Surface-Order Mediated Assembly of π-Conjugated Molecules on Self

Jun 22, 2015 - Tuning Electrical Properties of 2D Materials by Self-Assembled Monolayers. Wi Hyoung Lee , Yeong Don Park. Advanced Materials Interface...
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Surface-Order Mediated Assembly of π‑Conjugated Molecules on Self-Assembled Monolayers with Controlled Grain Structures Boseok Kang,†,§ Namwoo Park,‡,§ Jeonghwi Lee,‡ Honggi Min,‡ Hyun Ho Choi,† Hwa Sung Lee,*,‡ and Kilwon Cho*,† †

Department of Chemical Engineering, Pohang University of Science and Technology, Pohang 790−784, Korea Department of Chemical and Biological Engineering, Hanbat National University, Daejeon 305-719, Korea

‡

S Supporting Information *

ABSTRACT: This study systematically demonstrates the effects of the grain structure of crystalline self-assembled monolayers (SAMs) on the growth of organic semiconductor thin films on such monolayers, as well as the electrical characteristics of the resulting semiconductor films. The grain structure of the octadecyltrichlorosilane (OTS) monolayers could be tailored by constructing the monolayers at three different temperatures: −30 °C (−30 °C OTS), −5 °C (−5 °C OTS), and 20 °C (20 °C OTS). Among the three layers, −30 °C OTS exhibited the largest crystalline grains and longestrange homogeneity of alkyl chain arrays. We found that pentacene films deposited on −30 °C OTS monolayers show larger crystalline grains with higher degrees of crystallinity and lateral alignment compared to that of films deposited on −5 °C OTS or 20 °C OTS monolayers, following the surface characteristics of the underlying OTS monolayers. Furthermore, pentacene fieldeffect transistors fabricated with −30 °C OTS monolayers showed lower charge trap densities and higher field-effect mobility values than devices fabricated using −5 or 20 °C OTS monolayers. These results are explained in terms of enhanced quasiepitaxial growth of pentacene films on OTS monolayers with large grains.

1. INTRODUCTION Organic field-effect transistors (OFETs) have undergone extensive development over the past decade, enabling the fabrication of high-performance devices with mobilities comparable to those of amorphous silicon devices.1−7 Especially, vacuum-evaporated organic semiconductor thin films provide OFETs with particularly high mobility values and high on−off ratios.8−10 The performance of these OFETs is significantly affected by the microstructural characteristics of the organic semiconductor thin film, such as molecular packing, crystalline structure, and morphology.11−14 In a bottom-gate configuration, such structural characteristics of the organic semiconductor layer are highly sensitive to the surface characteristics (surface energy,9,15,16 roughness,17−19 and surface molecular structure12,20−23) of the underlying gate dielectric, with different gate dielectric surfaces giving rise to different morphologies and crystalline polymorphs of the deposited semiconductor layer. Accordingly, considerable effort has been devoted to studying the relationship between the thin film microstructures of organic semiconductor layers and the surface properties of the underlying dielectric layers with the aim of fabricating high-performance OFETs.13 As part of efforts to improve the performance of OFETs, a variety of self-assembled monolayers (SAMs) have been explored to modify the surface characteristics of SiO2,24−26 the most frequently used material as a gate dielectric layer. Among the SAMs examined to date, octadecyltrichlorosilane (OTS) has been one of the best-selling SAMs until now.17,27,28 © 2015 American Chemical Society

The OTS monolayer uniformly passivates SiO2 surfaces with alkyl-terminated functional groups. The hydrophobicity of the monolayer reduces the number of interfacial charge trapping states by covering hydroxyl groups (−OH) of SiO2 and can induce a high degree of molecular ordering in semiconductor materials deposited on the SAM.27,29−31 However, studies on this topic have mainly focused on the relationship between OFET performance (or semiconductor thin film structure) and the surface hydrophobicity of the dielectric layer. Previous work by our group looked at these systems from a different viewpoint by examining the effects of phase states, the presence of ordered (crystalline) and disordered (amorphous) alkyl-chain structures, in an underlying OTS monolayer on the growth process and crystalline structure of deposited pentacene layers, and the performance of devices based on such layers.28 A crystalline OTS monolayer generally consists of several grains composed of ordered alkyl chain arrays tilted 10−15° relative to the surface normal.32 Because the orientation of the alkyl chains in each grain differs, grain boundaries inevitably emerge between the grains during the growth of an OTS monolayer on an oxide substrate (Figure 1a). Organic admolecules deposited on such substrates are expected to be heterogeneously adsorbed and diffuse since their physical movements would differ depending on whether they are located at grain boundaries Received: March 31, 2015 Revised: June 20, 2015 Published: June 22, 2015 4669

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OTS) were characterized by various surface characterization techniques. Figure 1b shows the Fourier-transform infrared (FT-IR) spectra of the OTS monolayers. The bands at 2917 and 2850 cm−1 were assigned to the asymmetric and symmetric methylene (−CH2−) stretching modes, respectively, and originated from the CH2 stretching vibrations in well-ordered alkyl chains of the OTS monolayers.27,28,35 These characteristic peaks shift toward higher wavenumbers (2924 and 2854 cm−1) in disordered monolayers.28,36 Such a peak shift was not present in the spectra, indicating that the OTS monolayers prepared at the three temperatures all comprised ordered chain structures with a high fraction of molecules in the trans conformation (i.e., crystalline or ordered OTS monolayers). X-ray photoelectron spectroscopy (XPS) analysis was conducted to evaluate the chain packing density of the OTS monolayers. The Si2p and C1s XPS spectra of the OTS monolayers on SiO2/Si substrates are shown in Figure 1c. The XPS spectrum in the Si2p range showed peaks at 99.5 and 103.5 eV, corresponding to elemental silicon (Si0) and SiO2 (Si4+), respectively.26 The single C1s peak observed at 284.6 eV was consistent with a single alkane environment in the molecule. The C1s/Si2p peak ratio can be used as a measure of the relative chain packing density of OTS monolayers on the SiO2/Si substrates. The C1s/Si2p peak ratio increased slightly as the preparation temperature of the OTS monolayers decreased (Table 1), indicating that lower preparation temperatures yield

Figure 1. (a) Schematic drawing that shows the self-assembly of OTS monolayers with grain structures. (b) IR external reflection spectra, (c) C1s and Si2p XPS spectra, and (d) in-plane GIXD patterns of OTS monolayers prepared at −30 °C (black), −5 °C (dark gray), and 20 °C (gray). The dotted lines are included as a guide, and the solid lines are fitted curves with a Gaussian function.

or in the interior regions of grains.28,33,34 These subtle variations in the movements of admolecules can lead to huge differences in their molecular packing and growth, thereby leading to changes in the crystalline and electrical characteristics of the deposited organic semiconductor film. Nevertheless, the effect of the grain structures of crystalline SAMs on the growth and structural evolution of deposited organic semiconductor films has not yet been elucidated. Clarifying this issue would greatly assist researchers seeking to establish the structure−property−performance relationship and further improve the performance of OFETs, and thus would contribute to the commercialization of OFET-based highly integrated circuits and their applications. Here, we systematically addressed this issue by constructing OTS monolayers on SiO2/Si substrates at three different temperatures (−30, −5, and 20 °C) in order to control the average grain size of the OTS monolayers. The resultant monolayers had similar alkyl chain ordering and surface physical properties but different average grain sizes. The structural evolution of SAMs was monitored by characterizing their surface morphologies using atomic force microscopy (AFM). The microstructures of deposited pentacene films were determined using AFM, two-dimensional grazing-incidence Xray diffraction (2D GIXD), rocking scans, and Raman spectroscopy. Relatively thin pentacene films (thickness 10 nm) were used for the structural analysis in order to focus on crystalline structures of the film that were directly affected by the underlying dielectric surface, i.e., the OTS monolayer. The electrical characteristics of the pentacene films were investigated by fabricating FET devices based on the films. In addition, the temperature-dependent mobilities of the pentacene FETs were determined to suggest a carrier transport mechanism in the pentacene films and to demonstrate the relationship between the structural characteristics and electrical properties.

Table 1. Characteristics of OTS Monolayers on SiO2/Si Substrates Prepared at Various Temperatures sample −30 °C OTS −5 °C OTS 20 °C OTS

thicknessa (Å)

surface energyb (mJ/m2)

rms roughnessc (Å)

C(1s)/ Si(2p) ratiod

relative crystallinitye

23.8

23.1

2.8

7.77

2.40

23.7

23.3

3.2

7.72

1.34

23.5

23.4

3.3

7.66

1

a Measured by ellipsometry. bCalculated from the hexadecane contact angles using the Good−Girifalco−Fowkes equation, γsv = γlv (1 + cos θ)2/4.28 cMeasured by AFM. dNormalized with respect to the thickness of the OTS monolayers. eNormalized areal intensities with respect to the reflection peak of 20 °C OTS monolayers.

a monolayer with a higher packing density.20 However, considering the huge difference in C1s/Si2p peak ratio between amorphous and crystalline OTS monolayers (ratios of 4.5 and 7.3, respectively),28 the differences among the ratios observed in the present work are almost negligible. We therefore concluded that the packing densities of alkyl chains in monolayers prepared at −30, −5, and 20 °C are similar and markedly different from that of amorphous OTS monolayers. Additionally, various physical properties of the three OTS monolayers, including the thickness, surface energy, and rootmean-square (rms) roughness, were characterized using ellipsometry, contact angle measurements, and AFM (Table 1). The thickness of the OTS monolayers was about 24 Å, regardless of the preparation temperature, which well matches the thickness previously reported for ordered OTS monolayers.28 The surface energy and roughness values were also similar for the three OTS monolayers, implying that these layers have similar wettabilities.36 The crystalline structures of the three OTS monolayers were characterized using 1D GIXD (Figure 1d). The OTS

2. RESULTS AND DISCUSSION 2.1. Characteristics of OTS Monolayers. The molecular ordering and packing density of OTS monolayers prepared at −30 (−30 °C OTS), −5 (−5 °C OTS), and 20 °C (20 °C 4670

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Chemistry of Materials monolayers all showed a peak at qxy = 1.49 Å−1, further confirming their crystalline characteristics. The peaks corresponded to the (10) diffraction of the hexagonal crystalline lattice, with a d-spacing of 4.23 Å for the alkyl chains of OTS monolayers, consistent with unit cell dimensions of a = 4.23 Å, b = 7.33 Å, and γ = 90°.32,37 Interestingly, the areal intensities of the peaks increased with decreasing monolayer preparation temperature. By normalizing the areal intensities of the peaks with respect to that of 20 °C OTS, the relative crystallinities of the OTS monolayers were quantitatively determined (Table 1). The crystallinity of −30 °C OTS was 2.4 times higher than that of 20 °C OTS. Since no sign of disordered CH2 vibrations was observed in the FT-IR spectra (Figure 1b), the higher crystallinity of −30 °C OTS compared to 20 °C OTS can be mainly attributed to larger grains or fewer grain boundaries. Therefore, we concluded that the three OTS monolayers have the same unit cell structure of alkyl chains but that their crystalline qualities differ depending on the preparation temperature. The grain evolution of OTS monolayers was monitored using AFM measurements. Figure 2a displays AFM images of

grew laterally and coalesced, such that they nearly uniformly covered the SiO2 surface with each 2D island structure becoming a grain in the monolayer. When the OTS monolayer was prepared at 20 °C, however, numerous dot-like island structures were observed shortly after the initial immersion. Compared to the islands observed at −30 °C, these islands were much smaller (average size of 150−250 nm). In other words, the density of grain boundaries was much higher in the monolayer prepared at 20 °C than at −30 °C. Consistent with this, the −5 °C OTS showed characteristics that lay between those of the −30 and 20 °C OTS systems, with an average grain diameter of 1.5−2.5 μm.32,37,38 The alkyl chains in islands in OTS monolayers are usually tilted by about 15° relative to the substrate normal, and the growth of an OTS monolayer progresses via the self-similar packing of alkyl chains.38 The grains (or islands) of OTS monolayers in turn have different alkyl chain orientations or tilting directions, with the grain boundaries between adjacent islands acting as crystalline defects that suppress long-range ordering of the alkyl chains.37 This reduces the crystalline quality of the OTS monolayers. Accordingly, the present findings indicate that the −30 °C OTS monolayers have longerrange order and larger-scale homogeneity across the monolayer compared to that of the −5 °C OTS and 20 °C OTS monolayers, even though all of the monolayers displayed similar degrees of alkyl chain packing and the same unit cell structures. 2.2. Microstructure of Pentacene Films Deposited on OTS Monolayers. When depositing an organic semiconductor thin film onto a gate dielectric layer, the surface characteristics of the underlying substrate are critical determinants of the nucleation and growth of the organic semiconductor thin films.33,34,39 Representative AFM topography images of 5 nmand 10 nm-thick pentacene films deposited on the three OTS monolayers are shown in Figure 3. Laterally isotropic and flat

Figure 2. (a) AFM images and height profiles of OTS monolayers prepared at −30, −5, or 20 °C by briefly immersing a SiO2 wafer in an OTS solution (30 s ∼120 s). To observe the grain structure of the OTS monolayers, the immersion time was adjusted. (b) Final AFM morphologies and height profiles of OTS monolayers, which were formed by immersion for 1 h in an OTS solution. All images are 3 μm × 3 μm in size except for the inset. The bar in the inset is 300 nm. Figure 3. AFM topography images of 5 and 10 nm-thick pentacene films deposited on the various OTS monolayers. During pentacene growth, the temperature of the OTS-treated SiO2 was maintained at 30 °C.

the OTS monolayer as a function of immersion time at the three different preparation temperatures (−30, −5, and 20 °C). The growth processes of the monolayers differed significantly depending on the preparation temperature, although the final morphologies and physical properties of the monolayers were almost identical (Figure 2b and Table 1). The −30 °C OTS showed a few molecular adsorption points and microscale 2D island structures in the early stages of immersion. The step heights of these islands were 2.3−2.5 nm, which correspond to the thickness of an OTS monolayer. The −30 °C OTS islands had an average diameter of 2.5−3.5 μm and a faceted morphology. As the immersion time increased, the islands

pentacene islands were found to form during the initial stages of pentacene growth on all three OTS monolayers (upper row in Figure 3). The heights of the islands corresponded to a thickness of about two or three pentacene monolayers, based on the c-axis length of the pentacene unit cell.13 The islands grew laterally and vertically as the pentacene deposition progressed, consistent with a Stranski−Krastanov growth 4671

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Chemistry of Materials mode (bottom row in Figure 3).13 However, the morphologies of the pentacene films varied dramatically according to the grain size of the underlying OTS monolayers. As the grain size of the OTS monolayer increased, the average island size of the pentacene film also increased. This result indicates that the grain structure of the OTS monolayers significantly affects the nucleation, molecular diffusion, and morphological evolution of the pentacene film. 2D GIXD patterns of the pentacene films showed obvious differences in the crystalline characteristics of the pentacene films grown on the different OTS monolayers. Figures 4a and b

Figure 5. Raman spectra for 10 nm-thick pentacene films deposited on OTS monolayers prepared at −30 °C (black) and 20 °C (gray).

The peak at 1158 cm−1 (v0 band) splits into two vibrational modes centered at 1155 cm−1 (v1 band) and 1163 cm−1 (v2 band) due to intermolecular forces between adjacent molecules with different orientations in a unit cell, a phenomenon known as Davydov splitting.42,43 The v1 band was associated with inphase intermolecular coupling (twinning) vibrations of different pentacene molecules in a unit cell. For the pentacene film prepared on the −30 °C OTS monolayer, slight down-shifts were observed in the v0 band at 1158 cm−1 and the main peak at 1179 cm−1 compared to the pentacene film prepared on the 20 °C OTS monolayer (see the insets in Figure 5), implying that the pentacene molecules were more homogeneous, thus leading to a lower molecular relaxation energy.41,42,44 Furthermore, the pentacene film deposited on the −30 °C OTS monolayer showed a stronger v1 relative to v0 band intensity compared to the pentacene film prepared on the 20 °C OTS monolayer. This indicates the presence of stronger inphase (intralayer) intermolecular interactions or π-orbital overlaps between the pentacene molecules in the film on the −30 °C OTS monolayer, consistent with the 2D GIXD results.43 In addition to the 2D GIXD and Raman spectroscopy experiments, we analyzed the full-width at half-maximum (fwhm) of the specular diffraction peak and rocking scan curves to estimate the crystalline quality and the distribution of crystal alignment in the pentacene films. For these experiments, pentacene films were deposited on the −30 and 20 °C OTS monolayers at various substrate temperatures (30, 50, 70, and 90 °C). The specular XRD patterns and their fwhm’s for the (002) peak as a function of substrate temperature are shown in Figures S1 and S3a of the Supporting Information. The rocking curves recorded by fixing the 2θ angle at the (002) peak and their fwhm values are also provided in Figure S2 and S3b of the Supporting Information. Compared to the pentacene films deposited on 20 °C OTS monolayers, the pentacene films grown on the −30 °C OTS monolayers showed much smaller fwhm values for both the XRD patterns and the rocking scans, indicating higher crystallinity (larger coherent domain size) in the out-of-plane direction and better crystalline alignment along the in-plane direction.45,46 On the basis of the results of the 2D GIXD, Raman spectroscopy, specular XRD, and rocking scan experiments, we concluded that pentacene films on the largegrain OTS monolayers grew with larger crystalline grains, higher crystallinity, and enhanced in-plane intermolecular interactions, all of which could contribute to better FET performance.47

Figure 4. (a) 2D GIXD patterns of 10 nm-thick pentacene films deposited on the various OTS monolayers. (b) Out-of-plane and (c) in-plane X-ray intensity profiles from the 2D GIXD patterns.

show the crystal reflections in the out-of-plane and in-plane directions of 10 nm-thick pentacene films deposited on the three OTS monolayers, respectively. In the insets of the figures, the log-scaled and linear-scaled X-ray reflection intensity profiles are plotted along the qz and qxy axes, respectively. The pentacene crystals exhibited distinct reflection spots along the qz axis at qxy = 0 Å−1, indicating that the pentacene crystals were well ordered along the vertical direction (Figure 4a). Two intense in-plane reflections corresponding to the “thin-film” phase of pentacene were observed at a given qxy (>0) (Figure 4b); these reflections were indexed as {1, ± 1} and {0, 2}.8 Among the samples examined, the pentacene film deposited on a −30 °C OTS monolayer showed the highest out-of-plane and in-plane reflection intensities, indicating highly crystalline properties in both the vertical and lateral directions.40 To assess the intermolecular order among the π-conjugated molecules in the polycrystalline pentacene films, Raman spectra of 10 nm-thick pentacene films deposited on the OTS monolayers prepared at −30 and 20 °C were analyzed (Figure 5). The Raman peaks at 1158 and 1179 cm−1 observed in the spectra arose from the C−H in-plane vibrations of a pentacene molecule and were assigned to the motions of atoms located at the ends and on both sides of the molecule, respectively.41,42 4672

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where α (= Nc/Nt) is the ratio of the trap DOS, and Nc is the effective DOS at the transport band edge. The experimental results were used in conjunction with the above equation to calculate the activation energies (Ea) for the devices. The pentacene FETs with −30, −5, and 20 °C OTS monolayers showed Ea values of 18.8, 22.9, and 34.1 meV, respectively, indicating that the height of the energy barrier for the trap state increased with increasing preparation temperature of the OTS monolayer on which the pentacene film was grown. The MTR model was also used to compare the number of trap DOS based on the intercept (μ0α) in the linear plot. The ratios of Nt,−5°C OTS/Nt,−30°C OTS and Nt, 20°C OTS/Nt,−30°C OTS were 1.65 and 1.98, respectively. This indicates that the trap DOS of the pentacene films deposited on −5 °C OTS monolayers (or 20 °C OTS monolayers) were 1.66 times (or 1.98 times) greater than that of the pentacene films deposited on −30 °C OTS monolayers. The higher charge carrier mobilities, lower Ea, and fewer trap DOS in the pentacene FETs with −30 °C OTS monolayers can be contributed to the better crystalline quality (larger grains, higher crystallinity, and longer lateral and vertical coherence of crystallites) of pentacene films deposited on this type of monolayer (see the discussion on the correlation between grain size and field-effect mobility in Figure S4, Supporting Information).31,50 2.4. Quasi-Epitaxial Growth on Crystalline OTS Monolayers. The differences in pentacene crystallinity, morphology, and resultant device performance observed between systems with pentacene deposited on OTS monolayers with large and small grain sizes can be explained by quasiepitaxial (QE) growth of the pentacene films on the crystalline OTS monolayers.51 Previously, we demonstrated QE growth of pentacene films on crystalline OTS monolayers and attributed this phenomenon to quasi lattice matching and strong van der Waals interactions between crystalline OTS monolayers and pentacene films.28,52−54 Given our observation in the present work that the alkyl chain arrays of −30, −5, and 20 °C OTS monolayers have identical lattice structures and similar packing densities, the main difference among these systems is the size of the crystalline grains. Monolayers with larger grains will have a lower density of grain boundaries and a higher degree of homogeneity in the tilting direction of the alkyl chains. Considering that films formed by QE growth have a favorable lattice orientation with respect to the substrate lattice, it is reasonable to attribute larger grains in the deposited pentacene films to the high degree of homogeneity in the large-grain OTS monolayer. To further examine the homogeneity of the large-grain OTS monolayers, a chemical amplification experiment was performed. The relatively disordered alkyl chain structure and lowdensity chain packing in the grain boundary regions of the OTS monolayers should cause these regions to have lower resistance to wet-chemical etching compared to that of regions inside the grains.55 Figure 7 shows selected AFM images of OTS monolayers prepared at 20, −5, and −30 °C and subjected to etching in the diluted HF solution. In the 20 °C OTS monolayers, numerous round-shaped holes were formed over the surface during the initial etching stage. As the etching progressed, these holes increased in size and combined. The hole depth was about 2.6 nm, corresponding to the thickness of the OTS monolayer. By contrast, etching of the −30 °C OTS monolayers showed a markedly different trend. Rather than the round defects observed for the 20 °C OTS monolayers, the etched surface of the −30 °C OTS monolayer exhibited line-

2.3. Effect of OTS Grain Structure on Pentacene FETs. The relationship between the crystalline structures of the pentacene films and the charge carrier transport properties was investigated by comparing the field-effect mobilities of pentacene FETs fabricated on the −30, −5, and 20 °C OTS monolayers. Figure 6a shows the output characteristics of the

Figure 6. (a) Output characteristics (ID − VDS) and (b) transfer characteristics (ID − VGS) of pentacene FETs prepared with different OTS monolayers. (c) Histogram of field-effect mobilities of the pentacene FETs at room temperature. (d) Temperature-dependent mobilities (μ) and activation energy (Ea) of the pentacene FETs.

three FETs. All of the devices showed well-behaved p-channel operation with ohmic behavior in the linear region (low drainsource voltages (VGS)). The on-current levels (ID at VGS = −40 V) gradually increased with decreasing the preparation temperature of the OTS monolayer. The charge transport properties of the pentacene films were further characterized by measuring the transfer characteristics (Figure 6b). The room temperature field-effect mobilities of the pentacene FETs are summarized in Figure 6c. The FETs based on a pentacene layer grown on −30 °C OTS showed a maximum mobility of 0.61 cm2 V−1 s−1 and an average mobility of 0.46 cm2 V−1 s−1; this average mobility was higher than those of the pentacene FETs with −5 °C OTS (average mobility of 0.31 cm2 V−1 s−1) and 20 °C OTS (average mobility of 0.19 cm2 V−1 s−1). Charge transport in these devices was further studied by investigating the temperature-dependent mobility behavior of the pentacene FETs (Figure 6d). In this analysis, the multiple trapping and release (MTR) model was adopted to calculate the activation energy (Ea) and the relative trap density of states (DOS), Nt, distributed in the band gap. This model is described by the following equation:48,49 μ = μ0 α exp( −Ea /kT )

(1) 4673

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as structural defects and reduce the homogeneity of the alkyl chain arrays, which reduces the film quality of the deposited organic semiconductor layer. In the present work, the OTS monolayer prepared at −30 °C had larger crystalline grains than did the −5 and 20 °C OTS monolayers. Pentacene films deposited on −30 °C OTS monolayers were found to exhibit the superior morphological and crystalline characteristics, and FET devices based on such films exhibited dramatically improved performance, with efficient charge carrier transport in terms of high field-effect mobilities and low charge trap densities. The superior quality of pentacene films grown on large-grain OTS monolayers can be explained in terms of enhanced QE growth of the pentacene film as a result of the long-range homogeneity of the alkyl chain arrays in OTS monolayers with large grains. Our findings demonstrate that, when depositing organic thin films on SAMs for use in organic electronic devices, the growth behavior and morphology of the film can be controlled by varying the grain structure of the SAM, with organic thin films grown on SAMs with large grains affording improved device performance. We believe that these findings will provide important guidelines for the surface engineering of practically useful high-performance organic transistors.

Figure 7. AFM images and height profiles of various OTS monolayers chemically etched for a brief time (4 s−10 s).

shaped defects similar to the grain boundary structures observed in the growth of the OTS monolayers. These results indicate that the grain boundaries of the OTS monolayers act as structural defects and reduce the surface homogeneity with respect to alkyl chain packing.55 The differences in the growth behavior of pentacene films deposited on the −30 OTS and 20 °C OTS monolayers, and the resulting pentacene microstructures, are shown schematically in Figure 8. When the underlying OTS monolayer has a

4. EXPERIMENTAL SECTION 4.1. Preparation and Characterization of OTS Monolayers. A highly doped n-Si wafer with a 300 nm thick thermally grown oxide layer was used as the substrate for the fabrication of OFETs. The wafer served as a gate electrode, and the oxide layer acted as a gate dielectric. Prior to treating the silicon oxide layer surface, the wafer was cleaned with a piranha solution (70 vol % H2SO4 + 30 vol % H2O2) for 30 min at 100 °C followed by washing with copious amounts of distilled water. Octadecyltrichlorosilane (OTS, Gelest) was used as received to prepare an organic interlayer between the organic active material and the dielectric layer. OTS monolayers with different grain sizes were prepared using a dipping method at 30, −5, and 20 °C. To compare the chemical etching behaviors of prepared OTS monolayers, the OTS-treated substrates were immersed in 5% hydrofluoric acid (HF) and washed with copious distilled water. The surface wettability of the monolayers was determined by measuring the contact angle of a probe liquid (distilled water) on the surface using a contact angle meter (Krű ss BSA 10). The thickness of the OTS monolayers was determined using an ellipsometer (M-2000 V, J. A. Woollam Co., Inc.), and the root-mean-square (rms) surface roughness was measured by AFM (Digital Instruments Multimode). The chain conformations in the OTS monolayers were investigated using Fourier-transform infrared spectroscopy (FT-IR, Bruker IFS 66v) with p-polarized light and an incident angle of 80°. Chain conformations and crystalline structures were also determined using X-ray photoemission spectroscopy (XPS) and grazing incidence X-ray diffraction (GIXD) (4D and 8A2 beamlines for XPS measurements and 5A and 9C beamlines for GIXD experiments at the Pohang Accelerator Laboratory, Korea). 4.2. Pentacene Thin Film Preparation and Characterization. Pentacene (organic semiconductor, Aldrich Chemicals, no purification) was deposited from a quartz crucible onto the OTS-modified substrates at a rate of 0.2 Å s−1 and a substrate temperature of 30 °C using an organic molecular-beam deposition (OMBD) system in which the temperature of the crucible was maintained at 210 °C under a base pressure of 10−7 Torr. The deposition rate, film thickness, and substrate temperature were recorded during deposition. The microstructures of the pentacene films on the dielectric surfaces were characterized using relatively thin (10 nm-thick) pentacene films, which permitted measurement of the film inner structures. 2D GIXD (3C and 9A beamlines) and specular XRD measurements (5A and 9C beamlines) were performed at the Pohang Accelerator Laboratory, Korea. Raman measurements were performed in the backscattering

Figure 8. Schematic drawings of OTS monolayers prepared at −30 or 20 °C and of the growth processes of the pentacene films deposited on them.

high degree of surface homogeneity, and hence fewer grain boundaries, QE growth of the deposited pentacene film is enhanced, leading to the formation of a film with superior morphological and crystalline characteristics. However, when the OTS monolayer has poor surface homogeneity, the presence of numerous grain boundaries disturbs the QE growth of the pentacene film. In addition, the grain boundaries in the underlying OTS monolayer would be expected to limit the diffusion and rearrangement of pentacene molecules to form high quality thin films. Consequently, the grain structure of the OTS monolayers is related to the QE growth of pentacene films, with OTS monolayers with large grains giving rise to higher quality pentacene films.

3. CONCLUSIONS In summary, we have demonstrated the effect of the grain structure of crystalline SAM on the growth of a pentacene film, as well as on the electrical performance of FETs based on such films. Our results indicate that grain boundaries in the SAM act 4674

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Chemistry of Materials geometry with a JY LabRam HR fitted with a liquid nitrogen-cooled CCD detector. The spectra were collected under ambient conditions using the 633 nm line of a He−Ne laser. 4.3. Fabrication and Characterization of OFETs. The FET devices were completed by evaporating gold through a shadow mask onto a 50 nm-thick pentacene film to define the source and drain contacts. The channel length and width were fixed at 100 and 500 μm, respectively. The current−voltage characteristics of the fabricated devices were characterized by operating the OFETs in the accumulation mode under an applied negative gate bias. The source electrode was grounded, and the drain electrode provided a negative bias. The electrical characteristics of the FET devices were obtained at room temperature under ambient conditions in a dark environment using Keithley 2636A source/measure units. The temperaturedependent field-effect mobility measurements were performed using a cryostat system with liquid N2 in a vacuum chamber. The field-effect mobility (μ) and threshold voltage (Vth) were estimated in the saturation regime (VDS = −60 V) according to the following equation:1

ID =

W μCg(VGS − Vth)2 2L

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(2)

where ID is the drain current, Cg is the capacitance of the gate dielectric, and VGS is the gate−source voltage.

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ASSOCIATED CONTENT

S Supporting Information *

Specular XRD patterns and rocking scans at the (002) peak along the out-of-plane 10 nm-thick pentacene films deposited on the −30 and 20 °C OTS monolayers. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.5b01207.

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AUTHOR INFORMATION

Corresponding Authors

*(H.S.L.) E-mail: [email protected]. *(K.C.) E-mail: [email protected]. Author Contributions §

B.K. and N.P. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF2014R1A1A4A01009458) and a grant (Code No. 20110031628) from the Center for Advanced Soft Electronics under the Global Frontier Research Program of the Ministry of Science, ICT, and Future Planning, Korea. We thank the Pohang Acceleratory Laboratory for providing the synchrotron radiation source at the 3C, 4D, 5A, 8A2, 9A, and 9C beamlines used in this study.

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