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Hanno Dietrich, Simon Scheiner, Luis Portilla, Dirk Zahn,* and Marcus Halik* Self-assembled monolayers (SAMs) have been established as tuning and functional layers in organic electronic devices. They serve as promoting layers for semiconductor growth, injection layers at electrodes, ultrathin dielectrics, or even as monolayer semiconductors in SAM field-effect transistors.[1–5] Typically chained molecules with substrate-specific anchor groups selfassemble in a top-down approach, where the SAM functionality is already incorporated by the molecular structure. However, some molecules exhibit the potential of post-self-assembly modification due to certain binding or reaction motifs.[6] Examples are molecules consisting of oligo-ethylene-glycol (OEG) chains, well-known as promoter surfaces for protein adsorption.[7] Moreover, OEG monolayers provide a very suited environment to coordinate cations by oxygen atoms. These binding motifs tend to form cage structures within the SAM, comparable to crown ethers[8,9] or cryptates.[10] Recently it was shown that OEG-based molecules with thiol anchors—which form striped patterns on gold nanoparticles in combination with short n-alkyl thiols—can efficiently and selectively trap cations and thus serve as a sensitive sensor.[11] In addition to that, we suggest that a conceivable application of such ion doping could also be the enhancement of organic thin-film transistors. The widely used combination of phosphonic acids (PAs) self-assembled on alumina has been shown to be an excellent choice for such devices as the alumna layer provides an additional insulation toward the aluminum gate and the PAs bind strongly to the surface.[12] However, the packing of PAs differs from that of thiols on gold and the surface is flat as compared to the curved nanoparticles. This in turn raises three questions: first, what are the structures, dimensions, and abundances of vacancies in densely packed monolayers and will cation trapping from solution occur in a selective manner? Second, what is the nature of the trapped species in a dried SAM considering the overall preference of charge neutrality of
H. Dietrich, Prof. D. Zahn Theoretical Chemistry and Computer-Chemistry-Center (CCC) FAU Erlangen-Nürnberg Nägelsbachstraße, 25, 91052 Erlangen, Germany E-mail: [email protected] S. Scheiner, L. Portilla, Prof. M. Halik Organic Materials & Devices (OMD) Institute of Polymer Materials FAU Erlangen-Nürnberg Martensstraße, 7, 91058 Erlangen, Germany E-mail: [email protected]
DOI: 10.1002/adma.201503911
Adv. Mater. 2015, 27, 8023–8027
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Improving the Performance of Organic Thin-Film Transistors by Ion Doping of Ethylene-Glycol-Based Self-Assembled Monolayer Hybrid Dielectrics
the system? And finally, can ion doping be employed to boost device performance? Inspired by recent success in rationalizing SAMs of PAs on alumina from molecular dynamics (MD) simulations, in the present work we analyze ion trapping in SAMs from both theory and experiment. Along this line, molecular scale insights derived from modelling are used for rationalization and designing experimentally assessed device performance by the example of 2-(2-(2-methoxyethoxy)ethoxy)ethyl)phosphonic acid (CH3(OC2H4)3-PA). As a starting point, we prepared a densely packed ≈2.5 nm × 3 nm SAM of OEG molecules bound to aluminum oxide in the absence of solvent. The relaxed structure was compared to the density functional theory (DFT) and molecular mechanics calculations of Grunze and co-workers who established that OEG moieties assume a helical or all-trans conformation in hexagonally structured SAMs depending on the packing density.[13,14] From the structural analyses of this work, ion incorporation appears favorable for the all-trans arrangement of the OEG chains, which applies to molecular spacing in the SAM of less than 4.38 Å, whereas the molecules curl into helices for less densely packed systems. The latter is the case even for the dense packing of PAs on sapphire (0001) with a spacing of 4.77 Å and indeed we observe helical structures for the pristine OEG SAM when modelled in the absence of solvent. However, since ion doping is commonly conducted in a wet-chemical approach, we decided to explicitly model a SAM– water interface. The following analyses are based on “solvated” SAM models, subjected to relaxation and sampling of statistical data from MD simulation runs. In the course of relaxation, the upper part of the OEG moieties soaks with water by incorporating small clusters of three to four water molecules. In this process, the OEG moieties bend and their conformational order is considerably disturbed. These findings agree well with the results of a Monte Carlo study carried out by Grunze and co-workers[15] We then explored the association of small cations and anions to OEG SAMs on the basis of a set of MD simulations as listed in Table 1. Ion pairs of alkali and ammonium chloride ions were placed in the layer of water molecules interfacial to the SAM. Rapid dissolution of the ion pairs was observed which is in line with the high solubility of the chosen compounds. While the chloride ion is migrated to the water phase, we observed that the cations may form salt bridges to the OEG SAM if ionic size is sufficiently small. Indeed, in our list of compounds, the only exception to such binding is given by N(CH3)4+ whose excessive size disfavors coordination to the ether oxygen atoms of
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www.MaterialsViews.com Table 1. Data from molecular dynamics simulations of OEG SAMs with different ion pairs in water and in vacuum: average coordination bond lengths dcoord between the cation (nitrogen atom for NH4+ and N(CH3)4+) and oxygen atoms from glycol and water. The prevailing coordination number is given for all types of oxygen atoms, while the number of coordinating water molecules is provided in parenthesis for solvated systems. The relative dipole moment prel is calculated as the overall change of polarization by ion pair incorporation and thus accounts for structural relaxation of the SAM. In the ion-free SAM, the dipole moment per OEG molecule amounts to −2.61 ± 0.16 D. dcoord [Å]
Preval. Coord. Nr.
prel [D]
Water
Vac
Water (H2O)
Vac
Vac
LiCl
2.06
2.03
5 (3)
4
−5.81
NaCl
2.41
2.39
6 (1)
5
−7.15
KCl
2.70
2.69
7 (1)
6
−8.53
NH4Cl
2.79
2.86
7 (0)
6/7
−11.34
N(CH3)4Cl
4.34
3.72
18–21
9
−16.68
the ethylene glycol moieties. On the other hand, the Li+, Na+, K+, and ammonium ions are trapped in the SAM despite still being partially coordinated by water molecules (Figure 1, see also Table 1 for distance analyses). The number of coordinating water molecules decreases with increasing ion radius, whereas the overall number of coordinating oxygen atoms (including those of OEG) increases due to the larger radius of the coordinating sphere. These two reverse trends entail an increase in the number of coordinating bonds formed by the OEG moieties leading to a stronger chelate effect. This implies a stronger binding of larger cations and accounts for the selectivity for certain ions. On the other hand, the example of N(CH3)4+ shows that too large ions do not fit into the OEG- spacing and would require excessive deformation of the SAM. The final step in wet-chemistry-based syntheses reflects the drying of the SAM devices. Removing all water molecules in
our simulation models leads to the association of all ions to the SAM. Nevertheless, the loosely attached tetramethylammonium-chloride ion pair is expected as too unstable to affect the SAM properties, whilst we suggest strong binding of the smaller cation–chloride ions pairs (Table 1). Along this line, persistent dipoles result from cation salt bridges to the OEG moieties, hence favoring penetration into the SAM, whereas chloride exclusively binds to the cation and is repelled by the SAM. This interplay of electrostatics and ion size effects is depicted exemplarily for LiCl and KCl in Figure 2. Using statistical data from 200 ns MD simulations we also calculated histograms of the z-position of the chloride ion and corresponding cation (Figure 2). The most significant implication here is the directional dipole moment that faces towards the SAM surface. The formation process implies that the cations bind first (i.e., in the solvated system) and the anions only after drying. As a consequence, dipole moments of individual ion pairs systematically sum up and eventually give rise to a large net dipole moment perpendicular to the surface. In Table 1, we show the average dipole moments prel of various ion pairs as calculated for dried SAMs. For better rationalization, all values are given with respect to that of the pristine (ion-free) SAM. A clear trend towards stronger dipole moments for larger cation species is observed. This can be attributed to the increase in the ion–ion distance caused by the larger radius of the cation. In combination with the beforehand discussed chelate and size effects, we expect the enhancement effect to be strongest for potassium chloride and ammonium chloride. It is intuitive to expect the observed permanent dipoles to affect the conducting properties of SAM-based electronic devices. In particular, the outermost negative charge of the ion pairs should cause an enrichment of electron holes, and thus of charge carriers if a p-type semiconducting layer is deposited above the SAMs. To test this hypothesis, ion doping of the SAMs was experimentally performed via a simple two step immersion process
Figure 1. Overview of CH3-(OC2H4)3-PA-SAM with sodium chloride and water and detail snippet showing the binding situation of the sodium ion.
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COMMUNICATION Figure 2. Center: histograms of z-position of the cation (blue) and chloride counter-ion (green) in the OEG–SAM in vacuum with respect to the approximate top of the SAM for lithium chloride (left) and potassium chloride (right). Left, right: accurate scale snapshots of the two systems showing the orientation and binding of the ion pair.
of the previously deposited SAM in an aqueous solution of alkali salts, lithium chloride (LiCl), sodium chloride (NaCl), and potassium chloride (KCl), respectively. The molecules in the SAM have a PA functional anchor group for binding on the AlOx surface, a short alkyl chain and three oxygen atoms in the chain for interaction with the cations similar to crown ether complex formation. Due to the thin SAM, the amount of adsorbed ions is expected to be low and thus challenging to detect. Hence, for XPS measurements the accessible surface was increased by using AlOx nanoparticles. A relatively large diameter of 50 nm was chosen in order to retain a low curvature and thus offer the same binding conditions. The particles were functionalized with the ethylene-glycol-based molecules, doped by the same immersion process with an ionic solution, washed, re-dispersed and spray-coated on silicon wafers. The XPS measurements show that all the different cations bind in the layer and also the anion is detected which supports the simulation results in vacuum (Figure SI-4, Supporting Information). The electrical evaluation of the devices revealed a direct effect of doping in the performance. The capacitance of the different layers was determined in a simple Al–AlOx–SAM–Au stack. Thin-film transistor devices in a bottom gate, staggered (top contact) setup (Figure 3a) were used with evaporated aluminum gate structures, which were partially plasma oxidized and covered with the SAM. This hybrid layer of a thin layer of AlOx and a CH3-(OC2H4)3-PA SAM acts as dielectric. As semiconducting material the asymmetric 2-tridecyl[1]benzothieno[3,2-b][1]benzothiophene (C13-BTBT) was utilized, a small molecule which showed excellent mobilities in previous experiments.[16,17] Figure 3b shows the changes in capacitance as direct effect of the doping of the SAM. In Table 2 the corresponding capacitance data of the layers at 1 V at 500 kHz can be seen. The reference capacitance of the hybrid AlOx/SAM layer is 1.68 µF cm−1 and by incorporation of the different cations the capacitance can be reproducibly raised by about 0.1 µF cm−1. From a comparison of the capacitance values for the alkali ions in Table 1 it can be seen that the ion radius hardly affects the capacitance. Furthermore, experiments with different concentrations of ions in the stock solution (ranging from 0.05 × 10−3 to 1.0 × 10−3 M) revealed that the capacitance is not dependent on the concentration of ions in
Adv. Mater. 2015, 27, 8023–8027
solution. This indicates a low saturation doping concentration of the SAMs. In Figure 3c, the influence of the doping on transistor performance, especially on the drain current and the threshold voltage is illustrated exemplarily. The change of the drain current related to the cation doping occurs with statistical relevance, documented by at least six transfer curves depicted in Figure SI-5 (Supporting Information). The corresponding data in Table 2 also show that most importantly the mobility of the semiconductor is increased. The reference mobility of the pure CH3-(OC2H4)3-PA SAM devices is around 2.30 cm2 V−1 s−1. The doping of lithium chloride only slightly increases the mobility to 2.39 cm2 V−1 s−1 whereas the sodium chloride increases the mobility to 3.22 cm2 V−1 s−1. In excellent agreement with the predictions from our MD simulations, the best enhancement of the performance was reached by potassium doping which increased the mobility up to 5.07 cm2 V−1 s−1 in average with a peak value of 8.32 cm2 V−1 s−1. We address the increased mobility to an increased charge carrier density induced by the local dipole field.[18] The slightly reduced “overall performance” of transistors compared to devices with the same semiconductor but on other SAMs[16] can be attributed to the surface energy of the SAM which influences the semiconductor orientation and morphology at the interface which in turn directly influences the performance.[19,20] Additionally, the decrease of the absolute threshold voltage Vth for the doped samples can be seen in Figure 3c. Vth of the reference sample is −2.91 V whereas the doping with potassium shifts it to −2.78 V, for lithium to −2.70 V, and for sodium to −2.68 V. These changes in performance cannot solely be explained by a change in the morphology of the semiconductor since AFM images (Figure SI-6, Supporting Information) show a similar growth for doped and pristine SAM. The threshold voltage shift can be explained by the increased dipole moment of the doped SAMs, observed in the simulations (Table 1). This also fits previous observations of changing Vth at different dipole moments of the dielectric SAM layer.[21] The transfer curves in Figure 3c show a moderate hysteresis of the drain current. The hysteresis however is not directly related to the cation doping— compare reference sample—and is attributed to transient
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Figure 3. a) Schematic setup of the used transistor stack. b) Electrical evaluation of the ion-doped SAMs in a fabricated Al–AlOx–SAM–Au capacitor stacks at different frequencies and c) characteristic transfer curves of the transistors with corresponding gate current.
charging the semiconductor–dielectric interface. Thus, the magnitude of the hysteresis is influenced by the measuring speed on the device and can be reduced by slower measuring routines. Also the leakage current is not significantly increased by the cation doping of the SAM, suggesting that the devices do not exhibit ion conductivity through the doped SAM. Additional experiments were carried out with the potassium halides fluoride, chloride and bromide to investigate the influence of the anion size on binding affinity and device properties. An overview of transistor characteristics with different anions can be found in Supporting Information Figure SI-7. However, the type of anion did not affect the device performance nearly as much as the choice of cation. In summary, we demonstrate the doping of SAMs with ion pairs. The MD simulations correctly predict the binding affinities of various cations and the order for the magnitude of impact on device properties. The corresponding experiments showed that the charge carrier mobility in organic thin-film transistors can be enhanced by a factor of up to 3. In addition, the threshold voltage is shifted to lower absolute values. Doping by suitable ions is hence suggested as a simple-to-use method Table 2. Measured and calculated characteristic values of the fabricated reference and ion-doped devices with standard deviation.
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Mobility [cm2 V−1 s−1]
Capacitance [µF cm−2]
Vth [V]
On/off ratio
Id/Ig
Pure SAM
2.30 ± 0.41
1.68 ± 0.02
−2.91 ± 0.05
4 × 105
3 × 103
LiCl
2.39 ± 0.62
1.76 ± 0.02
−2.70 ± 0.08
1×
106
2 × 103
NaCl
3.22 ± 0.58
1.79 ± 0.03
−2.68 ± 0.02
1 × 106
1 × 103
KCl
5.07 ± 0.61
1.76 ± 0.04
−2.78 ± 0.12
3×
2 × 103
wileyonlinelibrary.com
105
for tuning the dielectric layer and thus improving device performance and lower voltage demand.
Experimental Section As substrate surface of a rigid model, a hydroxyl terminated 2.48 nm × 2.86 nm sapphire (0001) slab with a thickness of one unit cell was used. To this 2D periodic structure, a total of 36 CH3-(OC2H4)3-PA molecules was bound to yield a SAM with a packing density of 5.07 nm−2 via monodentate binding. At the PA-substrate contact, a hydroxide ion was removed to account for the condensation reactions as reported in references.[22,23] Prior to the addition of ions and/or water, the SAM was equilibrated and heated to 300 K. A slab of water molecules was added above the pre-equilibrated OEG SAM amounting to a film thickness of about 30 Å. The Generalized Amber Force Field[24] was used for the PAs with improved O–P–O angle interactions as suggested by Meagher et al.[25] in combination with the TIP3P water model[26] and corresponding ion parameters by Åqvist.[27] The aluminum oxide was handled with the CLAYFF force field[28] during relaxation and kept fixed during the course of the simulations to save computational effort. For the binding of PAs, a Morse function was fitted to match DFT calculations outlined in the supporting information. Electrostatics were treated with the shifted force algorithm using a cutoff of 12 Å. The MD simulations were carried out using the DL_POLY CLASSIC code version 1.9[29] with slight modifications to allow for a 2D-periodic isothermal-isobaric (NPT) simulation imposing an impenetrable boundary to the surface of the water film.[23] For the MD simulations, a time step of 1 fs was chosen. SAM hydration runs were performed for a total of 20 ns and the dried SAMs were characterized from 200 ns. The CH3-(OC2H4)3-PA was purchased from Sikémia. LiCl, NaCl, and KCl were purchased from Sigma-Aldrich. The semiconductor 2-tridecyl[1]benzothieno[3,2-b][1]benzothiophene (C13-BTBT) was synthesized by Heraeus Precious Metals GmbH & Co. KG.
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Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements H.D. and S.S. contributed equally to this work. The authors acknowledge gratefully the German Research Council (DFG), the Cluster of Excellence “Engineering of Advanced Materials— EXC 315” (www.eam.uni-erlangen.de), the Erlangen Graduate School of Molecular Science (GSMS) and the Graduate School 1161 “Disperse Systems for Electronic Applications” funded by DFG and Evonik Degussa GmbH, Germany for financial support. Received: August 11, 2015 Revised: September 29, 2015 Published online: November 2, 2015
[1] K. P. Pernstich, S. Haas, D. Oberhoff, C. Goldmann, D. J. Gundlach, B. Batlogg, A. N. Rashid, G. Schitter, J. Appl. Phys. 2004, 96, 6431. [2] M. Alt, J. Schinke, S. Hillebrandt, M. Hänsel, G. Hernandez-Sosa, N. Mechau, T. Glaser, E. Mankel, M. Hamburger, K. Deing, W. Jaegermann, A. Pucci, W. Kowalsky, U. Lemmer, R. Lovrincic, ACS Appl. Mater. Interfaces 2014, 6, 20234. [3] H. Klauk, U. Zschieschang, J. Pflaum, M. Halik, Nature 2007, 445, 745.
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[4] T. Schmaltz, A. Y. Amin, A. Khassanov, T. Meyer-Friedrichsen, H.-G. Steinrück, A. Magerl, J. J. Segura, K. Voitchovsky, F. Stellacci, M. Halik, Adv. Mater. 2013, 25, 4511. [5] M. Halik, A. Hirsch, Adv. Mater. 2011, 23, 2689. [6] C. Nicosia, J. Huskens, Mater. Horiz. 2014, 1, 32. [7] K. L. Prime, G. M. Whitesides, Science 1991, 252, 1164. [8] S. Flink, B. A. Boukamp, A. Van Den Berg, F. C. J. M. Van Veggel, D. N. Reinhoudt, J. Am. Chem. Soc. 1998, 120, 4652. [9] S. Flink, F. C. J. M. Van Veggel, D. N. Reinhoudt, J. Phys. Chem. B 1999, 103, 6515. [10] R.-C. Brachvogel, F. Hampel, M. von Delius, Nat. Commun. 2015, 6, 7129. [11] E. S. Cho, J. Kim, B. Tejerina, T. M. Hermans, H. Jiang, H. Nakanishi, M. Yu, A. Z. Patashinski, S. C. Glotzer, F. Stellacci, B. A. Grzybowski, Nat. Mater. 2012, 11, 978. [12] S. P. Pujari, L. Scheres, A. T. M. Marcelis, H. Zuilhof, Angew. Chem. Int. Ed. 2014, 53, 6322. [13] A. J. Pertsin, M. Grunze, I. A. Garbuzova, J. Phys. Chem. B 1998, 102, 4918. [14] R. L. C. Wang, H. J. Kreuzer, M. Grunze, J. Phys. Chem. B 1997, 101, 9767. [15] A. J. Pertsin, M. Grunze, Langmuir 2000, 16, 8829. [16] A. Y. Amin, A. Khassanov, K. Reuter, T. Meyer-Friedrichsen, M. Halik, J. Am. Chem. Soc. 2012, 134, 16548. [17] L. Portilla, S. H. Etschel, R. R. Tykwinski, M. Halik, Adv. Mater. 2015, 27, 5950. [18] S. Kobayashi, T. Nishikawa, T. Takenobu, S. Mori, T. Shimoda, T. Mitani, H. Shimotani, N. Yoshimoto, S. Ogawa, Y. Iwasa, Nat. Mater. 2004, 3, 317. [19] M. Novak, T. Schmaltz, H. Faber, M. Halik, Appl. Phys. Lett. 2011, 98, 093302. [20] A. Y. Amin, K. Reuter, T. Meyer-Friedrichsen, M. Halik, Langmuir 2011, 27, 15340. [21] M. Salinas, C. M. Jäger, A. Y. Amin, P. O. Dral, T. Meyer-Friedrichsen, A. Hirsch, T. Clark, M. Halik, J. Am. Chem. Soc. 2012, 134, 12648. [22] C. Meltzer, J. Paul, H. Dietrich, C. M. Jäger, T. Clark, D. Zahn, B. Braunschweig, W. Peukert, J. Am. Chem. Soc. 2014, 136, 10718. [23] C. Meltzer, H. Dietrich, D. Zahn, W. Peukert, B. Braunschweig, Langmuir 2015, 31, 4678. [24] J. M. Wang, R. M. Wolf, J. W. Caldwell, P. A. Kollman, D. A. Case, J. Comput. Chem. 2004, 25, 1157. [25] K. L. Meagher, L. T. Redman, H. A. Carlson, J. Comput. Chem. 2003, 24, 1016. [26] W. L. Jorgensen, J. Chandrasekhar, J. D. Madura, R. W. Impey, M. L. Klein, J. Chem. Phys. 1983, 79, 926. [27] J. Åqvist, J. Phys. Chem. 1990, 94, 8021. [28] R. T. Cygan, J.-J. Liang, A. G. Kalinichev, J. Phys. Chem. B 2004, 108, 1255. [29] W. Smith, T. Forester, I. Todorov, The DL POLY Classic User Manual, STFC Daresbury Laboratory, Daresbury, UK 2012.
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Aluminum gate structures were fabricated via thermal evaporation in vacuum on Si/SiO2 wafers. The Al structures were shortly plasma oxidized to form a thin 3.6 nm thick AlOx dielectric layer. Densely packed SAMs were deposited by a solution-based immersion process of CH3-(OC2H4)3-PA in 2-propanol (0.2 × 10−3 M, 24 h, RT). The SAMs were then characterized with static contact angle measurements to ensure the high quality of the SAMs. The SAM layers were afterward immersed in 1 × 10−3 M aqueous solutions of lithium chloride, potassium chloride, and sodium chloride for 120 s, subsequently rinsed with H2O and nitrogen dried. The capacitances were determined on a simple capacitor setup of Al–AlOx–SAM–Au with a capacitor area of 2500 µm2. Transistor devices were fabricated by vacuum sublimation of 20–25 nm of C13-BTBT onto the SAM at a substrate temperature of 60 °C followed by the deposition of 30 nm of gold (Au) as source/drain electrodes. The structuring of the electrodes and the semiconductor layer was done by the use of shadow masks. The considered transistor devices have a channel width of 500 µm and a channel length of 300 µm. The electrical evaluation of the devices was done with a micromanipulator setup coupled to an Agilent B1500A parameter analyzer. X-ray photoelectron scattering data were obtained using a Physical Electronics 5600 XPS tool.
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Hanno Dietrich, Simon Scheiner, Luis Portilla, Dirk Zahn,* and Marcus Halik* Self-assembled monolayers (SAMs) have been established as tuning and functional layers in organic electronic devices. They serve as promoting layers for semiconductor growth, injection layers at electrodes, ultrathin dielectrics, or even as monolayer semiconductors in SAM field-effect transistors.[1–5] Typically chained molecules with substrate-specific anchor groups selfassemble in a top-down approach, where the SAM functionality is already incorporated by the molecular structure. However, some molecules exhibit the potential of post-self-assembly modification due to certain binding or reaction motifs.[6] Examples are molecules consisting of oligo-ethylene-glycol (OEG) chains, well-known as promoter surfaces for protein adsorption.[7] Moreover, OEG monolayers provide a very suited environment to coordinate cations by oxygen atoms. These binding motifs tend to form cage structures within the SAM, comparable to crown ethers[8,9] or cryptates.[10] Recently it was shown that OEG-based molecules with thiol anchors—which form striped patterns on gold nanoparticles in combination with short n-alkyl thiols—can efficiently and selectively trap cations and thus serve as a sensitive sensor.[11] In addition to that, we suggest that a conceivable application of such ion doping could also be the enhancement of organic thin-film transistors. The widely used combination of phosphonic acids (PAs) self-assembled on alumina has been shown to be an excellent choice for such devices as the alumna layer provides an additional insulation toward the aluminum gate and the PAs bind strongly to the surface.[12] However, the packing of PAs differs from that of thiols on gold and the surface is flat as compared to the curved nanoparticles. This in turn raises three questions: first, what are the structures, dimensions, and abundances of vacancies in densely packed monolayers and will cation trapping from solution occur in a selective manner? Second, what is the nature of the trapped species in a dried SAM considering the overall preference of charge neutrality of
H. Dietrich, Prof. D. Zahn Theoretical Chemistry and Computer-Chemistry-Center (CCC) FAU Erlangen-Nürnberg Nägelsbachstraße, 25, 91052 Erlangen, Germany E-mail: [email protected] S. Scheiner, L. Portilla, Prof. M. Halik Organic Materials & Devices (OMD) Institute of Polymer Materials FAU Erlangen-Nürnberg Martensstraße, 7, 91058 Erlangen, Germany E-mail: [email protected]
DOI: 10.1002/adma.201503911
Adv. Mater. 2015, 27, 8023–8027
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Improving the Performance of Organic Thin-Film Transistors by Ion Doping of Ethylene-Glycol-Based Self-Assembled Monolayer Hybrid Dielectrics
the system? And finally, can ion doping be employed to boost device performance? Inspired by recent success in rationalizing SAMs of PAs on alumina from molecular dynamics (MD) simulations, in the present work we analyze ion trapping in SAMs from both theory and experiment. Along this line, molecular scale insights derived from modelling are used for rationalization and designing experimentally assessed device performance by the example of 2-(2-(2-methoxyethoxy)ethoxy)ethyl)phosphonic acid (CH3(OC2H4)3-PA). As a starting point, we prepared a densely packed ≈2.5 nm × 3 nm SAM of OEG molecules bound to aluminum oxide in the absence of solvent. The relaxed structure was compared to the density functional theory (DFT) and molecular mechanics calculations of Grunze and co-workers who established that OEG moieties assume a helical or all-trans conformation in hexagonally structured SAMs depending on the packing density.[13,14] From the structural analyses of this work, ion incorporation appears favorable for the all-trans arrangement of the OEG chains, which applies to molecular spacing in the SAM of less than 4.38 Å, whereas the molecules curl into helices for less densely packed systems. The latter is the case even for the dense packing of PAs on sapphire (0001) with a spacing of 4.77 Å and indeed we observe helical structures for the pristine OEG SAM when modelled in the absence of solvent. However, since ion doping is commonly conducted in a wet-chemical approach, we decided to explicitly model a SAM– water interface. The following analyses are based on “solvated” SAM models, subjected to relaxation and sampling of statistical data from MD simulation runs. In the course of relaxation, the upper part of the OEG moieties soaks with water by incorporating small clusters of three to four water molecules. In this process, the OEG moieties bend and their conformational order is considerably disturbed. These findings agree well with the results of a Monte Carlo study carried out by Grunze and co-workers[15] We then explored the association of small cations and anions to OEG SAMs on the basis of a set of MD simulations as listed in Table 1. Ion pairs of alkali and ammonium chloride ions were placed in the layer of water molecules interfacial to the SAM. Rapid dissolution of the ion pairs was observed which is in line with the high solubility of the chosen compounds. While the chloride ion is migrated to the water phase, we observed that the cations may form salt bridges to the OEG SAM if ionic size is sufficiently small. Indeed, in our list of compounds, the only exception to such binding is given by N(CH3)4+ whose excessive size disfavors coordination to the ether oxygen atoms of
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www.MaterialsViews.com Table 1. Data from molecular dynamics simulations of OEG SAMs with different ion pairs in water and in vacuum: average coordination bond lengths dcoord between the cation (nitrogen atom for NH4+ and N(CH3)4+) and oxygen atoms from glycol and water. The prevailing coordination number is given for all types of oxygen atoms, while the number of coordinating water molecules is provided in parenthesis for solvated systems. The relative dipole moment prel is calculated as the overall change of polarization by ion pair incorporation and thus accounts for structural relaxation of the SAM. In the ion-free SAM, the dipole moment per OEG molecule amounts to −2.61 ± 0.16 D. dcoord [Å]
Preval. Coord. Nr.
prel [D]
Water
Vac
Water (H2O)
Vac
Vac
LiCl
2.06
2.03
5 (3)
4
−5.81
NaCl
2.41
2.39
6 (1)
5
−7.15
KCl
2.70
2.69
7 (1)
6
−8.53
NH4Cl
2.79
2.86
7 (0)
6/7
−11.34
N(CH3)4Cl
4.34
3.72
18–21
9
−16.68
the ethylene glycol moieties. On the other hand, the Li+, Na+, K+, and ammonium ions are trapped in the SAM despite still being partially coordinated by water molecules (Figure 1, see also Table 1 for distance analyses). The number of coordinating water molecules decreases with increasing ion radius, whereas the overall number of coordinating oxygen atoms (including those of OEG) increases due to the larger radius of the coordinating sphere. These two reverse trends entail an increase in the number of coordinating bonds formed by the OEG moieties leading to a stronger chelate effect. This implies a stronger binding of larger cations and accounts for the selectivity for certain ions. On the other hand, the example of N(CH3)4+ shows that too large ions do not fit into the OEG- spacing and would require excessive deformation of the SAM. The final step in wet-chemistry-based syntheses reflects the drying of the SAM devices. Removing all water molecules in
our simulation models leads to the association of all ions to the SAM. Nevertheless, the loosely attached tetramethylammonium-chloride ion pair is expected as too unstable to affect the SAM properties, whilst we suggest strong binding of the smaller cation–chloride ions pairs (Table 1). Along this line, persistent dipoles result from cation salt bridges to the OEG moieties, hence favoring penetration into the SAM, whereas chloride exclusively binds to the cation and is repelled by the SAM. This interplay of electrostatics and ion size effects is depicted exemplarily for LiCl and KCl in Figure 2. Using statistical data from 200 ns MD simulations we also calculated histograms of the z-position of the chloride ion and corresponding cation (Figure 2). The most significant implication here is the directional dipole moment that faces towards the SAM surface. The formation process implies that the cations bind first (i.e., in the solvated system) and the anions only after drying. As a consequence, dipole moments of individual ion pairs systematically sum up and eventually give rise to a large net dipole moment perpendicular to the surface. In Table 1, we show the average dipole moments prel of various ion pairs as calculated for dried SAMs. For better rationalization, all values are given with respect to that of the pristine (ion-free) SAM. A clear trend towards stronger dipole moments for larger cation species is observed. This can be attributed to the increase in the ion–ion distance caused by the larger radius of the cation. In combination with the beforehand discussed chelate and size effects, we expect the enhancement effect to be strongest for potassium chloride and ammonium chloride. It is intuitive to expect the observed permanent dipoles to affect the conducting properties of SAM-based electronic devices. In particular, the outermost negative charge of the ion pairs should cause an enrichment of electron holes, and thus of charge carriers if a p-type semiconducting layer is deposited above the SAMs. To test this hypothesis, ion doping of the SAMs was experimentally performed via a simple two step immersion process
Figure 1. Overview of CH3-(OC2H4)3-PA-SAM with sodium chloride and water and detail snippet showing the binding situation of the sodium ion.
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COMMUNICATION Figure 2. Center: histograms of z-position of the cation (blue) and chloride counter-ion (green) in the OEG–SAM in vacuum with respect to the approximate top of the SAM for lithium chloride (left) and potassium chloride (right). Left, right: accurate scale snapshots of the two systems showing the orientation and binding of the ion pair.
of the previously deposited SAM in an aqueous solution of alkali salts, lithium chloride (LiCl), sodium chloride (NaCl), and potassium chloride (KCl), respectively. The molecules in the SAM have a PA functional anchor group for binding on the AlOx surface, a short alkyl chain and three oxygen atoms in the chain for interaction with the cations similar to crown ether complex formation. Due to the thin SAM, the amount of adsorbed ions is expected to be low and thus challenging to detect. Hence, for XPS measurements the accessible surface was increased by using AlOx nanoparticles. A relatively large diameter of 50 nm was chosen in order to retain a low curvature and thus offer the same binding conditions. The particles were functionalized with the ethylene-glycol-based molecules, doped by the same immersion process with an ionic solution, washed, re-dispersed and spray-coated on silicon wafers. The XPS measurements show that all the different cations bind in the layer and also the anion is detected which supports the simulation results in vacuum (Figure SI-4, Supporting Information). The electrical evaluation of the devices revealed a direct effect of doping in the performance. The capacitance of the different layers was determined in a simple Al–AlOx–SAM–Au stack. Thin-film transistor devices in a bottom gate, staggered (top contact) setup (Figure 3a) were used with evaporated aluminum gate structures, which were partially plasma oxidized and covered with the SAM. This hybrid layer of a thin layer of AlOx and a CH3-(OC2H4)3-PA SAM acts as dielectric. As semiconducting material the asymmetric 2-tridecyl[1]benzothieno[3,2-b][1]benzothiophene (C13-BTBT) was utilized, a small molecule which showed excellent mobilities in previous experiments.[16,17] Figure 3b shows the changes in capacitance as direct effect of the doping of the SAM. In Table 2 the corresponding capacitance data of the layers at 1 V at 500 kHz can be seen. The reference capacitance of the hybrid AlOx/SAM layer is 1.68 µF cm−1 and by incorporation of the different cations the capacitance can be reproducibly raised by about 0.1 µF cm−1. From a comparison of the capacitance values for the alkali ions in Table 1 it can be seen that the ion radius hardly affects the capacitance. Furthermore, experiments with different concentrations of ions in the stock solution (ranging from 0.05 × 10−3 to 1.0 × 10−3 M) revealed that the capacitance is not dependent on the concentration of ions in
Adv. Mater. 2015, 27, 8023–8027
solution. This indicates a low saturation doping concentration of the SAMs. In Figure 3c, the influence of the doping on transistor performance, especially on the drain current and the threshold voltage is illustrated exemplarily. The change of the drain current related to the cation doping occurs with statistical relevance, documented by at least six transfer curves depicted in Figure SI-5 (Supporting Information). The corresponding data in Table 2 also show that most importantly the mobility of the semiconductor is increased. The reference mobility of the pure CH3-(OC2H4)3-PA SAM devices is around 2.30 cm2 V−1 s−1. The doping of lithium chloride only slightly increases the mobility to 2.39 cm2 V−1 s−1 whereas the sodium chloride increases the mobility to 3.22 cm2 V−1 s−1. In excellent agreement with the predictions from our MD simulations, the best enhancement of the performance was reached by potassium doping which increased the mobility up to 5.07 cm2 V−1 s−1 in average with a peak value of 8.32 cm2 V−1 s−1. We address the increased mobility to an increased charge carrier density induced by the local dipole field.[18] The slightly reduced “overall performance” of transistors compared to devices with the same semiconductor but on other SAMs[16] can be attributed to the surface energy of the SAM which influences the semiconductor orientation and morphology at the interface which in turn directly influences the performance.[19,20] Additionally, the decrease of the absolute threshold voltage Vth for the doped samples can be seen in Figure 3c. Vth of the reference sample is −2.91 V whereas the doping with potassium shifts it to −2.78 V, for lithium to −2.70 V, and for sodium to −2.68 V. These changes in performance cannot solely be explained by a change in the morphology of the semiconductor since AFM images (Figure SI-6, Supporting Information) show a similar growth for doped and pristine SAM. The threshold voltage shift can be explained by the increased dipole moment of the doped SAMs, observed in the simulations (Table 1). This also fits previous observations of changing Vth at different dipole moments of the dielectric SAM layer.[21] The transfer curves in Figure 3c show a moderate hysteresis of the drain current. The hysteresis however is not directly related to the cation doping— compare reference sample—and is attributed to transient
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Figure 3. a) Schematic setup of the used transistor stack. b) Electrical evaluation of the ion-doped SAMs in a fabricated Al–AlOx–SAM–Au capacitor stacks at different frequencies and c) characteristic transfer curves of the transistors with corresponding gate current.
charging the semiconductor–dielectric interface. Thus, the magnitude of the hysteresis is influenced by the measuring speed on the device and can be reduced by slower measuring routines. Also the leakage current is not significantly increased by the cation doping of the SAM, suggesting that the devices do not exhibit ion conductivity through the doped SAM. Additional experiments were carried out with the potassium halides fluoride, chloride and bromide to investigate the influence of the anion size on binding affinity and device properties. An overview of transistor characteristics with different anions can be found in Supporting Information Figure SI-7. However, the type of anion did not affect the device performance nearly as much as the choice of cation. In summary, we demonstrate the doping of SAMs with ion pairs. The MD simulations correctly predict the binding affinities of various cations and the order for the magnitude of impact on device properties. The corresponding experiments showed that the charge carrier mobility in organic thin-film transistors can be enhanced by a factor of up to 3. In addition, the threshold voltage is shifted to lower absolute values. Doping by suitable ions is hence suggested as a simple-to-use method Table 2. Measured and calculated characteristic values of the fabricated reference and ion-doped devices with standard deviation.
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Mobility [cm2 V−1 s−1]
Capacitance [µF cm−2]
Vth [V]
On/off ratio
Id/Ig
Pure SAM
2.30 ± 0.41
1.68 ± 0.02
−2.91 ± 0.05
4 × 105
3 × 103
LiCl
2.39 ± 0.62
1.76 ± 0.02
−2.70 ± 0.08
1×
106
2 × 103
NaCl
3.22 ± 0.58
1.79 ± 0.03
−2.68 ± 0.02
1 × 106
1 × 103
KCl
5.07 ± 0.61
1.76 ± 0.04
−2.78 ± 0.12
3×
2 × 103
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105
for tuning the dielectric layer and thus improving device performance and lower voltage demand.
Experimental Section As substrate surface of a rigid model, a hydroxyl terminated 2.48 nm × 2.86 nm sapphire (0001) slab with a thickness of one unit cell was used. To this 2D periodic structure, a total of 36 CH3-(OC2H4)3-PA molecules was bound to yield a SAM with a packing density of 5.07 nm−2 via monodentate binding. At the PA-substrate contact, a hydroxide ion was removed to account for the condensation reactions as reported in references.[22,23] Prior to the addition of ions and/or water, the SAM was equilibrated and heated to 300 K. A slab of water molecules was added above the pre-equilibrated OEG SAM amounting to a film thickness of about 30 Å. The Generalized Amber Force Field[24] was used for the PAs with improved O–P–O angle interactions as suggested by Meagher et al.[25] in combination with the TIP3P water model[26] and corresponding ion parameters by Åqvist.[27] The aluminum oxide was handled with the CLAYFF force field[28] during relaxation and kept fixed during the course of the simulations to save computational effort. For the binding of PAs, a Morse function was fitted to match DFT calculations outlined in the supporting information. Electrostatics were treated with the shifted force algorithm using a cutoff of 12 Å. The MD simulations were carried out using the DL_POLY CLASSIC code version 1.9[29] with slight modifications to allow for a 2D-periodic isothermal-isobaric (NPT) simulation imposing an impenetrable boundary to the surface of the water film.[23] For the MD simulations, a time step of 1 fs was chosen. SAM hydration runs were performed for a total of 20 ns and the dried SAMs were characterized from 200 ns. The CH3-(OC2H4)3-PA was purchased from Sikémia. LiCl, NaCl, and KCl were purchased from Sigma-Aldrich. The semiconductor 2-tridecyl[1]benzothieno[3,2-b][1]benzothiophene (C13-BTBT) was synthesized by Heraeus Precious Metals GmbH & Co. KG.
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Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements H.D. and S.S. contributed equally to this work. The authors acknowledge gratefully the German Research Council (DFG), the Cluster of Excellence “Engineering of Advanced Materials— EXC 315” (www.eam.uni-erlangen.de), the Erlangen Graduate School of Molecular Science (GSMS) and the Graduate School 1161 “Disperse Systems for Electronic Applications” funded by DFG and Evonik Degussa GmbH, Germany for financial support. Received: August 11, 2015 Revised: September 29, 2015 Published online: November 2, 2015
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Aluminum gate structures were fabricated via thermal evaporation in vacuum on Si/SiO2 wafers. The Al structures were shortly plasma oxidized to form a thin 3.6 nm thick AlOx dielectric layer. Densely packed SAMs were deposited by a solution-based immersion process of CH3-(OC2H4)3-PA in 2-propanol (0.2 × 10−3 M, 24 h, RT). The SAMs were then characterized with static contact angle measurements to ensure the high quality of the SAMs. The SAM layers were afterward immersed in 1 × 10−3 M aqueous solutions of lithium chloride, potassium chloride, and sodium chloride for 120 s, subsequently rinsed with H2O and nitrogen dried. The capacitances were determined on a simple capacitor setup of Al–AlOx–SAM–Au with a capacitor area of 2500 µm2. Transistor devices were fabricated by vacuum sublimation of 20–25 nm of C13-BTBT onto the SAM at a substrate temperature of 60 °C followed by the deposition of 30 nm of gold (Au) as source/drain electrodes. The structuring of the electrodes and the semiconductor layer was done by the use of shadow masks. The considered transistor devices have a channel width of 500 µm and a channel length of 300 µm. The electrical evaluation of the devices was done with a micromanipulator setup coupled to an Agilent B1500A parameter analyzer. X-ray photoelectron scattering data were obtained using a Physical Electronics 5600 XPS tool.
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