Self-Assembled Monolayer Exchange Reactions as a Tool for

Article pubs.acs.org/Langmuir

Self-Assembled Monolayer Exchange Reactions as a Tool for Channel Interface Engineering in Low-Voltage Organic Thin-Film Transistors Thomas Lenz, Thomas Schmaltz, Michael Novak, and Marcus Halik* Organic Materials & Devices, Institute of Polymer Materials, Department of Materials Science, University of Erlangen-Nürnberg, Erlangen, Martensstr. 7, 91058, Germany S Supporting Information *

ABSTRACT: In this work, we compared the kinetics of monolayer self-assembly longchained carboxylic acids and phosphonic acids on thin aluminum oxide surfaces and investigated their dielectric properties in capacitors and low-voltage organic thin-film transistors. Phosphonic acid anchor groups tend to substitute carboxylic acid molecules on aluminum oxide surfaces and thus allow the formation of mixed or fully exchanged monolayers. With different alkyl chain substituents (n-alkyl or fluorinated alkyl chains), the exchange reaction can be monitored as a function of time by static contact angle measurements. The threshold voltage in α,α′-dihexyl-sexithiophene thin-film transistors composed of such mixed layer dielectrics correlates with the exchange progress and can be tuned from negative to positive values or vice versa depending on the dipole moment of the alkyl chain substituents. The change in the dipole moment with increasing exchange time also shifts the capacitance of these devices. The rate constants for exchange reactions determined by the time-dependent shift of static contact angle, threshold voltage, and capacitance exhibit virtually the same value thus proving the exchange kinetics to be highly controllable. In general, the exchange approach is a powerful tool in interface engineering, displaying a great potential for tailoring of device characteristics.

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monitored by measuring the static contact angle (SCA) of water at proceeding immersion times of up to 3 days. Exchange reactions and the corresponding kinetics were investigated by immersing substrates, which were fully covered with a C17−CA SAM, in a diluted solution of F15C18−PA (Figure 1c). Samples were removed after different reaction times to determine the progress of the SAM exchange reaction. F15C18−PA was chosen for exchange reactions in order to obtain a significant difference in SCA compared to the initial value of the C17−CA decorated surface. We proved in a cross experiment under comparable conditions that the exchange reaction only occurs from carboxylic acid to phosphonic acid and not vice versa. Apart from contact angle measurements, we also investigated the exchange reaction by introducing the mixed SAMs (removed after different exchange reaction times) on patterned Al/AlOx structures as part of the hybrid dielectric in capacitor devices (Figure 2a) and organic thin-film transistors (TFTs; Figure 3a). Due to the different chain substituents of C17−CA and F15C18− PA, which lead to different dipole moments and dielectric constants of the pure monolayers, the composition of the mixed SAM at a given exchange reaction time determines the capacitance as well as the threshold voltage of organic TFTs.7,8

elf-assembled monolayers (SAMs) of functional organic molecules have been proven to be an important tool for interface engineering in organic electronics, tuning device properties, or even serving as active functional device layers on the molecular scale.1−5 The SAM concept was demonstrated successfully for a large variety of molecular structures (e.g., insulating alkyl chains, dipolar chains, π-conjugated substituents, etc.) with several different anchor groups (e.g., silanes, phosphonic acids, thiols, etc.) providing a stable SAM on various surfaces (e.g., metals, conductive, or insulating oxides, etc.).4 So far, most investigations have described certain species of densely packed molecules in SAMs in equilibrium to obtain reliable results for the devices.6 However, organic electronics is supposed to benefit from inexpensive processing. That is why the time dependency of molecular self-assembly is of enormous interest, since it ensures a fast deposition for such selfassembled layers. Thus, knowledge of the kinetics of monolayer formation (Figure 1a) enables optimized process conditions for self-assembly. Different binding strengths of suitable anchor groups for a given surface allow for an innovative method of obtaining mixed monolayers by partially exchanging layers, that is, one molecular species is substituted by another one in a controlled exchange reaction. In order to explore the time dependency of molecular selfassembly, we investigated the deposition kinetics for a longchained carboxylic acid (stearic acid; C17−CA) and a phosphonic acid (1H,1H,2H,2H,3H,3H,4H,4H,5H,5H,6H,6H,7H,7H,8H,8H,9H,9H,10H,10H,11H,11H-perfluoro-octadecylphosphonic acid; F15C18−PA) (Figure 1b) on thin layers of aluminum oxide (AlOx). The self-assembly was © 2012 American Chemical Society

Received: July 11, 2012 Revised: September 6, 2012 Published: September 10, 2012 13900

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Figure 1. (a) Schematic image of the molecular self-assembly and the exchange reaction. (b) Molecular structure of C17−CA and F15C18−PA. (c) Static contact angle of SAM surfaces as a function of immersion time for molecular self-assembly and exchange reaction. 2-propanol (IPA) solutions using a concentration of 1 mM for C17− CA (used as received from Sigma−Aldrich GmbH) and concentrations of 0.05 or 0.005 mM for F15C18−PA (used as received from Dr. rer. nat. Matthias Schlörholz; www.schloerholz.com). Substrates were removed after specific times (between 1 min and 3 days), and the surfaces were gently rinsed with pure IPA and dried at 60 °C on a hot plate for 3 min. The investigation of the exchange kinetics was performed with samples initially immersed in C17−CA solution for 3 days and then immersed in F15C18−PA solution for various times (see Table 1). The cleaning and drying procedure of the samples was the same as described above. Static contact angles of H2O were measured using a contact angle system OCA from Dataphysics in sessile drop mode with a drop volume of 2.0 μL. Values of 107.0 ± 0.7° for the pure C17−CA SAM and 121.8 ± 0.4° for the pure F15C18−PA SAM were determined. Bottom gate top contact transistors with aluminum as the gate electrode, α,α′-dihexyl-sexithiophene (DH6T; used as received from Heraeus Clevios GmbH) as the semiconductor, and gold as the source and drain contacts were fabricated on silicon dioxide (substrate) utilizing thermal evaporation through shadow masks as reported elsewhere.9 We have chosen DH6T because of the reliable thin-film growth on different dielectric surfaces.10 For both types of devices, the hybrid dielectric consisted of either a mixed or a pure SAM as well as of aluminum oxide formed by means of oxygen plasma treatment as mentioned above. All TFT devices were electrically characterized in ambient air using an Agilent B1500A parameter analyzer. At least five representative transistors of each set with channel lengths of 40 μm and channel widths of 600 μm were measured. Capacitors were fabricated with thermally evaporated aluminum and gold as electrodes with an area of 2500 μm2 (50 μm × 50 μm). The capacitance was measured at a frequency of 100 kHz in a voltage range between −2 V and +2 V.

Figure 2. (a) Schematic setup of a capacitor device with hybrid dielectric (AlOx/SAM). (b) Breakdown characteristics of SAM hybrid dielectrics compared to AlOx (two measurements in each direction) and current density at negative supply voltage (inset).

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EXPERIMENTAL SECTION

In order to investigate the SAM formation kinetics on aluminum oxide surfaces, the SCA of water on top of the SAM-terminated surfaces was measured as a function of immersion time in SAM solution. The samples were fabricated by thermal evaporation of 30 nm of aluminum on p-silicon substrates with a rate of 2.5 Å/s at pressures below 2 × 10−6 mbar, followed by an oxygen plasma treatment at pressures of 0.2 mbar for two minutes (Diener Electronic Pico, 200 W) yielding an aluminum oxide layer of approximately 3.6 nm thickness and a permittivity of ε = 4.5.9 Subsequently, the SAMs were deposited from

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RESULTS AND DISCUSSION The kinetics of self-assembled monolayer formation has been systematically studied for thiols,11−16 carboxylic acids,13,17,18 and silanes,11−13,19−21 but so far, there are only a few 13901

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phosphonic acid (F15C18−PA) system with static contact angle measurements. Densely packed SAMs of molecules with organic chains consisting of alkyl or fluorinated alkyl groups exhibit hydrophobic surfaces with typical surface energies below 22 mN/m and SCA values larger than 100°.9 Aluminum oxide surfaces formed by plasma treatment and rinsed with IPA show an SCA value of typically 40°. Due to the growing hydrophobicity of the surface with increasing SAM surface coverage, measuring the SCA of water at proceeding immersion times is a functional tool to investigate SAMformation kinetics of the utilized molecular systems.17,25 We have chosen the value of IPA-treated AlOx (SCA ≈ 40°) instead of bare plasma-treated AlOx (SCA < 20°) as the initial value for the investigation because all depositions are performed in IPA solutions, where IPA as solvent is abound compared to the SAM molecules and thus will instantly cover the AlOx surface. Figure 1c shows the plot of the SCA of water versus immersion time in 0.05 and 0.005 mM F15C18−PA solutions (triangles and circles, respectively) and in a 1 mM C17−CA solution (squares). In 0.05 mM F15C18−PA solutions, the contact angle reaches a saturation regime in less than 30 min with a value of approximately 118°, which is only 4° below the value after 3 days of immersion. In contrast, the self-assembly of C17−CA is much slower. Even after 4 h of immersion, the SCA exhibits a value 10° below the saturation value (after 3 days). We note that the SCA value for C17−CA monolayers does not further increase after 3 days. In order to adequately compare the kinetics of the SAM formation, we calculated rate constants of self-assembly (ksaSCA) for the different systems based on the considerations of Peterlinz and Georgiadis14 (for calculations see the Supporting Information), which led to values of ksaSCA = 0.43 min−0.5 for 1 mM C17−CA and ksaSCA = 2.19 min−0.5 for 0.05 mM F15C18−PA. The rate constant for the kinetics of the phosphonic acid selfassembly exceeds that of the carboxylic acid by half an order of magnitude, although the concentration of F15C18−PA was significantly smaller than the concentration of C17−CA. Even the rate constant for the lowest concentrated 0.005 mM F15C18−PA solution (ksaSCA = 0.34 min−0.5) is in the same range as the carboxylic acid. The concentration of C17−CA however is 200 times higher. We report on rate constants for different concentrations of C17−CA (1 mM) and F15C18−PA (0.005 and 0.05 mM) due to the fact that these values closely relate to optimized concentrations for the practical purpose of those molecules. While lower concentrations for C17−CA lead to very slow SAM formation without saturation in a suitable time scale, higher concentrations (e.g., 1 mM) of phosphonic acids cause etching of the AlOx surface.26 Bearing in mind that the initial surface decoration with IPA is substituted by the corresponding acid in both cases, we conclude that the chemical bonding of the phosphonic acid anchor group with ksaSCA = 2.19 min−0.5 (0.05 mM) is faster compared to the carboxylic acid with ksaSCA = 0.43 min−0.5 (1 mM). Due to the different anchor group chemistry, a preferred binding of one species can be expected, which enables an exchange reaction. To investigate the SAM exchange, we immersed substrates decorated with a saturated C17−CA SAM (3 days of immersion time) in a 0.05 mM F15C18−PA solution. The results of SCA measurements as a function of exchange time are displayed in Figure 1c (rhombi). Starting from a contact angle of approximately 107.0 ± 0.7°, the curve progression of the

Figure 3. (a) Schematic setup of a TFT with hybrid dielectric (AlOx/ SAM) and DH6T semiconductor. (b) Square root of the drain current of TFT devices with pure SAMs and SAMs at different exchange times (forward and backward measurement). (c) Threshold voltage and capacitance of SAM devices as function of exchange time (closed symbols) and corresponding reference values (open symbols) for devices without exchange.

Table 1. Electrical Characteristics of Reference Devices and for Different Exchange Times (Extracted from at Least Five Devices for Each Set) sample

VTH (mV)

capacitance (μF/cm2)

Mobility ×10−2 (cm2/(V s))

reference: 3 days C17−CA exchange time (min) 1 3 5 10 20 30 60 240 3 days reference: 3 days F15C18−PA

−593 ± 41

1.85 ± 0.13

1.8 ± 0.03

−91 −90 −7.8 106 176 278 383 566 477 507

1.62 1.21 1.67 1.35 1.08 1.44 1.09 1.08 0.95 0.91

± ± ± ± ± ± ± ± ± ±

21 41 33 32 23 41 51 19 38 29

± ± ± ± ± ± ± ± ± ±

0.03 0.04 0.11 0.10 0.16 0.06 0.05 0.07 0.07 0.08

2.1 2.7 2.1 2.6 2.6 2.0 3.0 2.2 1.6 1.5

± ± ± ± ± ± ± ± ± ±

0.01 0.02 0.02 0.01 0.1 0.04 0.07 0.01 0.1 0.02

publications on the kinetics of phosphonic acid selfassembly.22−24 In this work, we investigated and compared the kinetics of SAM formation for a carboxylic acid (C17−CA) and a 13902

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exchange is similar to that of F15C18−PA in direct self-assembly on bare AlOx (triangles). After 3 days of exchange (121.9 ± 1.4°), the SCA is almost identical to the value gained for the direct self-assembly of F15C18−PA (121.8 ± 0.4°). The rate constant for exchange (kexSCA) of 0.21 min−0.5 is smaller than ksaSCA obtained for direct self-assembly but in the same range as the one obtained for lower concentrations of F15C18−PA or the self-assembly of C17−CA. These results suggest first that a carboxylic acid SAM can be fully exchanged by the phosphonic acid SAM and second that the preoccupied surface with C17− CA SAM slightly hinders the access of F15C18−PA (ksaSCA > kexSCA). We note again that, in the cross experiment (potential exchange of a F15C18−PA SAM for C17−CA), the initial value of above 120° remains unaffected. Figure 2b shows the results of leakage current measurements on the hybrid dielectrics formed by direct self-assembly or exchange reactions compared to untreated AlOx. The current density determined at −1.5 V is reduced from 3.3 × 10−4 A/ cm2 for bare AlOx to 1.2 × 10−4 A/cm2 for C17−CA SAM hybrid dielectrics and to 7.1 × 10−6 A/cm2 for F15C18−PA hybrid dielectrics. The SAM created by the exchange reaction (3 days immersion in C17−CA followed by 3 days immersion in F15C18−PA) exhibits a value of 1.6 × 10−5 A/cm2, which is smaller than that of C17−CA but larger than that obtained for pure F15C18−PA (Figure 1b, inset). The values for current densities are in the same range as the ones obtained in previous experiments.26 In all cases, the SAM treatment leads to improved insulation behavior and breakdown voltages of the hybrid dielectrics compared to bare AlOx. This is particularly noteworthy for C17−CA, since carboxylic acid SAMs are not commonly used as part of the dielectric in organic TFTs.27 Pure and/or mixed SAMs of phosphonic acids can be used in organic TFTs in a hybrid dielectric stack to enable low voltage operation and to tune the threshold voltage (VTH).7,10,28,29 VTH describes the turn-on characteristics of a TFT. Plotting the square root of drain current versus the gate voltage (Figure 3b), VTH is determined by the intersection of the curvès slope with the x axis. Here, we show that not only phosphonic acids but also molecules with a carboxylic acid anchor group can act as dielectric SAMs in hybrid stacks (Figure 3a). This also applies to mixed SAMs consisting of C17−CA and F15C18−PA formed by an exchange reaction. According to the different dipole moments of the SAM molecules, we obtained a shift in the threshold voltage from VTH = −593 ± 41 mV for pure C17−CA SAM to VTH = +477 ± 37 mV for an initially C17−CA terminated surface, which was almost fully exchanged by fluorinated F15C18−PA. The values for different exchange times, which correlate to different mixing ratios of C17−CA and F15C18−PA, occur in between those boundary values (Figure 3b, c). A strong shift is obtained for short exchange times, before the VTH change decelerates for longer exchange times and finally saturates to the value obtained for devices fabricated by direct self-assembly of F15C18−PA without exchange reaction (VTH = +507 ± 29 mV). We note that all TFTs show average charge carrier mobilities of 0.015−0.03 cm2/(V s), which is typical for fully patterned TFTs with DH6T on hybrid dielectrics.30 The TFT characteristics are summarized in Table 1, and the dependency of VTH on the exchange time is illustrated in Figure 3c (squares). We adapted the procedure for the evaluation of a rate constant from the contact angle measurement to the time-dependent VTH shift and obtained a rate constant (kexVTH) of 0.29 min−0.5. This is in good

agreement with the value obtained by SCA measurements (kexSCA = 0.21 min−0.5). Additionally, the capacitances (measured in stacked devices with different exchange times) show comparable trends regarding the time-dependent decrease in capacitance due to the exchange reaction. We attained a decrease in capacitance from 1.85 ± 0.2 μF/cm2 for pure C17−CA to 0.95 ± 0.1 μF/ cm2 for fully exchanged layers of F15C18−PA. The reference device with directly self-assembled F15C18−PA exhibits a capacitance of 0.91 ± 0.1 μF/cm2. The rate constant of the exchange evaluated from capacitance (kexCAP = 0.23 min−0.5) is again in good agreement with kexSCA and kexVTH obtained from SCA and VTH measurements, respectively. Hence, the rate constant of exchange can be determined by means of three independent approaches with very high accuracy (kexSCA ∼ kexVTH ∼ kexCAP ∼ 0.22 min−0.5). We investigated an additional molecular system in order to generalize the approach of SAM exchange reactions as powerful tool to tune device characteristics such as VTH. In this case, a partially fluorinated carboxylic acid (2H,2H,3H,3H-perfluoroundecanoic acid; F17C10−CA; used as received from Sigma− Aldrich GmbH; see the Supporting Information, Figure S1) was deposited on Al/AlOx up to saturation (SCA = 113.7 ± 0.8°) and investigated in TFTs with the same setup as described above. On a second substrate, a SAM of F17C10−CA was substituted with n-tetradecylphosphonic acid (C14−PA; used as received from PCI Synthesis; see the Supporting Information, Figure S1) (SCA = 111.6 ± 0.7°) followed by electrical characterization. Finally, SAMs of C14−PA were investigated as reference samples without exchange reactions (SCA = 108.8 ± 0.8°). The TFTs with F17C10−CA as part of the hybrid gate dielectric show a threshold voltage of VTH = +355 ± 30 mV. After an exchange reaction with C14−PA (2 days), we obtained a shift in threshold voltage to VTH = −493 ± 20 mV. This confirms that we can shift VTH in any direction by varying the backbone substitution of the carboxylic acid and the phosphonic acid (and thus the dipole moments). The reference devices fabricated on SAMs of C14−PA show a VTH = −599 ± 35 mV, thus indicating that the exchange reaction is almost fully completed after 2 days.

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CONCLUSION

With these findings, we conclude that suitable molecules with a phosphonic acid or carboxylic acid anchor group tend to selfassemble on AlOx surfaces with different rate constants. Phosphonic acids self-assemble much faster than corresponding carboxylic acids. SAMs based on densely packed carboxylic acids or phosphonic acids can improve the insulating properties of hybrid dielectrics as demonstrated in capacitors and in functional organic TFTs with DH6T as semiconductor. Apart from the direct self-assembly of phosphonic acids, SAMs consisting of carboxylic acid molecules can be partially or fully substituted by phosphonic acid molecules. According to the molecular fashion of the chains and the time of the exchange reaction, the properties of the resulting devices (capacitors or TFTs) reflect the degree of substitution and the composition of the mixed SAM. Hence, key device parameters such as VTH can be shifted in different directions and thus tuned to a large extent depending on the molecular structure of the backbones. 13903

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ASSOCIATED CONTENT

S Supporting Information *

Supplementary details on calculations of the rate constants, the substitution of F17C10−CA with C14−PA, and transfer and output characteristics for the substitution of C17−CA with F15C18−PA. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 49 9131 8527732; fax: 49 9131 8528321; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the funding of the German Research Council (DFG), HA 2952/4-1, the “Excellence Initiative” supporting the Cluster of Excellence “Engineering of Advanced Materials” (www.eam.uni-erlangen.de), and the Erlangen Graduate School of Molecular Science (GSMS).

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