. Fitting to the full data set (C8 until C16) yielded a regression coefficient (R2) of 0.975, while dividing the fit into the two subgroups increased the 410

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the finding that the XPS-derived effective ML thickness values are smaller than expected (0.55L0 to 0.86L0, see Table 1) point to a significantly smaller density of adsorbed molecules in the shorter chains. The resulting structural variation with AP length is shown schematically in Figure 9. The “long” AP-MLs are thought to be fairly dense, with “all-trans” alkyl chains and a small tilt from the surface normal. The “short” APs are much more spaced, with slightly larger tilt, but still contain a significant fraction of “alltrans” chains. The unique arrangement of the AP ML and, especially, of the short ones is attributed to the tridentate binding of PO3 to the Al substrate. DFT computations by Luschtinetz et al. kept negatively charged O (deprotonated hydroxyl) next to each tridentate bonded PO3 to preserve electroneutrality, leading to a twice larger molecular footprint (∼0.45 nm2) than that of mono- or bidentate bonded APs on AlOx.60,82 The Coulombic repulsion between positively charged PO3 groups forces the footprint of the phosphonate tridentate binding on AlOx to be much larger than that of alkyl thiol on gold (0.22 nm2),83 or AP MLs on titania and zirconia powders, which give a footprint that approaches the cross-section of neutral PO3 (0.23 nm2).25,84 In contrast, AP MLs adsorbed on calcium hydroxyapatite (CaHAP) give a large footprint (0.41 nm2), regardless of alkyl length.23 The 0.28 nm2 footprint of C16, as deduced from XRR (Table 2), combined with the IR-deduced tilt angle of 25 (Figure 3c), implies a chain to chain distance of 0.48 nm, which is close to the theoretical diameter of an all-trans alkyl chain of 0.475 nm. We suggest that the variation in binding density with alkyl length results from the increased van der Waals attraction with increasing alkyl chain length, which compensates for the increased headgroup repulsion as the AP molecules are packed denser. Taking the C14 as the threshold length (see Figures 3c, 7, and 8c), the van der Waals energy for close-packed C14 alkanes is ∼90 kJ/mol,85 and this will roughly be the repulsion energy between the PO3 groups that has to be overcome. Our results suggest that DFT calculations of AP binding to AlOx82 are valid for the short chains, but that dispersion forces are important for the longer ones. For short chains, the large spacing between neighboring AP molecules decreases the van der Waals forces so that their effect can be neglected. Thus, the slight increase in the IR-derived tilt for short as compared to long APs (Figure 3c) is probably due to random disorder rather than attractionenhanced tilting. A somewhat similar interplay between “heads” and “tails”86 was reported by McGuiness et al.13 for adsorption of alkyl-thiols on GaAs(001). They argued that the native spacing between GaAs binding sites is larger than the van der Waals radius of the alkyl chains. For short chains (107) and extension of X-ray reflectivity oscillations up to high 2θ values (Figure 5). This high quality for all AP lengths is somewhat surprising, considering the almost 2-fold difference in packing density between C8 and C16 (Table 2). Distinguishing thinner (i.e., more tilted) from less dense MLs was not trivial. As explained above, XPS and ellipsometry actually measure the amount of material, and the result is then translated to thickness by assuming some “standard” materials properties (IMFP or refractive index), ignoring the unknown density dependence of the latter properties. In contrast, XRR directly provides real-space length values, and, therefore, we consider the XRR to yield results closer to the actual thickness and intermolecular spacing, within the accuracy of the multiparameter fitting to XRR data. Additional pieces of experimental evidence support the varying density effect. The span of FTIR-extracted tilt angles (33
24) is much smaller than the tilt variation required to explain the reduction in effective thickness in ellipsometry (68
29). The C
H IR vibrations (Figure 3) indicate that shorter APs form less dense MLs. At the same time, the interchain wagging mode (inset to Figure 4) suggests that already in a C8 alkyl ML a considerable part of the chain in the ML is in the all-trans mode. Moreover, 411

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Langmuir enhance the binding rate, as compared to “good solvents”.23 Special adsorption setups could even lead to formation of condensed monomer films at the solvent
air interface47 or at the solvent
substrate interface,87 which later bind as compact AP ML. The roughness of the Al substrates used in other works is generally higher than what is employed here, which could also affect the formed ML.25 Thus, the structural scenario proposed in Figure 9 is not necessarily a general one for any AP-ML on AlOx. AP-ML Is an Outstanding Insulator Due to Its Unique Morphology. Novak at al.34 suggested that entangled short chains should form MLs with relatively uniform surface coverage, while long chains would adopt a more cluster-like ordered structure that is locally denser than a spaghetti-like structure, but with voids between different clusters (as pointed by arrows in Figure 9). Our observed larger tendency to Hg/Al amalgamation across the molecularly modified samples for longer APs than for shorter ones is consistent with that picture. A similar trend was observed across a metal/bilayer/metal junction made of alkylthiol MLs on Ag substrates contacted by Hg. Amalgamation in these junctions was more likely to occur with longer chains on ultrasmooth Ag (template stripped), while the opposite effect, easier amalgamation with short chain MLs, was seen if a rough Ag substrate was used.39 These results were attributed to the higher density of open voids (or thinner patches) as the alkyl chain length increases.88 The fact that the poorly ordered short AP MLs did not show significant shorts (cf., also ref 34) can be understood by realizing that a liquid-like layer can provide more homogeneous coverage than a well-ordered one.86 The excellent insulating properties of the APs might, hence, be related to a balance between the PO3 and alkyl interactions that form robust, liquid-like, defect-healing MLs with air or vacuum (good and excellent insulating media, respectively) between the chains. The significantly better insulating properties of AP MLs than of trichloro silane MLs30 can be explained by the lack of headgroup cross-linking of phosphonates, which promotes a uniform monolayer growth rather than island-like growth of alkyl silanes.84,89 The weak surface binding of thiols to Au also leads to island-like growth90 and, therefore, to more defects in the adsorbed monolayer. The FTIR position of the antisymmetric CH2 stretch was g2920 cm
1 for all chain lengths (Figure 3b), as compared to 2918.3 cm
1 for directly bound C18
Si52 or 2918.5 cm
1 for C22
S
Au.91 In principle, prolonged adsorption time might drive the AP MLs toward denser coverage. Still, it is not obvious that denser MLs will be better insulators. Variation in Transport Attenuation (β) with the Density of AP-MLs. While our data do not allow us to derive clear net β values, we find that denser MLs (long APs) have a much smaller equilibrium length decay coefficient, β0 (Figure 7), with a stronger bias-dependence (Figure 8) than the less compact MLs (short APs), as is also clear from the raw data in Figure 6 (see also below). Comparing Figures 7 and 8 clarifies that the large difference in β0 values observed in Figure 7 is characteristic for a narrow bias range ((0.1 V) and changes abruptly with bias. The differences in the relative magnitude of β values with bias raise the question in how far one single extracted parameter (e.g., the near 0 V β0 value) allows one to characterize transport. The raw data (Figure 6) show that the spacing between the C14 and C16 J
V curves is much smaller than that between the other curves over the full bias range (see also Figure S4 of the Supporting Information). Therefore, adding C12 to the “long” AP ML subgroup (to avoid fitting to only two length points) does

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not express a genuine behavior. While this issue could be addressed by extending the electrical characterization to longer AP MLs, we have already noted that this is experimentally problematic because of low current signals. The observed change in β values was explained in various ways, including the difference between one and two chemicontacts,36 HOMO versus LUMO-mediated tunneling,92 and “intrachain” as compared to “interchain” transport.4,8
10 The first two reasons do not appear to be germane to this study, because the contacting mode and electrode materials that we used were identical for all samples. “Interchain” transport is also unlikely as an explanation of the higher β values for the short chain junctions, because of the wide spacing of these APs that we derive from our structural characterizations. Another possibility that has been forwarded concerns the presence of gauche defects, which will decrease the transport efficiency across the molecules, as compared to “all-trans” alkyl chains.5,93
95 In principle, longer chains have higher probability for gauche conformers,5,94 but in our case the wide spacing between short MLs provides more conformational freedom for gauche defect formation than what is the case for the tighter arranged longer APs (see Figure 9). We adopt this scenario as a reasonable explanation for the smaller β values for the longer APs. Electrostriction5,92 is not a reasonable explanation for the decrease in β of long APs because it is expected to occur, if at all, for wider spaced monolayers rather than for denser packed long ones. Explaining the smaller β of long AP MLs by a larger contribution from pinholes at grain boundaries to net current is inconsistent with the strong β
V dependence of long APs observed here because transport via direct shorts is expected to have negligible bias-dependence.96 Thus, we conclude that the difference in gauche defects is the likely cause for the variation of β with length reported here. The different β
V dependencies observed here shed light on the seemingly contradictory results in the literature. Within the (modified) WKB model, a too steep or flat β
V behavior translates into nonphysical effective mass values. In contrast, the dispersion model could account for a broad range of β
V relations, depending, among other parameters, on the position of the electrode’s Fermi level relative to the molecular HOMO
LUMO gap, EG (see section 6 of Supporting Information). Within this model, the β
V slope approaches 0 (constant β with bias) as the barrier height approaches one-half the gap (j f EG/2) and becomes strongly bias-dependent and even nonmonotonic, if the Fermi level is closer to one of the molecular levels.

’ CONCLUSIONS Systematic length-dependent structural changes have been found in MLs of alkyl phosphonates on Al substrates. Molecules longer than C12 appear to organize in relatively compact arrangements, while the shorter molecules adopt much larger intermolecular distances with significantly lower intermolecular interactions. The strong PO3
Al binding of the AP molecule leads to uniform MLs at both length limits, but the short molecules present an additional property that turns their MLs into excellent insulators: their flexible “liquid-like” short-range ordering can heal local defects and further avoid amalgamation by the top contact. These changes in molecular packing lead to relatively more efficient charge transport across dense MLs, as compared to looser packed ones. The bias effect on β was also very different at the two limits. Still both ends can be fitted within 412

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the dispersion model of β. This model, therefore, explains the seemingly contradicting β
V dependencies reported in the literature. Finally, we demonstrated that formation of alkyl MLs that is driven by strong substrate binding, rather than by intermolecular attraction, yields improved insulating performance for device applications.

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’ ASSOCIATED CONTENT

bS Supporting Information. (1) Variation of conductance and thickness of untreated AlOx with ambient exposure time; (2) FTIR indication for tridentate binding; (3) the effect of sulfuric etching on partition of O1s peak into Al
OH and Al
O
Al; (4) the XPSderived carbon to phosphor ratios; (5) variation of β(V) with bias scan direction, and fitting to different lengths subgroups; and (6) adaptation of eq 4 from ref 44. This material is available free of charge via the Internet at http://pubs.acs.org. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (D.C.); ayelet.vilan@weizmann. ac.il (A.V.).

’ ACKNOWLEDGMENT We thank A. Yoffe and S. Garusi for assisting with aluminum substrate preparation, and O. Yaffe, R. Har Lavan, R. Lovrincic, M. Deutsch (Bar-Ilan University), and A. Kahn (Princeton) for fruitful discussions. We thank the Israel Science Foundation, ISF, through its Centre of Excellence programs and the Grand Centre for Sensors and Security for partial support. This work was made possible in part by the historic generosity of the Harold Perlman family. D.C. holds the Sylvia and Rowland Schaefer Chair in Energy Research. ’ REFERENCES (1) Amy, F.; Chan, C. K.; Zhao, W.; Hyung, J.; Ono, M.; Sueyoshi, T.; Kera, S.; Nesher, G.; Salomon, A.; Segev, L.; Seitz, O.; Shpaisman, H.; Sch€oll, A.; Haeming, M.; Bocking, T.; Cahen, D.; Kronik, L.; Ueno, N.; E., U.; Kahn, A. Radiation damage to alkyl chain monolayers on semiconductor substrates investigated by electron spectroscopy. J. Phys. Chem. B 2006, 110, 21826–21832. (2) Akkerman, H. B.; de Boer, B. Electrical conduction through single molecules and self-assembled monolayers. J. Phys.: Condens. Matter 2008, 20, 013001. (3) Yaffe, O.; Qi, Y.; Segev, L.; Scheres, L.; Puniredd, S. R.; Ely, T.; Haick, H.; Zuilhof, H.; Kronik, L.; Kahn, A.; Vilan, A.; Cahen, D. Charge transport across metal/molecular (alkyl) monolayer-Si junctions is dominated by the LUMO level, submitted. (4) Slowinski, K.; Chamberlain, R. V.; Miller, C. J.; Majda, M. Through-bond and chain-to-chain coupling. Two pathways in electron tunneling through liquid alkanethiol monolayers on mercury electrodes. J. Am. Chem. Soc. 1997, 119, 11910. (5) Slowinski, K.; Majda, M. Mercury
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