Effect of Structure and Disorder on the Charge Transport in Defined

Effect of Structure and Disorder on the Charge Transport in Defined Self-Assembled Monolayers of Organic Semiconductors Thomas Schmaltz,†,⊥ Bastian Gothe,† Andreas Krause,‡ Susanne Leitherer,§ Hans-Georg Steinrück,∥ Michael Thoss,§ Timothy Clark,‡ and Marcus Halik*,† †

Organic Materials & Devices (OMD), Dept. of Materials Science, Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Martensstraße 7, 91058 Erlangen, Germany ‡ Computer-Chemie-Centrum and Interdisciplinary Center for Molecular Materials, FAU, Nägelsbachstraße 25, 91052 Erlangen, Germany § Institute for Theoretical Physics and Interdisciplinary Center for Molecular Materials (ICMM), FAU, Staudtstrasse 7/B2, 91058 Erlangen, Germany ∥ SSRL Materials Science Division, SLAC National Accelerator Laboratory, Menlo Park, California 94025, United States S Supporting Information *

ABSTRACT: Self-assembled monolayer field-effect transistors (SAMFETs) are not only a promising type of organic electronic device but also allow detailed analyses of structure−property correlations. The influence of the morphology on the charge transport is particularly pronounced, due to the confined monolayer of 2D-πstacked organic semiconductor molecules. The morphology, in turn, is governed by relatively weak van-der-Waals interactions and is thus prone to dynamic structural fluctuations. Accordingly, combining electronic and physical characterization and time-averaged X-ray analyses with the dynamic information available at atomic resolution from simulations allows us to characterize self-assembled monolayer (SAM) based devices in great detail. For this purpose, we have constructed transistors based on SAMs of two molecules that consist of the organic p-type semiconductor benzothieno[3,2-b][1]benzothiophene (BTBT), linked to a C11 or C12 alkylphosphonic acid. Both molecules form ordered SAMs; however, our experiments show that the size of the crystalline domains and the charge-transport properties vary considerably in the two systems. These findings were confirmed by molecular dynamics (MD) simulations and semiempirical molecular-orbital electronic-structure calculations, performed on snapshots from the MD simulations at different times, revealing, in atomistic detail, how the charge transport in organic semiconductors is influenced and limited by dynamic disorder. KEYWORDS: self-assembly, monolayer transistor, molecular electronics, MD simulations, charge-transport calculations

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between charge carriers and low-frequency intermolecular vibrational modes, a factor that is not found in conventional solid-state devices.2 Organic semiconductors inherently exhibit structural disorder, which appears as grain boundaries, lattice defects or vacancies, even in the absence of charge carriers. This relatively static disorder is further augmented by fluctuating positions of the organic molecules in their supramolecular arrangement, which in turn leads to variations in the electronic

rganic semiconductors will play a prominent role in next-generation electronics.1 They present challenges for both theory and experiment, mainly because of their conformational flexibility, which makes possible chargetransport paths very heterogeneous because they must cross intermolecular and/or domain boundaries. The complex and dynamic energetic landscape for charge carriers presents challenges not encountered for traditional semiconductors. As self-assembled monolayer (SAM)-based devices are inherently two-dimensional, they offer one-of-a-kind insight into the factors that influence electronic performance. They allow us to investigate phenomena of general importance in organic electronics: We can, for instance, expect strong coupling © XXXX American Chemical Society

Received: April 6, 2017 Accepted: August 16, 2017 Published: August 16, 2017 A

DOI: 10.1021/acsnano.7b02394 ACS Nano XXXX, XXX, XXX−XXX

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Figure 1. Chemical structure of the two semiconducting molecules BTBT-C11-PA and BTBT-C12-PA and the schematic setup of a selfassembled monolayer field-effect transistor device.

landscape.3−9 These fluctuations can even limit the electronic properties of organic semiconductors on the macroscopic scale, since the intermolecular electronic coupling critically depends on the electronic communication between, and consequently on the relative positions of, the molecules. Improving our understanding of the relationships between chemical structure, supramolecular order and electronic properties of organic semiconductors is of utmost importance in order to improve the performance of electronic devices, or even to design semiconductors with desired properties. This task requires both experimental structural characterization and simulations. The role of the simulations in such combined studies is pivotal, as they can provide information that is otherwise difficult or impossible to obtain, such as structural changes on nanosecond time scales. Molecular dynamics (MD) simulations based on modern force fields have now attained a level of accuracy that allows them to compete with many experimental characterization techniques.10 The strength of modern hard-/software combinations enables simulations to be extended to time scales up to microseconds compatible with the self-organization processes that determine the morphology of the electro-active layers of organic devices, making them a powerful source of information not available from experiments.11 Thus, when used judiciously with adequate experimental validation, MD simulations provide information essential in designing organic semiconductor devices.12 Especially, well-defined two-dimensional systems allow us to obtain important results for the inplane morphology and charge transport that is predominantly two-dimensional in a thin film transistor. SAMs with semiconducting functionality are excellent model systems in this regard, for studying general processes in organic semiconductor thin films in order to promote the understanding of structure− property relationships. The general chemical structure of SAM-forming molecules comprises an anchor group that binds to substrate surfaces specifically, a spacer (typically an alkyl-chain) and a functional end group that determines the electronic properties of the SAM. These molecules self-assemble into defined monolayers with a thickness that is determined by the chemical structure and packing motives of the constituting molecules. Their facile processing and defined structure have established such SAMs as powerful materials in organic electronic devices,13,14 as workfunction modifiers,15−18 dielectrics,19,20 charge-storing units in memory transistors,21,22 or even as active semiconducting layer in transistor devices.23−26 The dimensions of only few nanometer thickness of SAMs makes their precise experimental characterization challenging.

In particular, investigations of in-plane molecular interactions and nearest-neighbor arrangements are difficult. The combination of experiments (electrical characteristics and in-plane morphology) and simulations offers outstanding opportunities in this respect. MD simulations on SAMs have proven very powerful in understanding their structure on the molecular and atomistic levels.27−31 In addition to the morphological structure of the SAMs, electronic properties of these systems can be calculated and simulated, as shown for the charge transport across semiconducting SAMs.28,32,33 Here we report a combined experimental and simulationbased study of two semiconducting self-assembled monolayers. They consist of the organic p-type semiconductor benzothieno[3,2-b][1]benzothiophene (BTBT), attached to undecyl(BTBT-C11-PA), or dodecylphosphonic acid (BTBT-C12-PA) moieties (Figure 1). We have chosen these two SAMs because they only differ by one CH2-group in the alkyl chain (a vertically projected difference of only 1.27 Å in an all-trans alkyl chain). These were investigated in terms of morphology and electronic properties by both experiment and simulations. X-ray reflectivity (XRR) and grazing incidence X-ray diffraction (GIXD) experiments were performed to characterize the morphology of the SAMs and were compared with the results of MD simulations. The electronic properties of these semiconducting monolayers were investigated in self-assembled monolayer field-effect transistors (SAMFETs, Figure 1) and the results were compared with charge-transport simulations based on semiempirical electronic structure calculations and Landauer transport theory.

RESULTS AND DISCUSSION Self-assembled monolayers of BTBT-C11-PA and BTBT-C12PA (Figure 1) were created on aluminum oxide surfaces and their morphology and charge transport were investigated experimentally and by simulations. Sub-Ångstrøm resolution X-ray based analysis methods were used as a powerful tool for a detailed characterization of the morphology of few nanometer thick SAMs.34,35 MD simulations represent an ideal method to access the atomistic structure of the SAM from the theoretical side. In order to investigate the charge-transport properties, the SAMs must be integrated into electronic devices. Thin-film transistors are ideal devices for testing the lateral charge transport across these layers. Charge-transport calculations help to correlate the results from the macroscopic-scale measurements with processes on the molecular scale. This correlation helps to understand how charge transport in such molecular B


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Figure 2. Results of the morphological investigations of the SAMs. (a) XRR data (circles) and corresponding fits (lines) for BTBT-C11-PA and BTBT-C12-PA SAMs. (b) The fit-derived vertical electron density profiles. (c) Background-corrected profiles (circles) of GIXD experiments, and the corresponding Lorentzian fits (lines) of BTBT-C11-PA (blue) and BTBT-C12-PA (red) SAMs. (d) Average domain sizes of crystalline domains in BTBT-C11-PA and BTBT-C12-PA SAMs, extracted from the peak width of the diffraction data.

(02) and (12) in-plane peaks of a two-dimensional rectangular unit cell with herringbone structure were found in both SAMs (Figure 2c, d and Supporting Information Figure S1), as reported before in similar systems.23 The lattice constants of the unit cell were calculated to a = 6.05 ± 0.02 Å and b = 8.19 ± 0.02 Å. These values are approximately 2−5% higher than literature values of alkyl-substituted BTBT-derivatives in single crystals and thin films with lattice constants of approximately 5.9 and 7.8 Å.36−38 The slightly different lattice parameters can be explained by the covalent linking of the molecules to the substrate, which results in restrictions in their conformational freedom.39 Further information can be obtained from the peak width of the Bragg rods. The average domain size of the crystalline domains can be determined via the Debye−Scherrer equation. Domain sizes of less than 20 nm were found in BTBT-C12-PA SAMs, whereas BTBT-C11-PA SAMs exhibited domain sizes of approximately 50 nm. Typically, larger domains are expected to be beneficial for charge transport, since the number of grain boundaries is smaller.40 The packing density in the SAMs was determined from both XRR and GIXD. First, the molecular density was calculated from the electron density in the SAM, obtained by XRR measurements,35 from which the area occupied per molecule was determined to be 0.263 nm2 per BTBT-C11-PA molecule and 0.284 nm2 per BTBT-C12-PA molecule. In the crystalline regions, the area requirement per molecule was found to be 0.248 nm2 for both SAMs, as calculated from the lattice parameters of the GIXD measurements. This value is smaller than the ones determined by XRR, which suggests that the SAMs also contain amorphous regions, including grain

semiconductor systems is governed by processes on the nanoscale. Morphological Characterization. The morphology of the SAMs was characterized by XRR and GIXD. XRR allows for the analysis of the vertical structure of the SAM, based on the difference in electron density of the functional parts within the monolayer, i.e., the BTBT-group, the alkyl chain and the anchor group.35 A four-slab model, three for the SAM and one for the aluminum-oxide layer, was used to fit to the experimental data (Figure 2a, the resulting values for thickness, electron density and roughness for each of the layers are listed in Tables S1 and S2 in the Supporting Information). The vertical electron density profile of the two SAMs (Figure 2b) confirms the single-layer structure with a total thickness of 2.54 nm and upright standing molecules in both systems.35 Starting with a high value for the aluminum oxide substrate, the electron density decreases in the region of the alkyl-chain region of the SAMs to values between 0.23 and 0.24 Å−3. The BTBT functional end group exhibits a significantly higher electron density of 0.48 Å−3, which is represented by the peak at z ≈ 19 Å. The thickness of the alkyl-chain regions corresponds well with the theoretical length of the stretched molecules and is slightly smaller in the case of BTBT-C11-PA. Furthermore, the thickness of the BTBT functional end groups within the SAMs varies slightly between the two systems (8.5 Å in BTBT-C11-PA SAMs and 7.7 Å in BTBT-C12-PA SAMs). The difference can be attributed to the alkyl chain of BTBT-C11-PA being one CH2-group shorter, which potentially results in a less tilted BTBT-moiety in BTBT-C11-PA SAMs. The in-plane structure of the SAM was investigated by GIXD. Three Bragg rods that could be assigned to the (11), C


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Figure 3. Results of MD simulations of BTBT-C11-PA (a−c) and BTBT-C12-PA (d−f) SAMs on AlOx. (a,d) Side view of snapshots at 81 ns simulation time; (b,e) Side view of selected molecules and mean BTBT-group tilt angles toward the surface normal; (c,f) Calculated vertical electron-density profiles for the two SAMs (colored lines), compared with results obtained from XRR experiments (black lines).

different liquids, which allowed the surface energy of the SAMs to be determined. Although the two molecules have the same functional end group (BTBT) attached to an alkyl chain, the contact angles and surface energy values determined differ with a surface energy of 37.4 mN/m for BTBT-C11-PA and 40.0 mN/m for BTBT-C12-PA (Supporting Information, Figure S2). Considering the very similar chemical structures of the two molecules and our MD results shown below, we suggest that a different tilt angle of the molecules in the SAM, or at least of the BTBT-group, may be responsible for this observation, as has previously been observed for alkyl-SAMs.42 MD simulations were carried out in order to obtain a deeper insight into the morphology of the SAMs. MD Simulations. As outlined in the introduction, MD simulations provide a powerful opportunity to determine otherwise inaccessible atomistic details of the morphology of the SAM-layer. Simulation times under one microsecond proved to be adequate to characterize the domain-formation behavior of the SAM-forming molecules. Domain growth is not completed in this time but differences in aggregation behavior are well-defined. Snapshots of the simulated structures for BTBT-C11-PA and BTBT-C12-PA are shown in Figures 3 and 4. The simulations used only an electrostatic interaction between phosphonate groups and the surface, which leads to predominantly monodentate coordination of the phosphonate anchor groups to the AlOx substrate, as also found earlier to be a stable configuration in DFT calculations.41 Figure S3 in the Supporting Information illustrates how this bonding pattern leads to an orientation of the BTBT-groups perpendicular to the surface plane for odd numbers of CH2-groups in the alkyl chain and to an inclined orientation for even chains. This

boundaries, with smaller packing density than in the crystalline areas. The lower average packing density in the BTBT-C12-PA indicates a smaller overall amount of crystallinity than in the BTBT-C11-PA. In general, understanding crystallinity in self-assembled monolayers requires some considerations. First, the space requirement of the anchor group and the functional end group that crystallizes has to be similar. Second, the molecules must show a certain flexibility to compensate for small differences in the above-mentioned space requirement of the anchor group and the functional end group and for the fact that no epitaxial growth can be expected on the typically amorphous or polycrystalline substrates. And third, a certain mobility of the molecules on the substrate surface facilitates a dense packing and consequently the crystallization of the SAM-forming molecules. In our case, the binding strength of phosphonic acids on aluminum oxide substrates is strong.41 Nevertheless, a certain mobility of the molecules on the surface can be expected, as the binding is an equilibrium reaction. Hence, the possibility of a rearrangement of the molecules on the surface toward a denser packing can be assumed, which in turn facilitates their crystallization, driven by attractive interaction between the individual molecules. However, even without a considerable mobility of the molecules on the surface, the length of the alkyl chain linker (C11/C12) allows for a certain flexibility of the molecules, i.e., tilting, bending and twisting, which enables a crystallization despite their fixed lateral position defined by the anchor group, even on amorphous aluminum oxide substrates. In addition to XRR and GIXD analyses, static contact-angle measurements were performed on both SAMs, using three D


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the in-plane order in the two SAMs, we calculated the radial distribution functions for the two SAMs, which reveal the difference in structural order very clearly (Supporting Information, Figure S5). Beyond the static structure, MD simulations allow to access the structural development with time, i.e., how the morphology changes over time. This is particularly interesting for organic semiconductors, which are known to be susceptible to dynamic disorder.4,8,9 We observed such structural fluctuations in the MD simulations of the two SAMs and visualized them by overlaying snapshots after 74 to 89 ns simulation time (Figure 4c and d). The fluctuations are more pronounced in amorphous regions than in crystalline domains, in which only small variations of the positions of the molecules were observed. This result shows that a high degree of order and crystallinity is not only important for the sake of a high degree of static order, but also to limit the amount of dynamic disorder. In this regard, we found less dynamic disorder in the BTBT-C11-PA SAM compared to the BTBT-C12-PA SAM, which is expected to be beneficial for the electronic properties of the organic semiconductor system. Overall, the results from the MD simulations are in very good agreement with the experimental results of the morphology of the two SAMs. The general trends for the inand out-of-plane structures of the SAMs are the same for experiment and simulation. Both experimental and simulation results give reason to expect improved electronic properties of BTBT-C11-PA SAMs compared to BTBT-C12-PA SAMs, due to a smaller amount of static and dynamic disorder in BTBT-C11PA SAMs. Electrical Characterization. Thin film transistors with BTBT-C11-PA or BTBT-C12-PA as semiconducting layer were fabricated and characterized electronically to investigate the electronic properties of these SAMs. A bottom gate, top contact device layout was chosen with aluminum gate electrodes, a thin aluminum oxide dielectric grown by atomic layer deposition and gold electrodes (for details on device fabrication see Methods section and Supporting Information, Figure S6). The electrical characterization of the SAMFET devices was carried out under ambient conditions. The monolayer transistors with both BTBT-C11-PA and BTBT-C12-PA, exhibited excellent switching behavior (Figure 5a). Devices comprising BTBT-C11-PA exhibit a 3-fold better performance in terms of charge-carrier mobility, on-current and on/off-ratio than the BTBT-C12-PA-based TFTs (Supporting

Figure 4. Surface structure of BTBT-C11-PA (a) and BTBT-C12-PA (b) after 90 ns simulation time. For clarity, only the two sulfur atoms of the BTBT functional end groups are shown as dots, connected with a line; different crystalline domains are colorcoded. The BTBT-C11-PA simulation shows strong domain formation, whereas the equivalent BTBT-C12-PA structure shows little or no formation of crystalline domains. (c, d) Dynamic changes of the morphology. Top view of BTBT functional end groups at 74−89 ns. For simulation time see the color coding.

simple schematic representation visualizes and rationalizes the general effect that holds true also for other binding geometries, and is in accord with the simulation results: The influence of the additional CH2-group on the tilt angles of the molecules was also observed in the simulated structures of the two SAMs, which show different inclination angles of both the alkyl chains and the functional end groups. The mean tilt angles for the BTBT functional end groups in the simulated SAMs relative to the surface normal are 7.2 ± 0.9° and 15.2 ± 1.1° for BTBTC11-PA and BTBT-C12-PA, respectively (Figure 3). In addition to the average tilt angles, the vertical electron-density profiles were calculated from the 80 ns production simulations to give time-averaged electron-density profiles for comparison with those obtained from experiment. The electron-density profiles extracted from the MD simulations are in good agreement with the experimental ones (Figure 3c and f, Supporting Information, Figure S4). Small deviations between simulated and experimental results can be explained by the higher substrate roughness in the experimental samples and possibly a slightly denser packing of the molecules in the simulation. The two SAMs show a considerable difference in their crystallization behavior. Figure 4 shows snapshots after 90 ns simulation time for BTBT-C11 -PA and BTBT-C 12 -PA; crystalline regions are highlighted in black or orange. Whereas the BTBT-C11-PA SAM forms extensive crystalline domains within the 90 ns simulation, BTBT-C12-PA shows only few and small crystallites. One possible explanation for this observation is the different tilt angle of the BTBT-groups. Less tilted functional end groups, i.e., an orientation more perpendicular to the surface, are expected to promote the formation of crystalline domains, which can only attain full π−π overlap between the BTBT moieties if these are perpendicular to the surface plane. In order to obtain quantitative information for

Figure 5. Electrical characterization: (a) Representative transfer curves of BTBT-C11-PA (blue) and BTBT-C12-PA (red) SAMFETs, showing the drain current (thick lines) as well as the gate leakage current (thin lines). (b) Extracted charge carrier mobilities in the saturation regime of BTBT-C11-PA (blue) and BTBT-C12-PA (red) SAMFETs as a function of the channel length. E


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Figure 6. (a, b) Typical molecular orbitals that are contributing to the charge transport of BTBT-C11-PA (a) and of BTBT-C12-PA (b) snapshots. (c) Comparison of calculated IV-characteristics of BTBT-C11-PA and BTBT-C12-PA SAMs, extracted from the MD trajectories of 28 snapshots at different simulation times. The curves show a strong variation in current over time, which visualizes the effect of dynamic disorder on the electronic properties of organic semiconductor systems. (d) Averaged IV-characteristics of all 28 snapshots extracted from the MD trajectory. The solid line depicts the average current, while the shaded area represents the fluctuations.

advances transport methods.50,51 It is also noted that a quantitative description of transport in systems with strong fluctuations requires, in principle, an explicitly time-dependent treatment of charge transport.52 The methodology used here allows a qualitative analysis of charge transport and the relative differences between the two SAMs. 74 Å × 85 Å structures were considered for both systems. The size of the simulated system was chosen as a compromise to afford the transport calculations. We focus on charge transport within the SAM, where the influence of a gate potential and the AlOx dielectric is not taken into account. The coupling to the gold electrodes is included implicitly using selfenergies. The Fermi level of the electrodes is set to the workfunction of gold (EF = −5.1 eV), which is close to the occupied levels of the SAMs. Only the occupied levels of the SAM are considered in the simulations (i.e., only hole-transport is considered), in accord with the fact that the BTBT molecules are p-type semiconductors. We have analyzed the MO-structure of different BTBT-C11PA and BTBT-C12-PA snapshots. Structural disorder causes significant localization of the MOs. The orbitals that are mainly relevant for transport processes are localized on the BTBT functional end groups. Examples of such MOs are shown in Figure 6a and b. The MOs exhibit a strongly localized distribution in the amorphous regions of the SAMs, as shown for MOs of an exemplary snapshot of the BTBT-C12-PA SAM selected after 81 ns simulation time (Figure 6b). More delocalized MOs were found within the crystalline domains in the SAMs, exemplified by a BTBT-C11-PA snapshot at 81 ns (Figure 6a). The larger fraction of crystallinity in BTBT-C11-PA

Information, Figure S7). Charge-carrier mobilities depend strongly on the channel length and values of up to 3 × 10−2 cm2 V−1 s−1 were found in BTBT-C11-PA devices with long channels (Figure 5b), among the highest values reported for SAMFET devices.23,25,43 The difference in the electrical properties of these two SAMs is surprising, since they consist of almost identical molecules and have similar thin-film structures. However, we explain this difference in electrical performance with the larger domain size of the crystallites in BTBT-C11-PA SAM and the smaller tilt angle of the semiconducting BTBT functional end groups. Smaller crystalline domains result in a larger number of domain boundaries, which are typically detrimental for charge transport and thus yield poorer charge carrier mobilities.40,44,45 Charge-Transport Simulations. To deepen the understanding of the observed difference in electrical properties of BTBT-C11-PA and BTBT-C12-PA SAMs, we calculated current−voltage characteristics, using a 3-fold approach.28,33 First, the structure of the SAMs is obtained from MD simulations as described above. Second, the electronic structure of snapshots, selected at specific time steps in the MD simulation, is determined using AM146 semiempirical molecular orbital (MO) theory within the EMPIRE program.47 Third, for the transport calculations, we employ a fully quantum mechanical description. Specifically, we used Landauer transport theory, which provides a coherent transport treatment based on elastic scattering mechanisms,48,49 in which dephasing and inelastic processes due to electron−phonon or electron− electron interactions are neglected. The study of these processes requires an extension of the model and more F


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size and the lower tilt angle of the BTBT functional end groups in BTBT-C11-PA SAMs. The combination of MD simulations with snapshots at different times and charge transport simulations on these snapshots allowed information about the influence of dynamic structural changes in the SAM on the charge transport to be accessed. A strong influence of this dynamic disorder on the charge transport was found in the calculations. Averaging over many snapshots from different times of the MD simulation gave meaningful results, confirming the improved charge transport in BTBT-C11-PA SAMs compared to BTBT-C12-PA SAMs. With this study, we show that structural order is particularly important for the electronic properties of semiconducting self-assembled monolayers and predict that semiconducting SAMs with higher degree of crystallinity and larger crystalline regions will exhibit superior performance.

SAMs therefore leads to a larger number of delocalized MOs in the system compared to BTBT-C12-PA SAMs. Electrical currents were calculated for snapshots taken at different times along the MD trajectory, revealing significant fluctuations in the current magnitude in both systems, depending on which snapshot was used (Figure 6c). These changes demonstrate the strong impact of the dynamic disorder in the morphology of the SAMs on the charge transport, resulting in pronounced fluctuations in their electronic structure and their corresponding conductivity. To provide a comparison between the conductivity of BTBT-C11-PA and BTBT-C12-PA SAMs, we calculated the electrical currents, averaged over 28 snapshots (Figure 6d). The average currents are depicted as solid lines, while the shaded areas indicate the fluctuations in form of the variance. The currents increase as soon as the energy levels enter the conductance window, defined by the Fermi distributions in the left and right electrodes, which in our simulations is at approximately 2.8 V. While at small voltages the average current across the BTBTC11-PA SAM (blue) is slightly lower than the current across the BTBT-C12-PA SAM (red), in the high voltage regime the situation is reversed and the current across the BTBT-C11-PA SAM is 2-fold larger than that across the BTBT-C12-PA SAM. These current−voltage characteristics can be related to the underlying MO-structure. The pronounced localization of the contributing MOs of the BTBT-C12-PA SAMs, resulting from a weaker intermolecular interaction due to the structural disorder, leads to a low coupling to the electrodes and to low currents. At small voltages, the situation is similar for BTBTC11-PA SAMs, i.e., low currents due to strongly localized MOs. At high voltages, MOs delocalized within the crystalline domains in the BTBT-C11-PA SAMs become relevant for transport processes. However, their contributions are limited by the grain boundaries, as shown exemplarily in Figure 6a, where the delocalization of the MOs decays strongly at the boundaries of the crystalline domain, leading to a very low coupling to the electrodes. In accord with the experiment, we find an improved charge transport across BTBT-C11-PA SAMs compared to BTBT-C12PA SAMs. The simulations clearly show how the larger domain size of crystalline structures facilitates charge transport. The existence of domain boundaries in both SAMs explains why the differences are not more pronounced.

METHODS Morphological Characterization. X-ray reflectivity (XRR) and grazing incidence X-ray diffraction (GIXD) measurements and the corresponding data analysis was performed as described previously.26,53 Samples for XRR and GIXD experiments were prepared on silicon wafers with 10 nm aluminum oxide films, grown by atomic layer deposition (ALD). These substrates provide a smoother surface (rms roughness of 0.23 nm, determined by AFM) than the substrates used for transistor fabrication, which include a thermally evaporated aluminum layer. Low surface roughness is crucial for XRR and GIXD in order to obtain atomic resolution data. BTBT-C11-PA and BTBTC12-PA were synthesized according to literature procedures,26 and selfassembled on these surfaces by immersion in 0.05 mM solutions in 2propanol for 72 h, as for the device fabrication. Specular X-ray reflectivity (XRR) measurements were performed using a Bruker D8 reflectometer with a Mo tube and with Goebel mirrors before and after the sample, providing a characteristic wavelength of 0.709 Å and an angular resolution of 0.03°. XRR was measured with the detector placed in the reflection plane and outgoing angle β equal to incoming angle α. In this geometry, the momentum transfer vector is normal to the surface, qz = 4π/λ sin α.54,55 The reflected intensity was measured as a function of qz, and was normalized to the incident intensity. The resulting reflectivity curves R(qz) are sensitive to the surface normal electron density profile. The data was fitted using the Abeles matrix method56 as implemented in Motofit.57 Grazing Incidence X-ray Diffraction (GIXD) experiments were carried out at beamline ID10 at the European Synchrotron Radiation Facility (ESRF) in Grenoble using 22 keV X-rays. The data was collected with a Pilatus 300 K area detector with an illumination time of 10 s. The incident angle α was set to 0.080°, which is just below the critical angle of silicon, αc = 0.082°. The corresponding critical momentum transfer vector is qc = 0.032 Å−1. Thus, the X-rays are totally externally reflected, and only the evanescent wave interacts with the sample. In this setup, the scattered intensity from the surface is maximized, whereas the bulk scattering is reduced to a minimum.58,59 In this geometry, the momentum transfer vector q has a vertical and horizontal component, qz = 2π/λ(sin α + sin β), where β is the outgoing angle, and qr ≃ 4π/λ(sin 2θ/2), where 2θ is the horizontal scattering angle.60−62 Since the present self-assembled monolayers consist of crystallites, which are randomly oriented in the plane, any Bragg peak can be measured by mapping the (2θ, β)-space.63 All peaks can be described well by a Lorentzian line-shape κ r / 2π I(qr ) = I0 2 2 . Such line-shapes have been used success-

CONCLUSIONS Semiconducting self-assembled monolayers of the two molecules BTBT-C11-PA and BTBT-C12-PA were investigated in terms of morphology and charge transport by experiment and simulations. A highly ordered structure with vertically aligned molecules and in-plane crystallinity with a herringbone arrangement of the BTBT functional end groups was found in both SAMs. Larger domain sizes of the crystalline regions were observed for the BTBT-C11-PA SAMs. MD simulations also revealed ordered SAMs with a very similar structure as those found in the experiments and slightly tilted BTBT functional end groups with, however, a different tilt angle for the two SAMs. Furthermore, we reveal a higher tendency to crystallize in case of BTBT-C11-PA SAMs. Excellent charge transport and switching behavior in transistors was found in charge-transport measurements for both SAMs, with 3-fold higher on-currents and charge carrier mobilities in the case of BTBT-C11-PA SAMs, with values as high as 3 × 10−2 cm2 V−1 s−1. This improved performance can be explained by the larger domain

(qr − q0) + (κr / 2)

fully to describe the scattering profiles of OTS self-assembled monolayers.64 To obtain the intrinsic scattering widths, we convoluted this profile with the Gaussian resolution function of the spectrometer. This analysis gives the peak position q0 and a crystalline correlation length, i.e., the size of the crystallites, which is related to the inverse of the full-width-half-maximum κr through the Debye−Scherrer formula G


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0.9·2π 65,66 . κr

the Fermi distributions of the left and right electrodes, and t(E) = tr{GMΓLG†MΓR} is the transmission function. This expression involves the Green’s function of the SAM, defined by GM = (E+ − HS − ∑L − ∑R)−1, where ∑L/R are the self-energies describing the influence of the left and right electrodes. The density of states in the electrodes is described in the wide-band approximation. Specifically, the matrix elements of the self-energies ∑L/R in a local basis (represented by −i atomic orbitals χν) are given by (Σ L/R )νν = 2 (ΓL/R )νν , with (ΓL/R)νν = 1 eV for orbitals ν corresponding to the atoms of the aromatic plane of the outermost BTBT-PA molecules at the left and right boundary of the SAM and (ΓL/R)νν′ = 0 otherwise. Neither the influence of the gate electrode nor that of the AlOx dielectric layer are taken into account. 1 Averaged currents are given by I ̅ = ∑n N In , and fluctuations are

The values reported in this manuscript are the mean

values of the best fits and the uncertainties obtained from independent measurements of different samples. MD Simulations. MD simulations were carried out using DL_POLY classic67 with the general Amber force field (GAFF).68 Two-dimensional periodic boundary conditions were used with 7.40 × 8.59 nm cells. The short-range van-der-Waals cutoff was 1.2 nm and bonds to hydrogen were constrained with SHAKE,69 which allowed a time step of 2 fs. Long-range electrostatics were treated with the Particle-Mesh Ewald (PME) method.70−72 20 000 steps of steepestdescent minimization were performed before heating up the system in 20 K steps within 12.5 ps per step using a Berendsen thermostat73 with a relaxation constant of 0.5 ps. 100 ns of production simulation, the last 80 ns of which were used for analysis, were carried out at 300 K and 1 Atm. The GAFF force field for BTBT was parametrized to reproduce BH&H74/aug−cc−pVDZ75,76 calculations on BTBT dimers. The force field is defined in the Supporting Information. Special attention was paid to reproducing the π−π interactions correctly. BH&H has been shown to perform well in this respect.77 No covalent bonds were defined between the phosphonate oxygens and the AlOx surface, so that the interaction between the substrate and the anchor groups is purely electrostatic. This allows both mono- and bidentate coordination and migration of the anchor groups if necessary. Device Fabrication and Electrical Characterization. The device fabrication process was performed on silicon wafers with 100 nm thermally grown oxide. Thirty nm thick aluminum-gate electrodes were thermally evaporated and patterned via a standard photolithographic process and wet-etching of the aluminum. An oxygen plasma treatment was performed at pressures of 0.2 mbar for 5 min (Diener Electronic Pico plasma oven, 200 W, 40 kHz) yielding a dense aluminum oxide AlOx film of a little below 4 nm thickness,78 followed by an additional aluminum oxide deposition via an atomic layer deposition (ALD) process (Savannah S100 ALD tool from Ultratech/ Cambridge Nanotech using trimethylaluminum and water as precursor, a substrate temperature of 200 °C and 100 deposition cycles). These aluminum oxide films exhibit a roughness of 1.6 nm (rms), as measured by AFM, and are amorphous, as known from literature.79 Afterward, the substrates were again treated with oxygen plasma to activate the surface and immersed in a solution of the corresponding BTBT-PA (0.05 mM in 2-propanol, 72 h) for selfassembly of the semiconducting monolayers. Subsequently, the substrates were removed from solution and thoroughly rinsed with 2-propanol to wash off unbound molecules. The substrates were blown dry in a stream of nitrogen and placed on a hot plate (60 °C) for 3 min to remove residual solvent. This process follows a standard procedure and has been reported in many publications by various groups.20,78,80−82 To pattern the source/drain electrodes, a standard photolithographic lift-off process was employed. Thirty nm gold were thermally evaporated and the lift-off was performed by immersion in acetone. In this process, it is critical that the active layer comes in direct contact with the photoresist, the NaOH-based developer and the acetone during lift-off. It cannot be excluded that the electrical properties of the semiconducting monolayers are influenced by these wet chemical process steps. Thus, the substrates were immersed again in corresponding BTBT-PA solutions for approximately 44 h, after the lift-off process. A graphical representation of the process flow is provided in Figure S6. All electrical characterizations were performed in ambient air on a manual probe station with an Agilent B1500A parameter analyzer. Charge Transport Simulations. We have used an effective singleparticle model, the parameters of which were determined by semiempirical MO calculations using the restricted Hartree−Fock formalism and the AM1 Hamiltonian.46 These calculations were performed using the parallel EMPIRE program.47 Correspondingly, for a given snapshot of the structure of the SAM, the Hamiltonian of the SAM reads HS = ∑j |ϕj⟩ϵj⟨ϕj|, where ϵj denotes the energy of an electron in the jth MO |ϕ j ⟩. The current is given by 2e I(V ) = h ∫ dE t(E)(fL (E) − fR (E)),48,49 where f L/R(E) denotes

defined by the variance σ =

1 N (N − 1)

∑n (I ̅ − In)2 .

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b02394. Additional figures with information on the experimental and simulation results, a schematic description of the device processing, and the force field parameters of the MD simulation (PDF)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Timothy Clark: 0000-0001-7931-4659 Marcus Halik: 0000-0001-5976-0862 Present Address

École Polytechnique Fédérale de Lausanne (EPFL), Institute of Materials, Station 12, 1015 Lausanne, Switzerland.

⊥

Author Contributions

M.H., T.C. and M.T. conceived and designed the experiments and simulations. T.S., B.G., H.G.S. performed the experiments and evaluated the data. A.K. and S.L. carried out the simulations. All authors discussed the results, commented and cowrote the paper. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors gratefully acknowledge the funding of the “Excellence Initiative” of the German Research Council (DFG) supporting the Cluster of Excellence “Engineering of Advanced Materials” (www.eam.uni-erlangen.de), the Erlangen Graduate School of Molecular Science (GSMS), and the “Solar Energy goes Hybrid” (SolTech) initiative of the Bavarian Ministry of Culture, Science and Education. Furthermore, the authors give thanks to Dr. T. Meyer-Friedrichsen and Heraeus Precious Metals GmbH&Co. KG for providing the BTBT-C11PA and BTBT-C12-PA molecules. The GIXD experiments were performed on beamline ID10 at the European Synchrotron Radiation Facility (ESRF), Grenoble, France. We are grateful to Federico Zontone and Oleg Konovalov at the ESRF for providing assistance in using beamline ID10. T.S. gratefully acknowledges funding by the Alexander von HumboldtStiftung/Foundation. Generous allocation of computing time at the computing centers in Erlangen (RRZE), Munich (LRZ), and Jülich (JSC) is gratefully acknowledged. H


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