Electrodeposition of Long-Chain Alkylaryl Layers on Au Surfaces

Article pubs.acs.org/JPCC

Electrodeposition of Long-Chain Alkylaryl Layers on Au Surfaces Sun Lin, Ching-Wei Lin, Jin-Hao Jhang, and Wei-Hsiu Hung* Department of Chemistry, National Taiwan Normal University, Taipei 116, Taiwan S Supporting Information *

ABSTRACT: 4-Alkylaryl layers were electrodeposited on Au surface via formation in situ of diazonium cations and the subsequent electroreduction in solution of 4-alkylanilines (4CH3(CH2)nC6H4NH2, n = 5−13). The electrodeposited layers were characterized with X-ray photoelectron spectra (XPS), an atomic force microscope (AFM), attenuated-total-reflection infrared spectra (ATR-IR), spectroscopic ellipsometry, and water contact angle. The thickness of the deposited alkylaryl layer increased with the length of the alkyl chain, which was about 1.6 times the molecular length of the alkylaniline precursor. The surfaces of deposited adlayers were uniform and smooth without aggregation and exhibited hydrophobic character with a water contact angle ∼95°. Charge transport across the alkylaryl layer was characterized with a measurement of a conductive AFM. The current−voltage characteristics at low voltages are describable with a nonresonant tunneling mechanism, according to which the resistance increased exponentially with the thickness of the adlayer. As a measure of the efficiency of the charge transport, the average attenuation factor (β) was 0.63 Å−1, which was less than that observed for the alkyl SAM because of the presence of the aryl ring. The breakdown voltage of the adlayer also increased with the length of the alkyl chain. The 4-tetradocylaryl layer exhibited a resistance and breakdown voltage comparable to that observed for a 1-octadecanethiolate SAM.

1. INTRODUCTION The deposition of organic molecules or layers on surfaces is much investigated because of their prospective application in molecular electronic devices or in organic field-effect transistors.1−4 For instance, a monolayer or multilayer of insulating organic (e.g., long-chain hydrocarbon) is an alternative to SiO2 used as a gate dielectric in an ultrathinfilm transistor.5−8 These organic layers find applications also in biosensors as matrices into which guest molecules insert and, more importantly, as insulating layers.9,10 The layers can be designed with tunable properties, providing unique processing advantages for device fabrication.11−14 The electric conduction of organic thin layers might involve both intramolecular and intermolecular transport of charge.15,16 The current−voltage (I−V) characteristics across the organic layer is greatly influenced by the intrinsic properties of the deposited molecules, which include the chemical structures and the conformations of grafted molecules or groups.15,17−20 Saturated organic molecules are reported to exhibit lower efficiency of charge transport than conjugated ones.5,21,22 For various measurements of charge transport, the current through the alkylthiolate chains chemisorbed on surfaces is found to decrease exponentially with increasing length of the alkyl chain.15,23 The electrochemical deposition of organic molecules has received much attention as a versatile method to functionalize the surfaces of metals and semiconductors.24−26 The organic molecules can be grafted covalently to the surfaces without the © 2012 American Chemical Society

need for high-temperature processing via an electrochemical method. Of established electrochemical reactions, the electroreduction of aryl diazonium salts results in the covalent attachment of the aryl derivatives onto the conducting surface.27−32 This approach allows the presence of selected functional groups on the aryl group, which imparts diverse properties on the modified surface and elaborates more complicated chemical structures for various applications.33,34 For example, the interfacial chemistry of aryldaizonium compounds has been utilized in surfaces patterning, grafting of reactive and functional polymers, immobilization of nanostructured electrocatalysts, and fabrication of molecular electronics.35−38 Recently, the 4-dodecylphenyl adlayer was grafted onto a Au surface and served as the hydrogen donor in surface-initiating photopolymerization.39 Between the aryl group and the surface by the electrochemical reaction, an Au−C bond is formed that is stronger than the Au−S bond of self-assembled monolayers (SAM) via chemisorption of alkanethiols.40,41 The aryldiazonium ion can be generated in situ in solution of aromatic amines (aniline and substituted anilines) by diazotization or in solution of diphenylhydrazine by oxidation and subsequent reaction with a secondary amine; the resulting solution is used directly to modify a surface by electroreduction.41−44 This method prevents tedious isolation Received: May 9, 2012 Revised: July 16, 2012 Published: July 18, 2012 17048

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Figure 1. (a) Consecutive cyclic voltammograms recorded for an Au electrode in an aqueous of 4-dodecylaniline (0.3 mM) and NaNO2 and HCl (1.0 M) at a scan rate 100 mV s−1. (b) Sample current and coverage of 4-dodecylaryl on the Au surface as a function of number of CV scans. The sample current was the cathodic current of a CV scan at the potential of −0.15 V.

(ATR-IR) spectrometer (Bomem, DA8.3) in attenuated-totalreflection mode under a vacuum condition. A germanium crystal was clipped between two samples to form the sandwich structure for the ATR-IR measurement of a multireflective mode. The thickness of the deposited adalyers was measured with an ellipsometer (Sopra) in a clean room (class 10,000). XPS were recorded with a hemispherical energy analyzer (Specs, Phiobos 100) in a vacuum chamber maintained at 5 × 10−10 Torr; as the source of X-ray excitation, a Mg anode was operated at 15 kV and 20 mA. The XPS spectra were recorded and analyzed with software (SpecsLab2). The hydrophilic property of the deposited adlayer was examined on measuring the water contact angle (Krüss DSA 100) and analyzed with the software (DSA). In the ellipsometric measurement, the angles of the incident light was 75° from the surface normal and the polarization angle was set to 45° from the surface. AFM and CAFM data were recorded (MultiMode SPM system, Veeco DI; Nanoscope Software Ver. 5) with AFM tips (fpN 01 type, NIIFP) and CAFM tips (PPP-CONPt type, Nanosensors). The CAFM tip was coated with an approximately double layer (thickness 25 nm) of Cr and PtIr5 on both sides of the cantilever, which has a typical force constant of 0.2 N/m. The AFM images of the deposited adlayers were recorded with the tapping mode to derive the surface morphology and roughness. The CAFM served to map the distribution of electrical conductivity and to measure the I−V curves for the deposited layers. For the CAFM measurement, the voltage was applied to the sample and the tip was electrically connected to ground. The resulting sample current was recorded with a module of current amplifier with a range between 1 pA and 1 μA. To measure the I−V curves, the CAFM tip was placed in a stationary point contact with the layer and the cantilever deflection was maintained at a constant value with a feedback loop. These I−V measurements were typically repeated at several positions across the deposited adlayer to ensure the reproducibility and to obtain the standard deviations of data.

and purification that is essentially required for the aryldiazonium salts before use. The conducting atomic force microscope (CAFM) provides a reliable method to investigate electron transfer through metal−supported layers, such as an alkanethiolate SAM on the Au surface.21,45 The characteristics of charge transfer across a junction of an organic layer were correlated with the molecular structure and conformation. The electrical studies of adlayers might hence provide clues for the selection or design of organic precursors for the deposition of an organic layer with desired properties. Here we report an electrochemical approach to form 4-alkylaryl layers (4-CH3(CH2)nC6H4−Au, n = 5−13) on a Au surface via intermediate diazonium cations. Electrochemical and CAFM measurements served to characterize the properties of electron transfer across the 4-alkylaryl layers deposited on the Au surface. We illustrate the effects of the alkyl length on the resistance and breakdown voltage across the alkylaryl layers.

2. EXPERIMENTAL SECTION Preparation of Alkylaryl Layers. The electrografting of alkylaryl groups on a Au surface was performed with electroreduction with generation of diazonium cations in situ in aqueous electrolytic solution. The latter solution was prepared on adding a 4-alkylaniline precursor (4CH3(CH2)nC6H4NH2, n = 5−13, 0.1 mmol) to HCl (35 mL, 1.0 M) solution. The diazonium cation was subsequently generated on adding NaNO2 solution (0.04 M, 2.5 mL). The Au film (50 nm) with a Ti adhesion layer (20 nm) was deposited on a Si(100) wafer (n-type, 1−10 Ω cm) with thermal evaporation. The Au-coated Si wafer was cleaved into 1 × 1 cm2 chips and employed as a substrate for the electrodeposition of alkylaryl groups. This electrodeposition was performed with a cyclic voltammeter (CHI 614B) equipped with a three-electrode system; a platinum wire served as counter electrode; the reference electrode was a silver/silver chloride (Ag/AgCl) electrode. The alkylaryl groups on the Au surface were electrodeposited at a rate of 0.1 V s−1 in a range between +0.4 and −0.6 V. After electrodeposition, the deposited electrode was abundantly rinsed with ethanol and deionized water to remove residual adsorbates and subsequently dried under flowing gaseous N2. Characterization of Alkylaryl Layers. The vibrational spectra of alkylaryl layers were measured with an infrared

3. RESULTS AND DISCUSSION Figure 1a shows consecutive CV scans of an Au electrode in an aqueous solution of 4-dodecylaniline (0.3 mM) and NaNO2 with HCl (1.0 M) at a scan rate of 0.1 V s−1. The diazonium 17049

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cation was formed on reduction of the NH2 group of the primary aromatic amine with NaNO2. alkylaryl−NH 2 + NO2− + 2H+ → alkylaryl−NN+ + 2H 2O

Two irreversible reduction signals (Epc) appeared about −0.15 and +0.30 V (vs Ag/AgCl) observed in the initial scans, as previously reported for other diazonium salts.33,46−48 The origin of these signals has been attributed to the reduction of the diazonium intermediate at varied crystallographic facets of a polycrystalline Au electrode.39 Figure 1b shows the reduction current at Epc as a function of the number of CV scans. The peak current is rapidly attenuated with increasing number of CV scans; the current nearly vanishes after 30 scans. This condition indicates that the insulating organic groups are grafted onto the surface during the electrodeposition. The relevant reactions involved a formation of diazonium cations and electroreduction for one electron.24 The 4-alkylaniline is converted to a diazonium cation that subsequently releases a dinitrogen molecule and forms a C−Au bond to the surface, as depicted in Figure 2.

Figure 3. FTIR spectra of (a) solid 4-tetradeocylaniline (4C14H29C6H4NH2) precursor and (b) 4-tetradecylaryl layer electrodeposited on Au (4-C14H29C6H4−Au).

the C−C bond in the aromatic ring.42 The peak at 3372 cm−1 observed in IR spectra of 4-tetradecylaniline, associated with the N−H stretching mode, disappears after the electrochemical grafting onto the Au surface. The positions of the va and vs signals have been widely interpreted to provide information on molecular ordering in the aliphatic layer.51,52 The va and vs modes of grafted tetradecylaryl are shifted to greater wavenumber than for the solid (crystalline) precursor. This observation indicates that the alkyl chains of tetradecylaryl groups are less well ordered in the deposited layer than in the crystalline solid.52,53 The elemental compositions of the electrodeposited layers were characterized with XPS measurements. Figure 4a shows a survey XPS of Au surfaces before and after electrografting of 4alkylaryl groups of varied alkyl length. The spectrum of the bare Au surface presents signals characteristic of Au 4f7/2 and 4f5/2 at 84.0 and 88.0 eV, respectively. A small signal for C 1s is observed at 284.6 eV due to a trace of organic contaminant during and after cleaning. The intensity of C 1s increases much

Figure 2. Schematic presentation of electrochemical reactions of 4alkylaniline on a Au surface.

As shown in Figure 1a, the current of the reduction peak decreased with the number of CV scans because the grafted 4dodecylaryl group (CH3(CH2)11C6H4) exhibited a blocking effect for further electrochemical reaction on the Au surface. The decreasing cathodic current thus corresponds to the increasing coverage of the electrodeposited 4-dodecylaryl groups. The coverage is simply estimated with the equation25,49,50 coverage = [(I1 − In)/I1] × 100%

in which I1 and In are the currents of the first and nth scans at Epc, respectively. Figure 1b shows that the calculated coverage of 4-dodecylaryl on the Au electrode is greater than 99% after 30 CV scans. The electrochemical reaction of the alkylaniline molecules occurs only on the uncovered region of the surface that is conductive and allows the reduction reaction to proceed. The surface becomes eventually fully covered with an insulting alkylaryl layer after electrodeposition. That alkylaryl groups had grafted onto the Au surface was confirmed with the measurement of vibrational spectra. Figure 3 shows an ATR-IR spectrum of the Au surface deposited with a 4-teradecylaryl (CH3(CH2)13C6H4) layer, together with an IR spectrum of 4-tetradecylaniline (4-CH3(CH2)13C6H4NH2) for comparison. Three distinct peaks are observed at 2928, 2857, and 1631 cm−1 for the tetradecylaryl-deposited Au surface. The former two peaks are attributed to the asymmetric (va) and symmetric (v s ) stretching modes of the CH 2 group, respectively, and the latter is due to the stretching mode of

Figure 4. (a) XPS survey spectra and (b) C 1s core-level spectra of bare Au and Au surfaces electrodeposited with varied alkylaryl layers with 30 CV scans. 17050

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According to the CV and CAFM measurements, the electrochemical deposition of 4-dodecylaryl (CH3(CH2)11C6H4) was saturated after 30 scans of CV in a solution of 4-dodecylamine (0.3 mM). Figure 5d shows a typical AFM image of an Au surface saturated with 4dodecylaryl groups. The surface has a root-mean-square roughness (Rrms) 1.55 nm, indicating a uniform and smooth morphology without aggregation. Measured with the ellipsometry, the thickness of the 4-dodecylaryl layer is 32.8 Å, which is about 1.6 times the estimated length (∼20 Å) of a 4dodecylaryl group.57 Preceding work showed that additional aryldiazonium cations might react with the aromatic rings of aryl groups directly tethered to the surface during electrodeposition, resulting in formation of a bilayer or multilayer film.28,42,58,59 However, the multilayer was not formed on electrodeposition of the aryl group that was substituted with a long alkyl group in this work. The aryl derivatives containing a long alkyl group might favor formation of a closely packed layer as observed in the formation of the long-chain organic SAM. As a result, the long dodecyl chain hindered the reaction of the substituted aromatic rings with additional dodecylaryl cations because of the steric effect during electrodeposition. The deposited dodecylaryl groups formed a layer of thickness between that of a monolayer and a bilayer as depicted in Figure 6. Conversely, the resulting surface roughness is slightly greater than that of an organic SAM obtained from chemical adsorption.60

with the increasing length of alkyl chains as shown in Figure 4b. A weak and broad peak centered at ∼291 eV is assigned the π−π* shakeup satellite which is characteristic of the aromatic group39 (Figure S1 in the Supporting Information). This also indicates the grafting of the alkylaryl group onto the surface. The signal of N 1s expected to appear at ∼400 eV is negligible for all deposited layers, consistent with the reaction mechanism illustrated in Figure 2: the grafting reaction of the alkylaryl group to the surface occurs with an elimination of N2. The intensities of the Au 4f signals are significantly attenuated by the deposited adlayer. As a result, the spectral background at the binding energy below Au 4f significantly increases because of inelastic scattering of Au 4f photoelectrons by the deposited adlayer, relative to the bare Au substrate.39,54−56 Figure 5 shows CAFM images of Au surfaces obtained after electrodeposition in a solution of 4-dodecylamine with varied

Figure 5. (a−c) CAFM images of Au surfaces obtained after 1, 2, and 30 scans of CV in an aqueous solution of 4-dodecylaniline (0.3 mM), NaNO2 (0.3 mM), and HCl (1.0 M). The current images were recorded at substrate bias of +0.5 V. (d) Tapping-mode AFM image of an Au surface as shown in (c).

number of CV scans. The sample current increased as the tiploading force increased and is greatly raised at a tip-loading force greater than ∼30 nN (Figures S2 and S3 in the Supporting Information).15 The rapidly increased current might result from a direct contact between the probing tip and the Au substrate. The tip-loading force was carefully chosen for the CAFM measurement to avoid structural damage of the deposited adlayer. The CAFM images were recorded in the contact mode with bias voltage of +0.5 V and an applied force of ∼10 nN on the probing tip. The obtained images represent the distribution of electric conductance on the surface, corresponding to the varied coverage of deposited 4dodecylaryl group. A considerable difference of resistance was observed between conductive and insulated areas of the Au surface deposited with 4-dodecylaryl, resulting in a clear contrast in the CAFM image. The areas exhibit a current between 15 and 25 nA; the area with a current less than 5 nA is attributed to the domains covered with 4-dodecylaryl groups. The entire Au surface became completely passivated after 30 cycles of deposition, consistent with the CV data shown in Figure 1a.

Figure 6. Schematic presentation of a proposed structure of electrodeposited alkylaryl layers.

Table 1 shows a comparison of Rrms and thickness between the deposited layers of alkylaryls with varied alkyl lengths: the thickness of the alkylaryl layer increases with the increasing alkyl length. This condition is consistent with the observation that the XPS intensity of C 1s increases with increasing length of the alkyl chain as shown in Figure 4b. Measured from the AFM measurements, the Rrms of the deposited layers increased slightly with the increasing alkyl length (Figure S4 in the Supporting Information). The water contact angles of the alkylaryl layers are listed also in Table 1. These angles are about 95 ± 2°, indicating that the surface exhibited a hydrophobic property due to the terminal CH3 of the grafted alkylaryl group. The conduction of electrons across the deposited alkylaryl layer was investigated with a I−V measurement. Figures 7a and 7b show typical I−V curves recorded with the CAFM tip in contact with the Au surfaces treated with varied CV cycles in the solutions of 4-decylaniline and 4-dodecylaniline, respec17051

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breakdown voltage and conductance. A grafted alkylaryl group with a longer alkyl chain exhibited a greater breakdown voltage, in agreement with observation for the alkylthiolate SAM.61,63,64 The I−V curves were asymmetric to the bias voltage: the current at forward bias was slightly greater than that at reverse bias (Figure S5 in the Supporting Information). The asymmetry might have originated because of the difference of work functions of metals and the interactions of alkylaryl groups with the metals.18,63,65,66 As shown in Figure 8a, the I−V curve displays a linear relation with bias voltage within a range between −0.3 and 0.3 V. The resistance across the deposited layer is accordingly estimated from the reverse of the slope of the I−V curve. Figure 8b shows a semilog plot of the junction resistance versus thickness of the alkylaryl layer; each data point represents the average resistance of several I−V curves collected with the same tip. The resistance increases with the thickness (i.e., length of alkyl chain) of the alkyaryl layer. At small voltages, the resistances of a M-I-M junction are describable with a typical nonresonant tunneling equation, R = R0 exp(βs), in which R0 is the effective contact resistance and s is the junction length that is directly associated with the thickness of the deposited layer.17,18,21,67,68 Exponential prefactor β is the attenuation factor of the electron-tunneling efficiency, which depends on the electronic and chemical structure of the deposited layer. As shown in Figure 8b, the logarithm of resistance versus thickness is well fitted with a straight line, the slope of which corresponds to the average β of the alkylaryl layers. The resulting β is 0.63 Å−1, which is smaller than that (∼1.1 Å−1) of alkylthiolate SAM on a Au surface.18,21,64,69 For comparison, phenyl groups were also grafted on the Au surface on electrodeposition in a solution of aniline. The resistance of the phenyl layer is 10−3 and 10−7 times that of 4hexylaryl and 4-tetradecylaryl, respectively (Figure S6 in the Supporting Information). The electrical conduction of a phenyl layer is much higher than that of an alkyl-substituted aryl layer because the conjugated phenyl group exhibits tunneling more efficient than that of the alkyl group.67 The resistance of an alkylaryl layer is thus predominantly contributed from the alkyl chain. As described above, the alkylaryl layer exhibits a smaller β than alkylthiolate SAM, attributed to the aromatic rings

Table 1. Experimental and Expected Thicknesses, RootMean-Square Roughness (Rrms), and Water Contact Angles of 4-Alkylaryl Layers Electrodeposited on a Au Surface sample

Rrms (nm)

measured thickness (Å)a

expected thickness (Å)b

water contact angle (deg)

bare Au CH3(CH2)5C6H4−Au CH3(CH2)7C6H4−Au CH3(CH2)9C6H4−Au CH3(CH2)11C6H4−Au CH3(CH2)13C6H4−Au

0.48 1.30 1.43 1.41 1.55 1.55

18.41 23.3 26.1 32.9 34.1

12.6 15.0 17.5 20.0 22.6

75.7 94.4 96.6 95.3 94.5 96.5

a

The values were obtained from ellipsometric measurements. bThe values were calculated with Chem3D Ultra 9.0.

tively. The current between the CAFM tip and the surface is critically dependent on the contact area and force (Figures S3 and S4 in the Supporting Information).15,21,61 Accordingly, the I−V curves were recorded with the same CAFM tip for all samples under a tip-loading force of 2 nN. The blocking effect of the grafted organic layer on the Au electrode is demonstrated by the I−V curves. At the same applied voltage, the current between the tip and the surface decreased as the number of CV scans increased. The I−V curve became nearly constant for Au surfaces after deposition of 30 CV cycles. This observation is consistent with the CAFM images shown in Figure 5, which illustrates that the Au surface was completely covered with dodecylaryl groups after 30 CV scans. A voltage increased beyond ∼1.5 V might lead to junction breakdown, resulting in a greatly increased current. The surface treated with CV scans of greater number exhibited a greater breakdown voltage because of greater coverage with alkylaryl groups. Figure 8a displays representative I−V curves for Au surfaces electrodeposited with alkylaryl layers of varied alkyl length. The measured layers were prepared with 30 CV scans to ensure a surface saturated with alkylaryl groups. During recording of the I−V curves, the measuring structure of the CAFM system is analogous to the junction of a metal−insulator−metal (M-I-M) structure.17,20,62 The insulating alkylaryl layer incorporated two electrodes of metals (i.e., Au substrate and IrPt5 AFM tip) with the chemical bonding and mechanical contact, respectively. Their resulting I−V curves exhibit significantly different

Figure 7. (a) Representative I−V curves of Au surfaces electrodeposited with (a) 4-decylaryl and (b) 4-dodecylaryl groups for varied number of CV scans. 17052

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Figure 8. (a) Representative I−V curves of Au surfaces deposited with varied alkylaryl layers (inset: enlarged scales of voltage and current). (b) Semilog plot of resistance as a function of thickness of the deposited alkylaryl layer. Each data point is an average of several I−V curves collected with a single CAFM tip. The error bars represent one standard deviation, and the straight line shows a linear fit to the data.

octadecanethiolate SAM and alkylaryl layers. This material is available free of charge via the Internet at http://pubs.acs.org.

contained in the alkylaryl layer exhibiting an efficient charge transfer through its conjugated π-bonds in a chain-to-chain path.15,17,64 The π−π intermolecular coupling between the aromatic rings might also contribute an additional channel of charge transport, resulting in a decreased resistance.70,71 Even though the effective resistance of 4-alkylaryl layers is still comparable to that obtained for alkylthiolate SAM of similar molecular lengths because the 4-alkylaryl groups form a thicker layer (∼1.6 monolayers). Our results show also that the breakdown voltage of the 4-tetradecylaryl layer is about 2.6 V, comparable to 1-octadecanethiolate (CH3(CH2)17S-) SAM obtained on chemical adsorption (Figure S6 in the Supporting Information).21 This large breakdown voltage might be attributed to a stable bond between the aryl group and the Au substrate.41 Thus, the I−V measurements show that the long-chain alkylaryl layers exhibit both excellent insulating properties and electrical stability.

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*E-mail [email protected], Tel +886-2-7734-6125, Fax +886-2-29324249. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the National Science Council for financial support, under Grant NSC 98-2113-M-003-004-MY3. REFERENCES

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4. CONCLUSIONS We fabricated alkylaryl layers on Au surfaces with a simple and rapid electrochemical method. The deposition of the alkylaryl layer attained saturation after 30 CV scans. The resulting layer exhibited a uniform morphology without aggregation. The thickness of the deposited alkylaryl layer was about 1.6 times the molecular length of the precursor. The alkylaryl layers with varied alkyl length exhibited significantly varied resistances and breakdown voltages. The I−V characteristics are described with a nonresonant tunneling mechanism such that the resistance increased exponentially with thickness at low voltage. The breakdown voltage increased with increasing alkyl length; the 4tetradocylaryl layer is demonstrated to have an electrical insulation and breakdown voltage comparable with that of 1octadecylthiolate SAM. The long-chain alkylaryl layers might thus possess electrical properties appropriate for use in the fabrication of organic thin-film devices.

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ASSOCIATED CONTENT

S Supporting Information *

XPS spectra of N 1s and C 1s, dependence of the sample current on the applied force onto a CAFM tip, AFM images of alkylaryl layers, and comparison of I−V curves between an 17053

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The Journal of Physical Chemistry C

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dx.doi.org/10.1021/jp304502e | J. Phys. Chem. C 2012, 116, 17048−17054