Self-Assembly of Mono-And Bidentate Oligoarylene Thiols onto

Article pubs.acs.org/Langmuir

Self-Assembly of Mono- And Bidentate Oligoarylene Thiols onto Polycrystalline Au S. Casalini,*,† M. Berto,‡ F. Leonardi,†,§ A. Operamolla,∥ C. A. Bortolotti,⊥,@ M. Borsari,‡ W. Sun,@ R. Di Felice,@ S. Corni,@ C. Albonetti,† O. Hassan Omar,# G. M. Farinola,∥,# and F. Biscarini⊥,† †

Consiglio Nazionale delle Ricerche - Istituto per lo Studio dei Materiali Nanostrutturati (CNR-ISMN), via P. Gobetti 101, 40129 Bologna, Italy ‡ Dipartimento di Scienze Chimiche e Geologiche, Università degli Studi di Modena e Reggio Emilia, via Campi 183, I-41100 Modena, Italy § Dipartimento di Chimica, ‘‘G. Ciamician’’, ‘‘Alma Mater Studiorum’’ Università di Bologna, via Zamboni 33, I-40126 Bologna, Italy ∥ Dipartimento di Chimica, Università degli Studi di Bari Aldo Moro, via Orabona 4, I-70126 Bari, Italy ⊥ Dipartimento di Scienze della Vita, Università degli Studi di Modena e Reggio Emilia, , via Campi 183, I-41100 Modena, Italy @ Centro S3, Consiglio Nazionale delle Ricerche - Istituto Nanoscienze (CNR-NANO), via Campi 213/A, I-41125 Modena, Italy # Consiglio Nazionale delle Ricerche-Istituto di Chimica dei Composti Organometallici-Bari (CNR-ICCOM-Bari), , via Orabona 4, I-70126 Bari, Italy S Supporting Information *

ABSTRACT: Four thiolated oligoarylene molecules (i) 4methoxy-terphenyl-4″-methanethiol (MTM), (ii) 4-methoxyterphenyl-3″,5″-dimethanethiol (MTD), (iii) 4-nitro-terphenyl-4″-methanethiol (NTM), and (iv) 4-nitro-terphenyl-3″,5″dimethanethiol (NTD) were synthesized and self-assembled as monolayers (SAMs) on polycrystalline Au electrodes of organic field-effect transistors (OFETs). SAMs were characterized by contact angle and AC/DC electrochemical measurements, whereas atomic force microscopy was used for imaging the pentacene films grown on the coated electrodes. The electrical properties of functionalized OFETs, the electrochemical SAMs features and the morphology of pentacene films were correlated to the molecular organization of the thiolated oligoarylenes on Au, as calculated by means of the density functional theory. This multi-methodological approach allows us to associate the systematic replacement of the SAM anchoring head group (viz. methanethiol and dimethanethiol) and/or terminal tail group (viz. nitro-, −NO2, and methoxy, −OCH3) with the change of the electrical features. The dimethanethiol head group endows SAMs with higher resistive features along with higher surface tensions compared with methanethiol. Furthermore, the different number of thiolated heads affects the kinetics of Au passivation as well as the pentacene morphology. On the other hand, the nitro group confers further distinctive properties, such as the positive shift of both threshold and critical voltages of OFETs with respect to the methoxy one. The latter experimental evidence arise from its electron-withdrawing capability, which has been verified by both DFT calculations and DC electrochemical measurements.

1. INTRODUCTION

tail groups for tuning the performances of organic devices employed as sensors, display backplanes, RFID tags, and any application where logic circuitry is used. Thus, the analysis of the properties of SAMs characterized by the aromatic backbone (i.e., low tunneling resistance) but different tail groups allows one not only to understand the SAM-induced charge-injection barrier phenomena but also to manipulate doping effects for

Self-assembled monolayers (SAMs) are a versatile tool for tuning the physical−chemical properties of metallic, semiconducting, and insulating surfaces.1,2 In organic electronics, SAMs play an important role in interface engineering of devices, since they are crucial for the control of charge injection/extraction in opto-electronic devices,3,4 for the minimization of charge trapping at dielectric-semiconductor interfaces,5−7 for grafting specific receptors in sensing devices,8,9 and for the unconventional large area fabrication.10−12 In spite of numerous studies on SAMs, few works were devoted to the role of both molecule backbone and head/ © 2013 American Chemical Society

Received: June 13, 2013 Revised: September 28, 2013 Published: September 30, 2013 13198

dx.doi.org/10.1021/la402217c | Langmuir 2013, 29, 13198−13208

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Figure 1. (i) MTM, (ii) MTD, (iii) NTM, and (iv) NTD backbones.

Scheme 1

advanced organic field-effect transistors (OFET)-based electronic devices. Within the interface engineering panorama, Stoliar et al.13 have studied the homologue series of linear alkanethiols (from 3 to 16 methylene units) assembled onto the Au source/drain electrodes of pentacene FETs. In this work, they introduced the concept of charge-injection organic gauge (CIOG)14 being the charge injection of functionalized OFETs driven by the SAM placed between electrodes and organic semiconductor. For instance, CIOG has been used as a straightforward method for calculating the tunneling exponent β of the homologue series of thiols, like other techniques such as the hanging mercury drop electrode,15 metal−SAM−metal junctions,16 and electrochemistry.17 By testing SAMs as electrode modifiers in bottom-gate bottom-contact OFETs and in large-area molecular junctions, Asadi et al.18 have demonstrated unambiguously how SAMs act as a tunneling resistance in-series to the resistance of the OFET channel. Further works have confirmed these results like oligothiophenes,19 oligoarylenes,14 and hydrophilic thiols.20

SAMs were successfully used to tune the charge-injection barrier in OFETs, by influencing important electrical parameters, such as field-effect mobility (μ), current density (J), and total resistance (R). These characteristics turned out to be effectively sensitive to the SAM structure and properties.14,19 Accordingly, in this work we have adopted a similar approach for the physical−chemical characterization of SAMs formed on gold by a new set of linear oligoarylenes. The two series of conjugated molecules under investigation are terarylene structures bearing a single methylthiol functionality as the anchoring group (methanethiol series) or two methylthiol moieties in relative 1,3-positions on the head ring (dimethanethiol series), as shown in Figure 1. Our previous studies, which were performed on terarylene structures featuring the same head groups, revealed the ability to use electrochemical experiments along with OFET measurements to investigate the effects of the number of anchoring groups on the electrical properties of the corresponding SAMs.14 13199


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Then it was charged with thioacetic acid S-(5″-acetylsulfanylmethyl-4nitro-[1,1′;4′,1″]terphenyl-3″-ylmethyl) ester 1 (0.46 mmol), dry dichloromethane (7 mL), and dry methanol (7 mL). The mixture was cooled using an ice bath, and 14 drops (700 μL) of concentrated H2SO4 were slowly added. The mixture was then warmed to room temperature and stirred for few minutes, then heated to 50 °C for 12 h. The reaction was monitored by TLC that showed the complete disappearance of the substrate. The reaction mixture was then cooled to room temperature and concentrated under reduced pressure. The acid excess was neutralized with a saturated aqueous solution of NaHCO3, and the mixture was extracted with dichloromethane (3 × 50 mL). The organic phase was collected, washed 3 times with saturated aqueous NaCl (3 × 100 mL), dried over anhydrous Na2SO4, filtered, and concentrated under reduced pressure. The crude product was purified by silica gel chromatography followed by precipitation from dichloromethane-hexane. A white solid was isolated (82% yield). Mp 155−157 °C . 1H NMR (500 MHz, CDCl3): δ 8.33−8.29 (d like, J ≈ 8.9 Hz, 2H), 7.79−7.76 (d like, J ≈ 8.9 Hz, 2H), 7.73−7.69 (m, 4H), 7.50−7.47(m, 2H), 7.34−7.31(m, 1H), 3.82 (d, J = 7.62 Hz, 4H), 1.86 (t, J = 7.61 Hz, 2H); 13C NMR (125 MHz, CDCl3): δ 147.2, 147.0, 142.3, 141.2, 141.1, 137.9, 127.9, 127.8, 127.7,127.2, 125.6, 124.2, 28.9. 2.1.3. Thioacetic Acid S-(4-nitro-[1,1′;4′,1″]terphenyl-4″-ylmethyl) Ester 4. An oven-dried Schlenk tube containing a magnetic stirrer was evacuated and backfilled with nitrogen (3 times). Then it was charged with Pd(OAc)2, 2-dicyclohexylphosphino-2′,6′-dimethoxybiphenyl (S-Phos, 12.5% mol), powdered anhydrous K3PO4 (2.0 equiv), thioacetic acid S-(4′-bromo-biphenyl-4-ylmethyl) ester 3, p-nitrophenylboronic acid 2 (1.5 equiv), and 10 mL of dry toluene. The mixture was stirred for few minutes at room temperature and then heated to 90 °C. The reaction was monitored by TLC that showed the complete disappearance of the substrate after 1 h. The reaction mixture was then cooled to room temperature and was worked up by filtration through a thin pad of silica gel (eluting with ethyl acetate and dichloromethane in a volumetric ratio of 1:1) and concentrated under reduced pressure. The crude material obtained was purified by preparative chromatography on silica gel eluting with petroleum ether: dichloromethane 1:1. A yellow solid was isolated. Mp 112−114 °C (dichloromethane/hexane); (65% yield). 1H NMR (500 MHz, CDCl3): δ 8.29−8.33 (d like J ≈ 8.9 Hz, 2H), 7.80−7.76 (d like, J ≈ 8.9 Hz, 2H), 7.73−7.69 (m, 4H), 7.60−7.56 (d like, J ≈ 8.3 Hz, 2H), 7.34−7.31 (d like, J ≈ 8.3 Hz, 2H), 4.18 (s, 2H), 2.38 (s, 3H). 13 C NMR (125 MHz, CDCl3): δ 195.0, 147.1, 147.0, 141.3, 139.1, 137.6, 137.4, 129.4, 127.8, 127.7, 127.6, 127.3, 124.1, 33.1, 30.3. 2.1.4. (4-Nitro-[1,1′;4′,1″]terphenyl-4″-yl)-methanethiol [NTM]. An oven-dried Schlenk tube containing a magnetic stirrer was evacuated and backfilled with nitrogen (3 times). Then it was charged with compound 4 (0.36 mmol), dry dichloromethane (10 mL), and dry methanol (10 mL). The mixture was cooled using an ice bath, and 35 drops of concentrated H2SO4 were slowly added. The mixture was then warmed to room temperature and stirred for a few minutes, then heated to 50 °C for 24 h. The reaction was monitored by TLC that showed the complete disappearance of the substrate. The reaction mixture was then cooled to room temperature and concentrated under reduced pressure. The acid excess was neutralized with a saturated aqueous solution of NaHCO3, and the mixture was extracted with dichloromethane (3 × 50 mL). The organic phase was collected, washed 3 times with saturated aqueous NaCl (3 × 100 mL), dried over anhydrous Na2SO4, filtered, and concentrated under reduced pressure. The crude product was purified by silica gel chromatography eluting with petroleum ether: ethyl acetate 55:45 followed by precipitation from dichloromethane−hexane. A white solid was isolated (83% yield). Mp 135−137 °C (dichloromethane/hexane). 1H NMR (500 MHz, CDCl3): δ 8.34−8.30 (d like, J ≈ 8.9 Hz, 2H), 7.81−7.76 (d like, J ≈ 8.9 Hz, 2H), 7.74−7.69 (m, 4H), 7.63−7.59 (d like, J ≈ 8.4 Hz, 2H), 7.46−7.42 (d like, J ≈ 8.4 Hz, 2H), 3.81(d, J = 7.6 Hz, 2H), 1.83 (t, J = 7.6 Hz, 1H); 13C NMR (125 MHz, CDCl3): δ 147.1 (two signals), 141.4, 140.9, 138.9, 137.6, 128.7, 127.8, 127.7, 127.6, 127.4, 124.2, 28.7.

In this paper, we combine experimental/theoretical approaches to investigate both structural and functional features of SAMs prepared onto polycrystalline gold using four linear oligoarylenes that possess the head functionalization previously described and bear different terminal groups on the tail ring. The latter are expected to impact the final properties of the respective Au-coated surfaces, in particular because of their different electron withdrawing capability. The molecules under investigation are (i) 4-Methoxy-Terphenyl-4″-Methanethiol (MTM), (ii) 4-Methoxy-Terphenyl-3″,5″-Dimethanethiol (MTD), (iii) 4-Nitro-Terphenyl-4″-Methanethiol (NTM), and (iv) 4-Nitro-Terphenyl-3″,5″-Dimethanethiol (NTD), and their structure is shown in Figure 1. These conjugated structures differ either for the number of thiolated head groups (e.g., MTM/MTD and NTM/NTD) or for the chemical nature of the functional tail group (viz., MTM/ NTM and MTD/NTD). All these molecules share a terarylene main backbone, which guarantees sufficient intermolecular πstacking interactions for an effective packing onto Au electrodes. The systematic replacement of the tail group (namely, −NO2 vs −OCH3) along with the head group (like methanethiol vs dimethanethiol) allows us to assess their effects on different physical−chemical properties, such as surface tension, capacitance, resistance, rate of Au passivation, pentacene growth mode, and electrical performances of functionalized OFETs.

2. EXPERIMENTAL SECTION 2.1. Synthesis. The synthesis of MTM and MTD was performed according to a methodology that we have recently reported.21,22 For the synthesis of NTM and NTD, we followed the same procedure, and we report herein synthetic details solely for the compounds that were not present in the previous reports. In accordance with Scheme 1, NTD was prepared by acid deprotection of the dithioacyl compound 1, whose synthesis is reported in previous paper.21 NTM was synthesized starting with a Suzuki−Miyaura palladium-catalyzed cross coupling between the commercial boronic acid 2 and the arylhalide 3.22 The cross-coupling was performed adopting the complex formed in situ by palladium with the biaryl phosphine S-Phos as catalyst and K3PO4 as base in anhydrous toluene at 95 °C. After isolating the S-acetyl precursor 4, NTM was obtained by acid deprotection with sulfuric acid in a solvent mixture of dichloromethane and methanol in the volumetric ratio of 1:1 at 50 °C. 2.1.1. General Experimental Procedure. All reactions were carried out under a nitrogen atmosphere in oven-dried glassware with dry solvents. All the solvents were immediately distilled prior to use. Toluene was distilled from benzophenone ketyl. Methanol was distilled from molecular sieves and dichloromethane from phosphorus pentoxide. Reagents were purchased at the highest commercial quality and used without further purification. Thioacetic acid S-(4′bromobiphenyl-4-ylmethyl) ester 3 was synthesized according to our previous work,22 while thioacetic acid S-(5″-acetylsulfanylmethyl-4nitro-[1,1′;4′,1″]terphenyl-3″-ylmethyl) ester 1 was synthesized according to our protocol.21 Anhydrous tribasic potassium phosphate was finely ground prior to use. Preparative column chromatography was carried out by using 0.04−0.063 mm silica gel. All new compounds were characterized by 1 H NMR and 13C NMR. 1 H NMR and 13C NMR were recorded at 500 MHz (1H NMR) and 125 MHz (13C NMR), using the residual proton peak of CDCl3 at 7.26 ppm as reference for 1H spectra and the signals of CDCl3 at 77 ppm for 13C spectra. Melting points (uncorrected) were obtained on a capillary melting point apparatus. 2.1.2. (5″-Mercaptomethyl-4-nitro-[1,1′;4′,1″]terphenyl-3″-yl)methanethiol [NTD]. An oven-dried Schlenk tube containing a magnetic stirrer was evacuated and backfilled with nitrogen (3 times). 13200


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assumption of a first-order reaction rate, the observed current must fulfill the following equation:

2.2. Electrical OFET Characterization. 2.2.1. Substrates. The test-patterns (TPs) were supplied by “Fondazione Bruno Kessler” (FBK, Trento, Italy) and they feature four transistors: two of them have widths (W) and channel lengths (L) equal to 11200 and 20 μm, respectively; the remaining two maintain the same geometry (namely a W/L ratio of 560) with W = 22400 μm and L = 40 μm. The common gate electrode is heavily n-doped silicon (resistivity in the range of 0.01−0.03 Ω cm), and the dielectric layer is a thermally grown 200 nm thick SiOx layer. The polycrystalline Au electrodes are patterned with interdigitated fingers of 150(±50) nm thick by photolithography and lift-off. A thin adhesive layer of Cr (2 nm thick) is prepatterned underneath the gold layer. 2.2.2. Functionalization of the Test-Pattern Surface. The cleaning protocol consists of (i) a rinsing (∼10 mL) with acetone to remove the photoresist layer (i.e., an organic and protective layer), (ii) etching in a piranha solution (1:1 ratio of H2SO4:H2O2) at 65 °C for 15 min in order to remove residual organic compounds on the surface, and (iii) quick wash in HF solution (2% v/v) for SiOx flattening. In this way, the SiOx dielectric is activated by the formation of a high density of hydroxyl groups. Test patterns were treated with hexamethyldisilazane (HMDS) vapor overnight. This functionalization reduced the number of traps at the dielectric/pentacene interface, and it conferred enhanced hydrophobicity compared to bare SiOx. After rinsing in acetone plus ethanol, the TPs were dipped in a 0.1 mM oligoarylene solution (CH2Cl2 as solvent) for 72 h. A final rinse with CH2Cl2 was done prior to pentacene deposition. 2.2.3. Pentacene Deposition. The pentacene deposition was performed by thermal sublimation in ultrahigh vacuum with a base pressure of 5 × 10−8 mBar at a rate of 7.5 Å min−1 on TPs held at room temperature. AFM measurements were used for calibrating the pentacene thickness, whereas the deposition rate was measured by quartz microbalance placed close to the sample holder. Regardless of SAMs, the pentacene film was kept constant to 15 nm (∼10 monolayers, ML). 2.2.4. Ex Situ Electrical Measurements. All the electrical measurements were carried out in an ambient probe station. The transfer characteristics were acquired by varying gate-source voltage (VGS) from +40 V to −40 V and drain-source voltage (VDS) −5 V/−40 V for linear and saturation regimes, respectively. The transfer characteristics of bare gold were acquired by varying VGS from +40 V to −40 V and VDS −1 V/−40 V for linear and saturation regimes. The output characteristics were performed by sweeping VDS from 0 V to −40 V and VGS from +10 V to −40 V with steps of 5 V. 2.3. Electrochemical Measurements. A potentiostat/galvanostat PAR model 273A was used for the cyclic voltammetry (CV) experiments, which were carried out using a three-electrode cell for small volume samples (0.5 mL) in argon atmosphere. A potentiostat/ galvanostat μ-Autolab III (Metrohm, Milan, Italy) was used for performing electrochemical impedance spectroscopy (EIS) measurements. A 1 mm diameter polycrystalline gold wire was used as a working electrode (WE), whereas a Pt sheet and a saturated calomel electrode (SCE) were chosen as counter electrodes (CE) and reference electrodes (RE). A Vycor (PAR) set ensured the electric contact between the SCE and the working solution. Potentials were calibrated against the MV2+/MV+ couple (MV methylviologen). All the redox potentials reported here are referred to as the standard hydrogen electrode (SHE). The gold WE was cleaned according to previously published procedures,23,24 and its active area has been estimated by means of the Randles−Sevçik equation. The electrode was functionalized by dipping the cleaned electrode into a 0.1 mM thiol solution (dichloromethane as solvent) at 20 °C for 20 h. To investigate the kinetics of Au passivation, the faradaic current (IF) of the [Fe(CN)6]3−/4− was recorded between WE and CE as a function of increasing incubation time of the WE in the solution containing the relevant SAM-forming molecule. An empirical method for quantitatively comparing the passivation kinetics of the different surfaces was designed and adopted. The method consists of monitoring the cathodic current, corresponding to the [Fe(CN)6]3−/4− reduction, as a function of the incubation time of the working electrode in the solution containing the SAM-forming molecules. Therefore, under the

ln i = ln i0 − k passt

(1)

where t is the incubation time, i0 and i represent the current intensity at a fixed potential value for bare and functionalized Au at time t, respectively. The cathodic peak current yields the i value as long as the cathodic peak is apparent. Once the voltammogram flattens, i was measured at the potential of the last observable peak. Charge transfer resistance to the reduction of [Fe(CN)6]3−/4− (RCT) and capacitance (C) of the SAMs formed by the four oligoarylene molecules have been measured through EIS. The measurements have been performed in 100 mM NaClO4 and 5 mM [Fe(CN)6]3−/4−, sweeping the frequency from 105 Hz up to 10−1 Hz, at a potential centered at the E0 value of ferricyanide with an amplitude of 10 mV. The obtained Nyquist plots have been successfully fitted by Randles circuit composed by the solution resistance (RS), a constant phase element (CPE), and the charge transfer resistance (RCT).25,26 This equivalent circuit lacks of the Warburg component: this fact is indicative of the high blocking capability of the oligoarylene-based SAMs. The surface coverage achieved by the SAM formation was estimated by performing reductive desorption experiments, according to the method of Porter et al.27−29 2.4. Contact Angle Measurements. Contact angle measurements were performed with the GBX model DS (Digidrop, Bourg de Peage, France). The adhesion work WA, viz., the energy required to separate two condensed phases forming an interface, is extracted by fitting experimental data with the Zisman equation: cos ϑ = (WA /γ ) − 1

(2)

where γ is the surface energy of a liquid and θ is the contact angle measured at the liquid/substrate interface. As liquids, we used bidistilled water, ethylene glycol, and nitromethane. Au substrates were purchased from Arrandee (Schlossstrasse 94, D-33824 Werther, Germany) with the following specifications: borosilicate glass of 0.7 (±0.1) mm coated by a Cr adhesive layer of 2.5 (±1.5) nm and the Au film of 250 (±50) nm. 2.5. AFM Imaging. Solver platform SMENA (NT-MDT Moscow, Russia) was used for AFM imaging of the pentacene thin film on the test-pattern surface. The instrument is equipped with a camera for scanning on selected areas of the devices (e.g., electrodes and/or channels). All the images were obtained in semicontact mode after the ex situ electrical measurements. For the data visualization and analysis, Gwyddion 2.32 SPM was used [http://gwyddion.net]. 2.6. Ab Initio Calculations. We performed first-principle calculations based on density functional theory (DFT), as implemented in the PWSCF module of Quantum ESPRESSO.30 The PBE-generalized gradient approximation was applied to the exchange-correlation functional,31 and ultrasoft pseudopotentials were used to describe the ion-electron interaction.32 Valence states included the 5d and 6s shells for Au, 2s and 2p for C, N, and O, and 3s and 3p for S. All pseudopotentials were generated with the PBE functional, including scalar relativistic effects for Au. A plane-wave basis set with a kinetic energy cutoff of 25 Ry was used for the electron wave functions. In this approach, the computed equilibrium Au lattice constant is 4.14 Å, which is in good agreement with the experimental value of 4.08 Å. To model each organic/metal interface, we employed a periodically repeated supercell in which the substrate was modeled by four Au(111) layers, each containing 16 atoms in a 4 × 4 lateral periodicity with the equilibrium lattice parameter. The supercell contained a vacuum thickness of 20 Å in the direction perpendicular to the surface, in which the adsorbate was hosted. Brillouin zone sums were done on a 2 × 2 × 1 Monkhorst−Pack k-point mesh. Structural optimization was performed by fixing the two bottom-most layers of the Au slab and allowing all other atoms to relax unconstrained until each force component was less than 0.027 eV/Å. 13201


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Table 1. Adhesion Work (WA), Apparent Passivation Rate Constant (kpass), Capacitance (C), Charge Transfer Resistance (RCT), and Coverage (Γ) are Shown for the Four Oligoarylene Molecules SAM

MTM

MTD

NTM

NTD

WA (mN/m) kpass × 104 (s−1) C (nF/cm2) RCT (MΩ/cm2) Γ × 1010 (mol/cm2)

46 (±5) 1.6 (±0.1) 227 (±11) 0.53 (±0.02) 5.3 (±0.4)

55 (±1) 1.2 (±0.1) 270 (±13) 3.12 (±0.12) 4.8 (±0.2)

48 (±5) 2.0 (±0.6) 400 (±20) 0.10 (±0.01) 6.0 (±0.2)

59 (±4) 0.70 (±0.06) 168 (±9) 4.04 (±0.16) 6.1 (±0.02)

Figure 2. (a) Cyclic voltammograms at different incubation times for MTD; (b) exponential dependence of the cathodic peak current vs immersion time. We simulated dissociative thiol-like adsorption of the target molecules, namely assuming that each S−H bond in the adsorbate species dissociates into a thiol radical and a H atom upon adsorption, which is a widely accepted scenario.33−35 The interface formation energy is then computed from the total energies (Eint) of the interface (composed of slab and adsorbate), Esur of the clean substrate, and Erad of the deprotonated gas-phase radical, as

Eads = E int − Esurf − Erad

whereas those relative to the other SAMs are depicted in Figure S.2 of the Supporting Information. A progressive distortion and decrease of the faradaic current relative to redox reaction of the [FeCN6]3−/4− is observed as a distinctive feature of Au surface passivation due to the progressive blocking of the electron transfer (ET) process, as a result of the SAMs formation (Figure 2a and Figure S.2a, Figure S.2c, and Figure S.2e of the Supporting Information). The cathodic peak first shifts toward more positive values and then disappears. When the Au passivation process was complete, only the capacitive contribution to the current is detected, as ET between the redox probe (ferricyanide) in solution and the gold electrode was hindered by the compact insulating SAM. As described in the Electrochemical Measurements, we have adopted an empirical approach to quantify the passivation mechanism occurring at the Au surface by determining an apparent constant kpass (see eq 1). This procedure was repeated for each investigated molecule (see Figure S.2 of the Supporting Information), and the corresponding passivation rate constants, kpass, are reported in Table 1. It is clear that the presence of two −SH groups instead of one slows down the Au passivation. In fact, internal comparison of kpass values for molecules featuring the same tail group, highlights how the passivation kinetics is faster for MTM than for MTD (1.6 × 10−4 s−1 vs 1.2 × 10−4 s−1) and for NTM compared to NTD (2.0 × 10−4 s−1 vs 0.7 × 10−4 s−1). It is reasonable to assume that the presence of two anchoring groups instead of one acts as an additional constraint, partially hindering the blocking properties of the oligoarylene molecules. This will be discussed further in the theoretical section as well as in the next paragraph dealing with the reductive desorption. As already observed in cyclic voltammetry, these pinhole-free SAMs display remarkable insulating properties. For this reason, the equivalent circuit used to fit the Nyquist plots (shown in Figure S.3 of the Supporting Information) requires only 3 elements: (i) the solution resistance (RS), (ii) the charge transfer resistance (RCT), and (iii) a constant phase element

(3)

Projected densities of states (PDOS) have been calculated for the adsorbed and radical molecules by projecting the electronic states over Slater atomic orbitals. A Gaussian broadening of 0.005 Ry has been used to smear the PDOS.

3. RESULTS AND DISCUSSION 3.1. Au−SAM Wettability. The adhesion work (WA), as extracted from eq 2 (described in the Experimental Section), is reported in Table 1. The experimental values of the contact angle are plotted in Figure S.1 and shown in Table S.1 of the Supporting Information. A systematic increase of WA is observed moving from one to two thiolated legs. This shows that WA is affected by the number of thiolated legs, while the chemical nature of the tail groups (nitro or methoxy) influences this parameter to a less important extent. This suggests a different packing arrangement on the surface for mono- and bidentate molecules. This result is in agreement with the electrochemical and theoretical results discussed in the next paragraphs. The difference in surface tension measured for the four ligands is also reflected by the observed changes of pentacene morphology on the functionalized source/drain electrodes. 3.2. Electrochemical Characterization. It is widely accepted that a pinhole-free and well-packed SAM is able to quench the redox signal of an electrochemically active probe in solution, such as [Fe(CN)6]3−/4−. For each SAM-forming molecule, we monitored the changes in the faradaic current of [Fe(CN)6]3−/4− as a function of increasing incubation time of the Au electrode, as described in the Experimental Section. The voltammograms relative to MTD are shown in Figure 2, 13202


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Figure 3. Reductive desorption graphs of (a) MTM, (b) MTD, (c) NTM, and (d) NTD. Each inset is the magnification of the potential range highlighted by the red circle.

cleaning, surface tension, and temperature.36 Thiolated SAMs are able to modify the surface tension of the bare Au, thus affecting the pentacene growth mode.37 This is crucial for the electrical performance of OFET.4 Hence, the morphology of pentacene grown on Au electrodes has been directly investigated by AFM imaging. The height−height correlation function H(r), with r spatial length scale, was monitored to extract the following morphological descriptors: (i) the rootmean-square roughness, σrms, (ii) the lateral correlation length, ξ, and (iii) the roughness exponent, α.38 σrms represents the standard deviation of the heights distribution; ξ is directly related to the domain size of pentacene and α exponent describes the slope of the decaying branch,39 as shown in Figure 4.

(CPE), which can be directly translated to the capacitance of the SAM. As already observed for adhesion work and kpass, the RCT values are clearly affected by the number of the anchoring group. In fact, a systematic increase of 1 order of magnitude (namely from hundreds of kΩ to units of MΩ) moving from methanethiol to dimethanethiol is observed. Although further investigations are required, the RCT and C values of NTM likely hint at higher-disordered regions within this SAM with respect to the other ones. Exploiting the reductive desorption as established by Porter et al., we have calculated the surface coverage, Γ. Although the reductive peaks are invariably placed in the onset of the electrolyte reduction, they are sharp enough to yield a reliable value for the reductive charge of the Au−S bond. (see Figure 3). The Γ values are similar among them (see Table 1) and slightly lower with respect to perfectly packed alkanethiol SAMs (Γ = 9.3 × 10−10 mol/cm2).27 The high compactness of the SAMs is in agreement with the rather negative value of the cathodic peak potential. The positions of MTD and NTD are centered at −1.53−1.55 V (vs Ag/AgCl), while MTM and NTM at −1.67 V (vs Ag/AgCl). Notably, the higher reduction charge density observed for MTD and NTD is almost twice as large as that of MTM and NTM, indicating that the binding of MTD and NTD occurs via both thiolate groups. It is also worth noting that a cathodic peak is invariably observed for NTM and NTD, prior to the S−Au cleavage but not for MTM and MTD. This signal could be confidently ascribed to the reduction of the −NO2 group, as suggested by DFT calculations, which show the energetic difference between the nitro and methoxy groups in terms of lowest unoccupied molecular orbital (see Ab Initio Modeling of the Adsorbed Molecules). Another interesting detail is the different positioning of the peak corresponding to the nitro group reduction of NTM and NTD. The former is placed at −0.869 V (vs Ag/ AgCl) and the latter at −1.089 V (vs Ag/AgCl). 3.3. Pentacene Morphology onto Functionalized Source/Drain Contacts. The pentacene growth is sensitive to different features of the substrate, such as roughness,

Figure 4. (a) Side of the figure displays the height−height correlation function of Au functionalized with MTM. The morphological descriptors α, ξ, and σrms are highlighted. (b) Corresponding image of the pentacene morphology (5 × 5 μm2).

All the estimated parameters are listed in Table S.2 of the Supporting Information, and the main result is the appearance of a second correlation length for NTM and MTM, which hints two characteristic length scales of morphological order for SAMs, featuring lower WA. This is also consistent with pentacene morphology onto Au coated by SAM based on [4′-(thiophen-2-yl)biphenyl-4-yl]methanethiol with respect to [4′-(thiophen-2-yl)biphenyl-3,5-diyl]dimethanethiol in our pre13203


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Figure 5. (a and c) Transfer and output (b and d) characteristics of (a and b) the reference OFET and (c and d) after CH2Cl2 immersion.

Figure 6. (a) Transfer and (b) output characteristics for OFET functionalized by MTM.

vious work.14 These evidences also suggest that bidentate SAMs yield pentacene morphology with a single level of order (namely a unique lateral correlation length). The contribution of different head groups emerges from α2 values, which are higher for NTM than MTM. This means a smoother pentacene morphology for −NO2-terminated SAMs than −OCH3terminated ones. 3.4. Electrical Characterization of Functionalized OFET. All the OFETs have been electrically characterized in air. The devices having solely the channel functionalization with HMDS are considered as references. In air, such devices have mobility of 0.41 (±0.04) cm2 V−1 s−1, threshold voltage of 7 (±2) V, and a negligible bias stress (data not shown). The functionalization protocol of source and drain electrodes with SAMs takes 72 h. In order to disentangle the solvent effect on the electrical properties from that of SAM formation, the OFETs have been firstly characterized right after being dipped in pure dichloromethane. The organic solvent treatment slightly decreases the electrical performances, with a mobility of 0.13 (±0.03) cm2 V−1 s−1 and threshold voltage of 0 (±1) V (see Figure 5). This is probably due to the introduction of traps within the channel.

Further studies must be performed in order to clarify the chemical interaction between SiOx functionalized by HMDS and CH2Cl2. The output characteristics of the SAM-coated OFETs clearly show how all the four oligoarylenes worsen the charge-injection (viz., nonohmic behavior in the linear regime of the output characteristics), as shown in Figure 6 (see also Figure A.5 of the Supporting Information). Mobility is not sensitive enough to enable us to discriminate the different SAMs, except for MTD that shows μ values almost 2 orders of magnitude lower than the other ones (see Table S.3 of the Supporting Information). However, threshold voltage is sensitive to the chemical nature of the headgroup at the charge injection interface. As a result, NTM and NTD molecules positively shift the threshold voltage according to an increase of the holes density (see Table S.2 of the Supporting Information). This is ascribable to the electron withdrawing feature of the NO2 group with respect to the −OCH3 one.40 This has been already observed in Electrochemical Characterization and will be widely discussed in Ab Initio Modeling of the Adsorbed Molecules. Another useful parameter is the sheet resistance (R), namely the sum of both channel and contacts resistances. From the output characteristics, R can be plotted as a function of VGS. The general trend features an exponential 13204


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transition from a high resistive behavior (VGS > Vth relative to an off state) and a low resistive one (VGS < Vth relative to the on state), as shown in Figure 7. The systematic difference of the

Table 3. Adsorption Eergies (Eads, in eV Per Molecule) and Au−S Distances (D, in Å) for the Four Optimized Interfacesa MTM MTD NTM NTD

Eads

D (Au1−Sa)

D (Au2−Sa)

D (Au3−Sb)

D (Au4−Sb)

−1.48 −2.58 −1.49 −2.61

2.43 2.46 2.43 2.44

2.43 2.49 2.43 2.52

− 2.45 − 2.48

− 2.50 − 2.54

a

The two (four) Au atoms considered are the nearest-neighbors to the S atom for the monothiolated (dithiolated) molecules. Sa and Sb refer to the two sulfur atoms of dithiolated molecules (for monothiolated only Sa is present); Au1 and Au2 refer to the Au atoms closest to Sa, while Au3 and Au4 are those closest to Sb. Figure 7. Sheet resistance for OFET functionalized by NTD-based SAM for smaller and bigger geometry.

For the double thiolate interfaces (MTD and NTD), the Au−S distances are slightly longer than in single-leg interfaces, between 2.44 and 2.54 Å, but still in the covalent regime. It should be noted that the distance between the two sulfurs in MTD and NTD radicals is 6.52 and 6.57 Å, respectively, which do not match a lattice constant of gold. In this sense, the MTD and NTD radicals could not dock on the Au(111) surface with high symmetry, which causes the different S−Au bonds to be inequivalent and somewhat strained. The requirement of finding the correct position for both S atoms is an additional constraint (besides the correct molecule−molecule packing) to be satisfied in the growing of the SAM for the dithiolated molecules, a constraint that does not exist for the monothiolated ones. As such, it is not surprising that the Au passivation of the dithiolated SAMs is slower than for the monothiolated as noted in Electrochemical Characterization, since the molecules should find a conformation to satisfy both the correct packing and the correct S−Au positioning. The adsorption energies are calculated according to eq 3. We find that the adsorption energy mainly depends on the number of anchoring bonds. In fact, for a given number of thiolate bonds, the difference (MTM-NTM and MTD-NTD) is rather small, which reveals a negligible effect of the tail group (i.e., metoxy vs nitro) in determining the energetic relations between the interfaces with the different adsorbate species. Notably, the adsorption strength for dithiolated molecules is sensibly lower than two times the value for thiolated molecules. This can be explained by the mismatch of the S−S distance and the gold lattice constant, as noted above. As a consequence of this mismatch, the two S atoms cannot stay both in exact bridge positions (as shown by Au−S distances in Table 1), and the resulting geometry is a compromise that causes the deformation of both adsorbate and gold substrate decreasing the bond energy. The thermodynamic driving force for the adsorption of one dithiolated molecule is larger than that for a monothiolated

resistive trend between the two geometries is a further proof of the contact-limited performances of the functionalized devices (see Figure A.6 of the Supporting Information) The following experimental fit, R = Rmax exp[−(VGS − Vmax)/ Vc], was used to quantify this resistive drop, ΔR (equal to 63% of Rmax), in terms of the critical voltage, Vc14, as shown in Table 2. Table 2. Critical Voltages Are Reported for Each Oligoarylenes smaller geometry

Vc (V)

MTM NTM MTD NTD

0.91 (±0.05) 1.22 (±0.04) 1.4 (±0.1) 1.9 (±0.3)

Vc is consistent with the electron withdrawing capability of the nitro group, previously observed for threshold voltage. 3.5. Ab Initio Modeling of the Adsorbed Molecules. First-principles simulations were used to determine the structure and energetics of the four molecules adsorbed on Au. The optimized atomic configurations are shown in Figure 8. All the sulfur atoms adopt the 2-fold bridge coordination. The distance between the sulfur atom and the two nearest gold atoms in single-leg interfaces (MTM and NTM) is 2.43 Å (Table 3), which is very close to the sum of covalent radii of sulfur (1.02 Å) and gold (1.34 Å). This is an indication of a strong covalent contribution to the thiolate bonding, as expected33,35 and further revealed in the electronic structure (not shown here).

Figure 8. Optimized geometries for the four interfaces. Color code: oxygen, red; nitrogen, blue; carbon, gray; hydrogen, white; gold, orange; sulfur, yellow. 13205


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Figure 9. PDOS for MTM (left panel) and NTM (right panel) projected on the tail group (upper panels) and phenyl 3 (lower panels). Red dashed lines refer to the gas-phase radical and black solid lines refer to the adsorbed radical on the surface. The zero of the energy is the Fermi energy of the system composed by the adsorbed molecule and the metal; the deepest molecular energy levels for the adsorbed and the detached molecule have been aligned.

geometric constraints that the bidentate molecule has to satisfy when it forms the SAM, which significantly slows down the Au surface passivation rate. Both experimental and theoretical results indicate that mono- and 1,3-dimethanethiols form selfassembled monolayers with different organization on polycrystalline gold surfaces. Changes in surface tension are also crucial for the morphology of pentacene grown onto the SAMcoated electrodes. Conversely, threshold and critical voltages turned out to be sensitive to the chemical nature of the tail group (−NO2 vs −OCH3), especially because of the higher electron withdrawing capability of the nitro group. To summarize, this work shows a comprehensive description of the chemical−physical features of a new set of oligoarylenes capable to self-assemble onto Au electrodes. In particular, we presented a multi-methodological approach able to disentangle the role played by both tail and head groups in terms of surface engineering. This approach can be employed for the design and fabrication of new materials as well as organic devices, whose figures of merit will have to be fine-tuned for specific purposes in different scientific fields like organic electronics.

one, although it is not the double as could be expected. Moreover, both S atoms should reach their most stable adsorption location to take full advantage of this driving force, a process that may conflict with the SAM packing, as mentioned before. Finally, we have examined the electronic structure to inquire on whether the expected larger hole injection capability of the nitro as compared with the methoxy group is found in the adsorbed molecular phase. This would fit nicely with the threshold potential trend discussed in Electrical Characterization of Functionalized OFET. To this end, from the electronic structure results, we have extracted the PDOS on the tail groups and on the last phenyl ring (phenyl 3 in Figure 1) (i.e., the portion of the molecules in contact with the pentacene layer in the OFET geometry). Results are shown in Figure 9. It is clear that the adsorption has a negligible effect on the states of the molecule in the considered region because the black-solid lines and the reddashed lines (before and after adsorption, respectively) almost coincide. Moreover, the position of the unoccupied states (i.e., the peaks in the positive energy range in Figure 9) confirms unequivocally that NTM is a better hole injector than MTM because the lowest unoccupied state in NTM is approximately 1 eV lower in energy than in MTM. As for the absolute values of the band gap, we remark that pure GGA functionals (including PBE) have a tendency of underestimating them. For such reasons, here the discussion is limited to a qualitative comparison between the effects of the two different head groups.

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ASSOCIATED CONTENT

* Supporting Information S

Plots for each functionalization, raw data of contact angle measurements, cyclic voltammograms, electrochemical impedance spectroscopy plots, height−height correlation functions, root mean square roughness, transfer and output characteristics, and total resistance as a function of VGS. This material is available free of charge via the Internet at http://pubs.acs.org.

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4. CONCLUSION We have synthesized and characterized four linear oligoarylenes differing for either the tail group (−NO2 vs −OCH3) or head functionality (mono- vs 1,3-dimethanethiol). The SAMs formed by the investigated molecules turn out to be compact and pinhole-free, as indicated by different electrochemical parameters such as surface coverage (Γ), high negative potentials of reductive desorption, and the lack of the Warburg impedance. The single methanethiol head ensures a faster passivation of the gold surface, regardless of the chemical nature of tail moiety. On the basis of ab initio modeling calculations, this can be ascribed to the larger number of

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research leading to these results has received funding from the European Union Seventh Framework Programme [FP7/ 2007-2013] under grant agreement no. 280772, project iONEFP7 and from EC-FP7-ONE-P large scale project under 13206


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agreement no. 212311. S.C., R.D.F., and W.S. acknowledge support from IIT Seed project MOPROSURF and IIT Platform “Computation”. A.O., O.H.O. and G.M.F. acknowledge the italian project PRIN 2009 prot. 2009PRAM8L.

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Self-Assembly of Mono-And Bidentate Oligoarylene Thiols onto

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