Organic Electronics 11 (2010) 1476–1482

Contents lists available at ScienceDirect

Organic Electronics

journal homepage: www.elsevier .com/locate /orgel

Letter

The morphology of integrated self-assembled monolayers and theirimpact on devices – A computational and experimental approach

Michael Novak a, Christof M. Jäger b, Armin Rumpel c,d, Henning Kropp b, Wolfgang Peukert c,d,Timothy Clark b, Marcus Halik a,*

a Department of Materials Science, University Erlangen – Nürnberg, Martensstr. 7, 91058 Erlangen, Germanyb Computer–Chemistry-Center and Interdisciplinary Center for Molecular Materials, University Erlangen – Nürnberg, Nägelsbachstrasse 25,91052 Erlangen, Germanyc Institute of Particle Technology, University Erlangen – Nürnberg, Cauerstrasse 4, 91058 Erlangen, Germanyd Erlangen Graduate School in Advanced Optical Technologies (SAOT), Germany

a r t i c l e i n f o a b s t r a c t

Article history:Received 9 March 2010Received in revised form 21 May 2010Accepted 21 May 2010Available online 2 June 2010

Keywords:Self-assembling monolayersMolecular dynamicsMolecular electronics

1566-1199/$ - see front matter � 2010 Elsevier B.Vdoi:10.1016/j.orgel.2010.05.009

* Corresponding author. Tel.: +49 9131 852778528321.

E-mail address: marcus.halik@ww.uni-erlangen.d

We report on a combined experimental and computational study on self-assembled mon-olayers (SAMs). The dielectric properties of SAMs based on n-alkyl phosphonic acids inlarge area thin film devices (organic field-effect transistors with good hole mobilities upto 0.3 cm2 V�1 s�1 at �1 V and capacitors) depend on their chain length, but not consis-tently to theoretical pictures of tunneling through saturated n-alkanes. An unexpected sat-uration of current density with increasing chain length was obtained in devices, whatimpact on the leakage current in transistors and current density in capacitors. This is attrib-uted to different self-assembled monolayer morphology ranging from an amorphous statefor short alkyl chains to a quasi-crystalline state for longer alkyl chains. Molecular dynam-ics (MD) simulations provide a deeper insight into the nature of the three-dimensionalintermolecular interactions and support the proposed SAM morphology. The change inmorphology leads to a reduced effective SAM thickness in devices, which could bedescribed by a Simmons approach quantitatively. The morphological aspect of self-assem-bled molecules is of enormous importance beyond the applications of SAMs in low-voltage,high mobility organic transistors because of it’s relevance for the hole field of molecularscale electronics. It demonstrates that even small changes in the molecular design canchange the molecular interactions and monolayer assembly.

� 2010 Elsevier B.V. All rights reserved.

1. Introduction

Molecules that self-organize into supra-molecularstructures (and that generally consist of an anchor group,hydrophobic chains and a head group) are promising com-ponents in high performance, low cost and/or flexible elec-tronic devices. Because of the specific molecular nature ofthe driving forces for spontaneous film formation, theprocess of self-assembly has the advantage of substrate

. All rights reserved.

32; fax: +49 9131

e (M. Halik).

selectivity and large area coverage, so that it yields denselypacked monomolecular films. Self-assembled monolayers(SAM) represent a key technology for organic electronics,so that several SAM-based devices with improved perfor-mance have been reported [1–6].

Organic transistors are an excellent example of devicesthat make use of the functionality of SAMs over relativelylarge areas. The channel region in particular offers anexcellent opportunity to explore the impact of SAMs. As�95% of the charge transport in the transistor channel oc-curs in the first two layers of the semiconductor moleculesclose to the semiconductor/dielectric interface, it is of greatimportance to understand the effects of interactions at the

M. Novak et al. / Organic Electronics 11 (2010) 1476–1482 1477

interface [7]. For example, Kobayashi et al. have describedthe modification and control of the electrical performancein p-type pentacene thin film transistors by varying thehead groups of the SAM molecules, and have characterizedthe impact of different dipole moments of the SAM mole-cules at the interface [8]. Full control on the threshold volt-age (VTH) and the impact of the thin film morphology of theorganic semiconductor has been demonstrated by using anextended series of self-assembled molecules with differentfunctional groups (chain and head groups) [9].

Apart from the changes in properties that have been ob-served by using different combinations of surfaces and an-chor groups (e.g., thiols, carboxylic acids, silanes, andphosphonic acids), homologous series of molecules withthe same anchor group and chemically comparable molec-ular structure in the chain and head groups (e.g., n-alkanes)behave differently depending on their chain length.Although a temperature-independent exponential decreaseof the tunneling current with increasing molecular chainlength of n-alkanes is expected, several groups have ob-served anomalous behavior with increasing moleculelength. It has been proposed that irregularities or defectsin the SAM composition (loops or collapsed sites) lead to adecreased tunneling distance between the electrodes [10–12]. However, even SAMs that have been shown to formhighly reliable systems, show remarkably reproduciblechanges in device characteristics that cannot be explainedby defects, formed during SAM deposition, but rather mustbe due to morphological changes caused by characteristicintermolecular interactions. Recently, it was shown thatthe properties of organic field-effect transistors vary whenthe chain length of self-assembled n-alkyl phosphonic acidsis modified. The charge carrier mobility and the morphologyof a subsequent pentacene layer are affected [13–15].Approximately, monotonic scaling is expected with increas-ing chain length. However, the leakage current (currentdensity through the monolayer) saturates at long chainlengths using different n-alkyl phosphonic acids in molecu-lar scale hybrid dielectrics. This effect is inconsistent withcurrent theoretical pictures of electron tunneling throughsaturated n-alkanes. Intermolecular interactions are thusclearly of great importance for the assembly process andare instrumental in determining the SAM characteristics.A deeper understanding of these correlations is necessaryin order to be able to construct molecular scale electronicsrationally and to design suitable component molecules.

We now report a combined experimental and computa-tional study of the morphology of SAMs based on n-alkylphosphonic acids. The insulating nature of the SAMs, inparticular those with longer chain lengths, on insulatingAl2O3, means that direct access at atomic scale resolution(e.g., scanning tunneling microscopy – STM) is not possi-ble. Other techniques require extremely flat surfaces (smallangle X-ray scattering – SAX) and/or provide average dataon thin films, lacking in local or 3D-resolution importantfor understanding the intermolecular effects of assembly(XPS, AFM). However, as the conduction mechanism inself-assembled monolayers of saturated alkyl chains hasbeen clearly identified as non-resonant tunneling, electri-cal data can give information about the structural proper-ties and the nature of the molecular ordering [16–19]. We

report divergence of the dependence of the electrical prop-erties of SAM-based capacitors and pentacene transistorsfrom that expected on the basis of extrapolations of thechain length. These observations are consistent with achange in the self-assembled monolayer morphology froman amorphous state for short alkyl chains (C6) to a quasi-crystalline state for alkyl chains from C10 to C18. The datafrom electrical measurements of the thin film devices wereinterpreted using the Simmons model to account for theobserved properties of these layers in terms of their struc-tural organization [19]. We have also used moleculardynamics (MD) simulations as tool to provide deeper in-sight into the nature of the intermolecular interactionsand to identify the most probable molecular assembly ofthe different monolayers. The proposed change in SAMmorphology is consistent with the measurements of thecurrent densities and supported by vibrational sum fre-quency generation (VSFG) spectroscopy results.

2. Experimental details

2.1. Device fabrication

For the fabrication of the organic/inorganic dielectrics,heavily p-doped silicon wafers act as substrate and serveas bottom (gate-) electrode. The inorganic part of the hy-brid dielectric is grown via atomic layer deposition, yield-ing uniform aluminum oxide films of 2 nm thickness. Thedeposition was realized in a commercial Cambridge Inc.ALD reactor at 250 �C with a constant deposition rate of1 Å per cycle. The layer thickness and composition was val-idated by ellipsometry and XPS measurements. The RMSroughness of the almost stoichiometric Al2O3 layer (rela-tive precursor concentration <5%) was determined byAFM (contact mode) to be in the range of 0.2 nm for1 lm2 (Veeco Digital Instruments Dimension 3100). Theorganic part of the dielectric is generated by a self-assem-bling monolayer of n-alkyl phosphonic acids from solution,according to literature [2,14]. The thickness of the self-assembled monolayer correlates to the length of thestretched molecule which was calculated with ChemBioOf-fice 2008 (tilt angles were not considered). The capacitorstacks, containing hybrid dielectric were completed by50 nm gold evaporated through a stencil mask (depositionrate: 0.1 nm s�1), yielding single devices with an electrodearea of 150 � 150 lm2. In the case of organic transistors,preliminary to the deposition of the gold source/drain con-tacts, defining a channel length of 20 lm and a channelwidth of 150 lm, 30 nm pentacene was thermally evapo-rated (rate of 0.01 nm) through another stencil mask.

2.2. Electrical characterization

The electrical characterization of the capacitors andtransistors was performed in ambient conditions by man-ually probing single devices (yield of �90%). The transis-tors and the capacitor stack were contacted from theback via the low noise chuck of the probe station. To guar-antee the intactness of the SAM, the top electrode wasprobed accurately by special probe tips for small targets

1478 M. Novak et al. / Organic Electronics 11 (2010) 1476–1482

(Micromanipulator Probe Tips 7F10). Capacitance mea-surements with a LCZ meter (HP4277) are proceeded inthe voltage range between �2 V and 2 V (double sweep,in steps of 100 mV) at a frequency of 100 kHz. Referencemeasurements to obtain the dielectric constant of theAl2O3 were performed previously (Supplementary data).Each measurement was repeated on at least five devices,yielding identical results. Electrical data of capacitor andtransistor devices were recorded with a semiconductorparameter analyzer (Agilent 4156C). A minimum of 20capacitors of each species, 10 in positive (0 to +2 V) and10 in negative (0 to �2 V) bias polarity were investigatedin single sweep mode, in steps of 40 mV. The thermal sta-bility of the tunneling current through the hybrid dielectricwas preliminary verified over a temperature range of�80 K. The conduction mechanism in the observed voltagerange was identified as direct tunneling, which is generallyaccepted for SAMs of saturated alkyl chains [16,17]. Trans-fer characteristics of at least 10 pentacene transistors foreach SAM are recorded for source–gate voltage VGS be-tween 0 and �2 V in double sweep mode at a source–drainvoltage of VDS = �1 V. For each TFT the charge carriermobility was calculated in the saturation regime usingthe standard formalism for field-effect transistors: l = 2 �L/(W � Cdiel) � (d

pID/dVGS)2, where Cdiel is the measured

dielectric capacitance, ID is the drain current, and VGS isthe gate-source voltage [14,20]. Benefiting from thesmooth substrate the deviations are negligible.

2.3. Simulation

All molecular dynamics simulations were carried out,using the program NAMD [21]. For the initial setup and

Fig. 1. The impact of sub-nanometer changes of the molecule length on device c�1 V (channel dimension 150 � 20 lm2) with different monolayers. (b) Currentmolecule length plot at a voltage of �1 V, extracted from (b). (d) Van der Wmolecular dynamics simulations of several n-alkyl phosphonic acids.

for parts of the analysis the programs antechamber, leap,and ptraj from the Amber package were used [22]. Theparameters are based on the general Amber force field(GAFF) [23]. The Al2O3 surface represents a regular 1 0 0surface with 850 atoms in total (52 � 38 nm) and wasbuild with Materials Studio [24]. The surface atoms wereconstrained in their position during the simulations. Foreach simulation 80 phosphonic acids were placed on thesurface. The phosphonic acids were treated singly deproto-nated (most reasonable approach, due to DFT calculations)and the atomic AM1-VESPA point charges were derivedfrom semiempirical calculations with VAMP 10.0 [25,26].Each system was minimized, equilibrated, and simulatedin vacuo with periodic boundary conditions. The electro-statics were treated with particle-mesh Ewald (PME) longrange electrostatics and a 10 Å cutoff for non-bonded inter-actions [27]. The width of the non-bonded skin was 2.0 Å.Langevin molecular dynamics with a target temperatureof 350 K were performed. The complete simulation timefor each system was 10 ns with an integration time stepof 1 fs.

3. Results and discussion

The morphology and device characteristics of SAMs ofn-alkyl phosphonic acids (C6-, C10-, C14- and C18-PA) wereinvestigated on 2 nm thick atomic layer deposited (ALD)Al2O3 deposited on p-doped single crystalline silicon asback electrode. This stack was chosen to achieve full con-trol of the Al2O3 thickness and to provide a smooth surfaceto ensure that the self-assembly is not significantly af-fected by surface roughness (typical ALD-Al2O3 surfaceroughness RMS of a 1 lm2 is 0.2 nm). In Fig. 1a the drain

haracteristics. (a) Transfer characteristics of pentacene transistors at VDS:density of capacitor devices (150 � 150 lm2). (c) Current density versusaals surface representation of time-averaged structures extracted from

M. Novak et al. / Organic Electronics 11 (2010) 1476–1482 1479

currents (ID) and gate currents (IG) of the pentacene tran-sistor devices with different SAMs are presented. As theelectrical characteristics (ID/IG, ON/OFF ratio, and VTH – Ta-ble 1) scale with the chain length as expected for C6-PA,C10-PA and C14-PA, the devices with C18-PA behave verysimilarly to those with C14-PA. The most remarkable fea-ture is the resemblance in the progress of IG, despite thefact that the C18-PA is approximately 0.5 nm longer thanthe C14-PA. This anomaly is even more pronounced incapacitor devices (Si/ALD-Al2O3/SAM/Au – Fig. 1b) and isconsistent with results found for integrated devices (Al/Al2O3/SAM/Au) with the same molecules [14]. While thevalues of capacitance (measured and calculated) fit thetheoretical monolayer thickness well, the current densities(J) do not follow the theoretical exponential decrease (for aconstant SAM morphology) with molecule length for long-er chains (Fig. 1c). Although the capacitance as an averagearea depending parameter is only sensitive to the meanthickness in an alternating electrical field, the current den-sity (J) is very sensitive to weakest point in the layer. Theresults of MD simulations of the corresponding SAMs onAl2O3 surfaces are shown in Fig. 1d. The MD simulationswere performed for a time scale of 10 ns, which shouldbe long enough to provide insight into the molecular struc-ture of the SAMs. The calculated SAM morphology changeswith increasing chain length from an amorphous state forC6-PA to a more ordered state for C10-PA to highly ordered

Table 1Device parameters of capacitors and pentacene transistors.

SAM Elongated moleculelength da (nm)

Capacitance calculatedb

(lF cm�2)Capacitance m(lF cm�2)

C6-PA 0.91 0.51 0.59C10-PA 1.43 0.46 0.55C14-PA 1.95 0.41 0.43C18-PA 2.44 0.38 0.37

a Calculated using ChemBioOffice 2008.b Capacitor devices.c Transistor devices, parameters calculated using the measured capacitances

Fig. 2. (a) Scheme of the proposed mechanism for C6-PA and C18-PA exemplarilydensity data of capacitor devices (area: 150 � 150 lm2) for n-alkyl phosphonic aC18-PA.

(crystalline) domains separated by gaps for C14-PA and C18-PA.

Considering that these morphologies, and in particularthe gaps, can be penetrated by the subsequently evapo-rated layer (e.g., Au in a capacitor device), a reduced dis-tance between the bottom (Si) and top electrode (Au) isexpected. Schematic sketches of the corresponding devicecross sections are shown in Fig. 2a. Using these assump-tions, the experimental results of the capacitor deviceswere correlated to the Simmons model for tunnelingthrough a thin isolating film to calculate the effectivemonolayer thickness [28]. No gaps are expected for amor-phous SAMs with short chains (C6-PA). The distance be-tween the electrodes is shorter at a gap in the SAM forlonger chains (C18-PA) because the thermally depositedgold can penetrate into the gap to reduce the effectivethickness of the layer. This increases the tunneling currentthrough the monolayer significantly [19,28]. The Simmonsmodel for direct tunneling through large area molecularjunctions was used to fit the experimental data. The tun-neling current density J is given by:

J ¼ J0 / � exp �Affiffiffiffi/

p� �� /þ eVð Þ � exp �A

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi/þ eV

p� �� �

ð1Þ

where

easuredb Mobilityc

(cm2 V�1 s�1)ID/IG

c ON/OFFratioc

Threshold voltageVTh

c (V)

0.20 <10 102 �0.510.21 10 103 �0.760.35 102 104 �0.890.33 102 104 �0.87

of [b].

. (b) Results of Simmons modeling and MD calculations. Fit of the currentcids. (c) Surface topography extracted from MD calculations for C6-PA and

1480 M. Novak et al. / Organic Electronics 11 (2010) 1476–1482

J0 ¼e

2phðbdÞ2ð2Þ

and

A ¼ 4pbdffiffiffiffiffiffiffiffiffi2m�e

ph

ð3Þ

with h being the Planck constant, e the electron charge, b acorrection parameter (0.96) and V the applied voltage(intermediate voltage range V < U/e). According to litera-ture a fixed effective electron mass of m�e = 0.28 me, com-mon for saturated n-alkanes and an arbitrary meanbarrier height of 5.1 eV, only one fit parameter remainsfor modeling the experimental tunneling current, thewidth d of the potential barrier [19,29]. Fig. 2b, showsthe fitting results for the chosen parameters and varyingbarrier widths. The total length of the tunneling path be-tween the two electrodes correlates to the different SAMthickness. The experimental values for J are described wellin the intermediate voltage range (�0.3 to �1 V) by theparameter set used. Details are provided in Table S1 ofthe Supplementary data. While the effective monolayerthickness of 0.9 nm for C6-PA and 1.2 nm for C10-PA agreewell with the calculated molecule length, there is a clearmismatch with respect to the real layer thickness for C14-PA and C18-PA. For extended molecular lengths of approx-imately 1.9 nm for C14-PA and 2.4 nm for C18-PA, the fittedbarrier width corresponds to an effective monolayer thick-ness of only 1.3 nm for C14-PA and 1.4 nm for C18-PA.

These reduced thicknesses can be attributed to the for-mation of leakage paths such as gaps in the monolayersand result in the observed increase of current density.Fig. 2c shows a quantitative evaluation of the MD data.By imposing an XYZ matrix grid of 2 Å, the cell occupancyand accessibility from the top can be obtained as functionsof the surface distance and topography. Exemplarily, thedata for C6-PA and C18-PA demonstrate the length-depen-dent trend to form gaps and crystalline domains. Longermolecules are more likely to organize into crystalline do-mains than shorter ones because of the increased van derWaals interactions between the longer chains, that alsocause the semi crystalline behavior of polyethylene (PE),which consists of the same building blocks than our SAMchains [30]. Further investigations of the SAMs by VSFG

Fig. 3. Distance to surface versus occupancy plot correlated to snapshots of theoccupancy of about 90% and the corresponding effective monolayer thickness.

lead to the same conclusion. By comparing the intensity ra-tio of the CH3 and CH2 symmetric stretch modes in the sumfrequency spectra, the ordering of a SAM can be qualita-tively determined. This ratio is a measure of molecular or-der since conformational disorder decreases the CH3

symmetric stretch intensity and increases the CH2 inten-sity because they lose their locally centrosymmetric struc-ture [31,32]. Our measurements reveal a clear tendency toa more ordered morphology for longer molecules indicatedby an increasing CH3 to CH2 intensity ratio (Supportinginformation).

We propose that the self-assembling of molecules withshorter alkyl chain is dominated by the footprint of thephosphonic acid and yields electrically dense and amor-phous monolayers. Molecules with longer alkyl chainsorganize into highly ordered quasi-crystalline domainsseparated by gaps, due to competition between the spacerequirements of the phosphonic acid and the van derWaals interactions of the alkyl chain [15]. Fig. 3 showssnapshots from the MD simulations that illustrate thechange in morphology for increased chain lengths.

The surface distance for each of the four phosphonicacid molecules investigated is plotted against the occu-pancy. The first few nanometers of each monolayer are clo-sely packed (occupancy of one) and impenetrable forarriving atoms and molecules of the subsequent layer. Asthe distance to the surface increases the occupancy valuedrops to zero at the stretched length of the molecule. Fordensely packed monolayers this decrease occurs abruptly,as shown for C6-PA at a surface distance of about 1 nm.For longer molecules (C10-PA to C18-PA) a more gradual de-crease is observed, indicating a closer stacking of the mol-ecules to form crystalline domains and free space. Thisgradual decrease is most pronounced for C18-PA and re-flects the anomalistic feature of long chain SAMs. As theoccupancy can be expressed as the geometrical dimensionof the gaps at a certain distance to the surface, a critical va-lue can be defined where gold atoms or semiconductormolecules are able to penetrate into the surface and conse-quently reduce the tunneling distance for the charge carri-ers. Setting the critical surface occupancy to an arbitraryvalue of about 0.90, the corresponding layer thicknesscan be determined from the plot. The monolayer thick-nesses thus obtained (C6-PA = 0.85 nm, C10-PA = 1.13 nm,

MD calculations for n-alkyl phosphonic acids. The black lines indicate an

M. Novak et al. / Organic Electronics 11 (2010) 1476–1482 1481

C14-PA = 1.39 nm, and C18-PA = 1.46 nm) agree well withthe calculated effective monolayer thickness obtained bythe Simmons model described above.

However, the influence of the top layer of the devices onthe ordering of the molecules and the effective monolayerthickness is not yet clear. In fact, both device setups, tran-sistors and capacitors, show the observed feature, indepen-dent of the subsequent layer. As the MD simulations do notinclude the top conductor layer, it is conceivable that thepenetrating molecules and atoms may expand the gapsor even cause additional gaps to form, thus increasingthe number of short tunneling pathways. The presence ofgaps and crystalline domains can have a strong impacton the morphology (grain size) of the subsequent penta-cene layer, as observed by Hill [13]. An increased numberof nucleation centers (edges of the crystalline domains)translate directly to smaller grain sizes in pentacene layerson C18-SAMs (Supplementary data – Fig. S2), which affectsthe transistor parameters as observed [33]. As the reportedfeature is attributed to the different space requirements ofanchor groups and alkyl chains, in other systems like thiolsor silanes, similar effects are not observed so far. However,a possible way to benefit from the stability of the phos-phonic acids and to overcome the structural mismatch isto synthesize molecules containing head groups or substit-uents that may compensate the different space require-ments [1].

4. Conclusion

In conclusion, we have investigated the molecularordering of self-assembled monolayers, both experimen-tally and using simulations. Combining the Simmons mod-el with the results of the molecular dynamics simulationsallows us to reproduce the observed anomalistic depen-dence of the electronic properties on the molecular chainlength in SAMs based on n-alkyl phosphonic acids. Thecombination of the two approaches results in a reducedeffective monolayer thickness compared to those expectedfor extended chains. This trend is more pronounced forlonger alkyl chains as the molecular interaction of the car-bon backbone facilitates the formation of highly ordereddomains with gaps between them. Simultaneously, achange in SAM morphology from amorphous to quasi-crys-talline from shorter to longer alkyl chains was observed byVSFG measurements. In general, our study shows that evensmall changes in the chain length can change the molecu-lar interactions and monolayer assembly significantly. Thisis of enormous importance in molecular scale electronicsas the device parameters are actively tunable by themolecular design (anchor group, chain, or possible ‘‘active”head group).

Acknowledgments

The authors gratefully acknowledge the funding of theGerman Research Council (DFG), which, within the frame-work of its ‘Excellence Initiative’ supports the Cluster ofExcellence ‘Engineering of Advanced Materials’ (www.eam.uni-erlangen.de), the Erlangen Graduate School of

Molecular Science (GSMS), the Erlangen Graduate Schoolin Advanced Optical Technologies (SAOT) at the Universityof Erlangen-Nuremberg and F. Ante and H. Klauk from MPIStuttgart.

Appendix A. Supplementary data

Supplementary data associated with this article can befound, in the online version, at doi:10.1016/j.orgel.2010.05.009.

References

[1] M. Halik, H. Klauk, U. Zschieschang, G. Schmid, C. Dehm, M. Schutz, S.Malsch, F. Effenberger, M. Brunnbauer, F. Stellacci, Low-voltageorganic transistors with an amorphous molecular gate dielectric,Nature 431 (2004) 963–966.

[2] H. Klauk, U. Zschieschang, J. Pflaum, M. Halik, Ultralow-powerorganic complementary circuits, Nature 445 (2007) 745–748.

[3] S.G.J. Mathijssen, E.C.P. Smits, P.A. van Hal, H.J. Wondergem, S.A.Ponomarenko, A. Moser, R. Resel, P.A. Bobbert, M. Kemerink, R.A.J.Janssen, et al., Nature Nanotechnologie 4 (2009) 674–680.

[4] E.C.P. Smits, S.G.J. Mathijssen, P.A. van Hal, S. Setayesh, T.C.T. Geuns,K.A.H.A. Mutsaers, E. Cantatore, H.J. Wondergem, O. Werzer, R. Resel,et al., Nature 455 (2008) 956–959.

[5] R.L. McCreery, A.J. Bergren, Progress with molecular electronicjunctions: meeting experimental challenges in design andfabrication, Advanced Materials 21 (2009) 4303–4322.

[6] D.J. Gundlach, J.E. Royer, S.K. Park, S. Subramanian, O.D. Jurchescu,B.H. Hamadani, A.J. Moad, R.J. Kline, L.C. Teague, O. Kirillov, et al.,Nature Materials 7 (2008) 216–221.

[7] M. Mottaghi, G. Horowitz, Field-induced mobility degradation inpentacene thin-film transistors, Organic Electronics 7 (2006) 528–536.

[8] S. Kobayashi, T. Nishikawa, T. Takenobu, S. Mori, T. Shimoda, T.Mitani, H. Shimotani, N. Yoshimoto, S. Ogawa, Y. Iwasa, Control ofcarrier density by self-assembled monolayers in organic field-effecttransistors, Nature Materials 3 (2004) 317–322.

[9] K.P. Pernstich, S. Haas, D. Oberhoff, C. Goldmann, D.J. Gundlach, B.Batlogg, Threshold voltage shift in organic field effect transistors bydipole monolayers on the gate insulator, Journal of Applied Physics96 (2004) 6431.

[10] H.B. Akkerman, A.J. Kronemeijer, P.A. van Hal, D.M. de Leeuw, P.W.M.Blom, B. de Boer, Self-assembled-monolayer formation of longalkanedithiols in molecular junctions, Small 4 (2008) 100–104.

[11] H.O. Finklea, S. Avery, M. Lynch, T. Furtsch, Blocking orientedmonolayers of alkyl mercaptans on gold electrodes, Langmuir 3(1987) 409–413.

[12] P. Diao, D. Jiang, X. Cui, D. Gu, R. Tong, B. Zhong, Studies of structuraldisorder of self-assembled thiol monolayers on gold by cyclicvoltammetry and ac impedance, Journal of ElectroanalyticalChemistry 464 (1999) 61–67.

[13] I.G. Hill, C.M. Weinert, L. Kreplak, B.P. van Zyl, Influence of self-assembled monolayer chain length on modified gate dielectricpentacene thin-film transistors, Applied Physics A 95 (2008) 81–87.

[14] A. Jedaa, M. Burkhardt, U. Zschieschang, H. Klauk, D. Habich, G.Schmid, M. Halik, The impact of self-assembled monolayer thicknessin hybrid gate dielectrics for organic thin-film transistors, OrganicElectronics 10 (2009) 1442–1447.

[15] K. Fukuda, T. Hamamoto, T. Yokota, T. Sekitani, U. Zschieschang, H.Klauk, T. Someya, Effects of the alkyl chain length in phosphonic acidself-assembled monolayer gate dielectrics on the performance andstability of low-voltage organic thin-film transistors, Applied PhysicsLetters 95 (2009) 203301/1–3.

[16] D.K. Aswal, S. Lenfant, D. Guerin, J.V. Yakhmi, D. Vuillaume, Selfassembled monolayers on silicon for molecular electronics,Analytica Chimica Acta 568 (2006) 84–108.

[17] S.A. DiBenedetto, A. Facchetti, M.A. Ratner, T.J. Marks, Chargeconduction and breakdown mechanisms in self-assemblednanodielectrics, Journal of the American Chemical Society 131(2009) 7158–7168.

[18] K. Seo, H. Lee, Molecular electron transport changes upon structuralphase transitions in alkanethiol molecular junctions, ACS Nano 3(2009) 2469–2476.

1482 M. Novak et al. / Organic Electronics 11 (2010) 1476–1482

[19] H.B. Akkerman, R.C.G. Naber, B. Jongbloed, P.A. van Hal, P.W.M.Blom, D.M. de Leeuw, B. de Boer, Electron tunneling throughalkanedithiol self-assembled monolayers in large-area molecularjunctions, PNAS 104 (2007) 11161–11166.

[20] G. Horowitz, P. Lang, M. Mottaghi, H. Aubin, Extracting parametersfrom the current–voltage characteristics of organic field-effecttransistors, Advanced Functional Materials 14 (2004) 1069–1074.

[21] J.C. Phillips, R. Braun, W. Wang, J. Gumbart, E. Tajkhorshid, E. Villa, C.Chipot, R.D. Skeel, L. Kale, K. Schulten, Scalable molecular dynamicswith NAMD, Journal of Computational Chemistry 26 (2005) 1781–1802.

[22] D.A. Case, T.A. Darden, T.E. Cheatham III, C.L. Simmerling, J. Wang,R.E. Duke, R. Luo, M. Crowley, R.C. Walker, et al., AMBER 10,University of California, San Francisco, 2008.

[23] J. Wang, R.M. Wolf, J.W. Caldwell, P.A. Kollman, D.A. Case,Development and Testing of a General Amber Force Field, Journalof Computational Chemistry 25 (2004) 1157–1174.

[24] Materials Studio 4.4, Accelrys Inc., San Diego (2008).[25] B. Beck, T. Clark, R.C. Glen, VESPA: A new, fast approach to

electrostatic potential-derived atomic charges from semiempiricalmethods, Journal of Computational Chemistry 18 (1997) 744–756.

[26] T. Clark, A. Alex, B. Beck, F. Burkhardt, J. Chandrasekhar, P. Gedeck, A.Horn, M. Hutter, B. Martin, G. Rauhut, et al., VAMP 10.0, Accelrys Inc.,San Diego, 2008.

[27] T. Darden, D. York, L. Pedersen, Particle mesh Ewald: an N log (N)method for Ewald sums in large systems, Journal of ChemicalPhysics 98 (1993) 10089–10092.

[28] J.G. Simmons, Generalized formula for the electric tunnel effectbetween similar electrodes separated by a thin insulating film,Journal of Applied Physics 34 (1963) 2581–2590.

[29] J.K. Tomfohr, O.F. Sankey, Complex band structure, decay lengths,and Fermi level alignment in simple molecular electronic systems,Physical Review B 65 (2002) 245105.

[30] D.M. Spori, V.N. Venkataraman, S.G.P. Tosatti, F. Durmaz, N.D.Spencer, S. Zürcher, Influence of alkyl chain length onphosphate self-assembled monolayers, Langmuir 23 (2007)8053.

[31] A.G. Lambert, P.B. Davies, D.J. Nievandt, Implementing thetheory of sum frequency generation vibrational spectroscopy: atutorial review, Applied Spectroscopy Reviews 40 (2005) 103–145.

[32] R.N. Ward, D.C. Duffy, P.B. Davies, C.D. Bain, Sum frequencyspectroscopy of surfactants adsorbed at a flat hydrophobicsurface, Journal of Physical and Chemical 98 (1994) 8536–8542.

[33] G. Horowitz, M.E. Hajlaoui, Grain size dependent mobility inpolycrystalline organic field-effect transistors, Synthetic Metals122 (2001) 185–189.