Article pubs.acs.org/cm
Unravelling the Self-Assembly of Hydrogen Bonded NDI Semiconductors in 2D and 3D Chaoying Fu,† Hua-ping Lin,† Jennifer M. Macleod,‡,§ Andrey Krayev,∥ Federico Rosei,‡,⊥ and Dmitrii F. Perepichka*,† †
Department of Chemistry and Center for Self-Assembled Chemical Structures, McGill University, 801 Sherbrooke Street West, Montreal, Quebec, Canada H3A 0B8 ‡ INRS−Energy, Materials and Telecommunications and Center for Self-Assembled Chemical Structures, Varennes, Quebec, Canada J3X 1S2 § School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology (QUT), Brisbane, 4001 QLD, Australia ∥ AIST-NT Inc., 359 Bel Marin Keys Blvd, Suite 20, Novato, California 94949, United States ⊥ Institute for Fundamental and Frontier Science, University of Electronic Science and Technology of China, Chengdu 610054, PR China S Supporting Information *
ABSTRACT: Supramolecular ordering of organic semiconductors is the key factor defining their electrical characteristics. Yet, it is extremely difficult to control, particularly at the interface with metal and dielectric surfaces in semiconducting devices. We have explored the growth of n-type semiconducting films based on hydrogen-bonded monoalkylnaphthalenediimide (NDI-R) from solution and through vapor deposition on both conductive and insulating surfaces. We combined scanning tunneling and atomic force microscopies with X-ray diffraction analysis to characterize, at the submolecular level, the evolution of the NDI-R molecular packing in going from monolayers to thin films. On a conducting (graphite) surface, the first monolayer of NDI-R molecules adsorbs in a flat-lying (face-on) geometry, whereas in subsequent layers the molecules pack edge-on in islands (Stranski− Krastanov-like growth). On SiO2, the NDI-R molecules form into islands comprising edge-on packed molecules (Volmer−Weber mode). Under all the explored conditions, self-complementary H bonding of the imide groups dictates the molecular assembly. The measured electron mobility of the resulting films is similar to that of dialkylated NDI molecules without H bonding. The work emphasizes the importance of H bonding interactions for controlling the ordering of organic semiconductors, and demonstrates a connection between on-surface self-assembly and the structural parameters of thin films used in electronic devices.
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have been extensively used to characterize the first monolayer (and sometimes multilayers7) of OSCs on conducting and insulating surfaces, respectively. For face-on oriented molecules (aromatic plane parallel to the surface), submolecular or atomic spatial resolution can be often achieved. Many benchmark semiconductors, including pentacene,8−10 rubrene,11,12 porphyrin derivatives,13−15 oligothiophene,16,17 perylenetetracarboxylic dianhydride (PTCDA),18−20 oligo(p-phenyleneethynylene) derivatives,21,22 etc., have been studied by STM, revealing the details of substrate−molecule and intermolecular interactions. Scanning Probe Microscopy (primarily AFM) is also used routinely to assess the overall morphology and mesoscopic
INTRODUCTION π-Conjugated molecules and polymers are widely used as organic semiconductors (OSC) in various types of optoelectronic devices, including organic light emitting diodes (OLED), organic field-effect transistors (OFET), and organic photovoltaic cells (OPV).1−3 The molecular arrangement of OSCs in thin films controls the intermolecular electronic couplings and ultimately determines charge and exciton mobility in these materials. The supramolecular structure at the interface with metallic and dielectric device components is particularly relevant. For example, the first few layers of the OSC near the gate dielectric surface in an OFET fully define the charge transport properties of the device.4 A variety of different techniques have been employed to study the structure of OSC films. Scanning Tunneling Microscopy (STM) and Atomic Force Microscopy (AFM)5,6 © XXXX American Chemical Society
Received: December 4, 2015 Revised: January 5, 2016
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DOI: 10.1021/acs.chemmater.5b04706 Chem. Mater. XXXX, XXX, XXX−XXX
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Figure 1. Self-assembly of NDI-C8 (10−5 M) at the octanoic acid/HOPG interface. (a) Overview STM image (25 × 25 nm2, Vb = 850 mV, It = 0.3 nA). (b) High resolution STM image (14.5 × 6.0 nm2: Vb = 800 mV, It = 0.3 nA). (c) Molecular model of the NDI-C2 SAMN. D denotes the displacement of the NDI core along the short molecular axis relative to the neighboring one.
films on both conducting (graphite) and insulating (SiO2) substrates, to bulk (3D) crystals. By carefully choosing the NDI-R molecule and tailoring the R groups, we identified growth conditions for preparing thin films suitable for all three analyses, and conducted the characterization under ambient conditions compatible with realistic OSC device applications. We explicitly correlate the initial stage of NDI-R thin film formation with the 2D self-assembly and the single crystal structure, hence bridging the gap between the fields of surface self-assembly and crystal engineering of OSCs.
structure of OSC films with edge-on orientation (aromatic plane perpendicular to the surface, or slightly tilted). However, molecular resolution is rarely achieved23,24 on these films due to the much smaller footprint/higher density of edge-on oriented molecules. As a result, bulk averaging techniques such as Grazing Incident X-ray Diffraction (GIXRD)25−27 and angle resolved Near-Edge X-ray Absorption Fine Structure (NEXAFS) are required to reveal in detail the orientation and molecular packing of OSC films.28,29 While relatively complex and expensive (typically requiring a synchrotron beamline), a combination of these studies has led to a reasonably good understanding of the growth of OSC films of simple rigid aromatic molecules. A common conclusion is that the supramolecular structure of thin films, particularly of the first monolayer, is often slightly or even dramatically different from that of bulk 3D crystals. On the other hand, supramolecular control of OSCs requires increased structural complexity of the molecular building blocks, and the details of the film growth of these more complex molecules are much less understood. Long alkyl chains30 and bulky substituents31 are commonly introduced to tweak solid state packing, enhance intermolecular interactions, and alter the π-stacking motif. Highly directional interactions, such as hydrogen bonding (H bonding), have been widely used in the self-assembly of π-functional molecules; however, until recently,32−38 their applicability for supramolecular control of semiconducting materials was unclear. Recently, we demonstrated the fabrication of dual-channel OFETs using OSC cocrystals assembled via complementary H bonding interactions between naphthalenetetracarboxydiimide (NDI) and dipyrrolopyridine derivatives.39 NDI, along with its derivatives and homologues, is among the most efficient and most wellstudied n-type OSCs.40 Alkylation of only one of the imide moieties confers sufficient solution processability to NDI while bringing a unique opportunity to explore the combined role of van der Waals, π-stacking, and H bonding interactions in defining the supramolecular order in active OSC materials. Herein, we combine STM and AFM measurements with single-crystal X-ray diffraction to characterize the supramolecular ordering of N-alkylnaphthalenediimide (NDI-R) semiconductors, from face-on monolayers to edge-on thin
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RESULTS Self-assembly at the liquid/HOPG interface. Applying a drop (6 μL) of NDI-C8 solution in octanoic acid onto a highly oriented pyrolytic graphite (HOPG) substrate leads to the immediate formation of monolayers of molecular networks, as observed by STM at the solid−liquid interface. High-resolution STM images (Figure 1a) reveal an oblique periodic arrangement of close-packed bright protrusions. This phase is stable within the studied concentration range (from 10−5 M to ∼10−2...10−3 M, i.e., up to saturated solutions). The discrete bright protrusions match the dimensions of the NDI aromatic cores. The measured unit cell contains two nonequivalent molecules and has parameters of a = 0.90 ± 0.05 nm, b = 1.97 ± 0.05 nm, and γ = 90 ± 2° (Figure 1a). The octyl chains of the NDI-C8 molecules are not observed in STM images, and the separation of aromatic cores is insufficient to accommodate them on the surface. A molecular mechanics model (assuming that the alkyl chains project above the surface) yields a very similar unit cell: a = 0.90 nm, b = 2.10 nm, γ = 87° (Figure 1b). In some STM images, the neighboring molecular features along the vector b show an alternation of brighter/dimmer contrast which might be attributed to the higher topography feature of the protruding alkyl chains41 asymmetrically located in the network (Figure 1b, Figures S1, S2). This out-of-plane orientation of the alkyl chains is somewhat unusual for selfassembled molecular networks (SAMNs) on HOPG, although not unprecedented.42,43 A similar behavior was earlier observed by Miyake et al. in the SAMN of dioctyl-NDI.44 It originates from unfavorable steric interactions between the imidic oxygen and the β-CH2 group of the alkyl chain which disfavors the B
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Figure 2. Self-assembly of NDI-C16 (10−5 M) at the octanoic acid/HOPG interface. (a) STM image of the initially formed lamellar structure (35 × 35 nm2; Vb = 900 mV, It = 0.25 nA). (b) STM image (50 × 50 nm2) after 40 min of scanning, showing the appearance of a new densely packed domain surrounded by lamellar structure. The inset (8 × 8 nm2) shows the zoomed-in area of the densely packed domain. Vb = 900 mV, It = 0.26 nA. (c) Models of the lamellar structure of NDI-C16. (d) AFM topography image (496 × 496 nm2) of NDI-C16 (5 × 10−5 M) at the octanoic acid/ HOPG interface obtained in PeakForce tapping mode. The insets show a 2D-FFT analysis of the image and height profile of the highlighted region.
coplanar conformation by ∼7 kcal/mol, according to DFT (Figure S3). This is comparable to the adsorption energy (10 kcal/mol45) of octane on HOPG, from the gas phase (and is most likely higher than the corresponding adsorption energy from solution). In addition, the conformationally unrestricted alkyl chains projecting in solution increase the entropy of the system, which, together with the above steric repulsion factors, counterbalance the loss of adsorption energy. To enforce the in-plane adsorption of NDI-R, we elongated the alkyl chain from octyl to hexadecyl (NDI-C16). After applying the 10−5 M solution of NDI-C16 in octanoic acid onto graphite, NDI-C16 self-assembles into a lamellar structure with unit cell parameters a = 0.93 ± 0.05 nm, b = 3.73 ± 0.05 nm, and γ = 86 ± 2° (Figure 2). The bright lamellae can be attributed to H-bonded dimers of the NDI aromatic cores, which form rows stabilized by weaker Csp2H...O interactions along the a direction. Molecular modeling suggests that this polymorph can be achieved by either (i) full planarization of the alkyl chain with the aromatic core or (ii) introducing gauche conformations in the chain (Figure 2c). The first scenario introduces a ∼ 7 kcal/mol steric repulsion between the carbonyl and α-CH2 group, but maximizes the alkyl chain− HOPG interactions; the second scenario avoids the above steric stress but introduces unfavorable gauche conformations and keeps at least two (α and β) CH2 from interacting with the
surface. The latter also leads to slightly higher lamellae density (0.58 molecule/nm2 vs 0.55 molecule/nm2), which increases the overall adsorption energy. A comparison of experimental and calculated unit cell parameters (Figure 2c) speaks in favor of model (ii) with gauche conformations. After continuous scanning for a prolonged time (>40 min), which leads to partial solvent evaporation, or by increasing the initial solution concentration to >10−4 M, a more densely packed polymorph emerges (Figures 2b and S4). The new phase aligns in the same direction as the surrounding lamellar phase. Its unit cell (a = 0.90 ± 0.05 nm, b = 1.95 ± 0.05 nm, and γ = 90 ± 2°) is indistinguishable from that of NDI-C8, strongly suggesting that the hexadecyl chains desorb from HOPG, forming a close-packed polymorph similar to that observed for NDI-C8. This transformation at increasing concentration must be driven by higher molecular densities of the close-packed (1.13 molecules/nm2) vs the lamellar phase (0.58 molecules/nm2).46,47 To exclude the possibility of multilayer formation, AFM in PeakForce mode was used to probe the self-assembly of NDI-C16 at the HOPG-liquid interface, confirming the coexistence of the lamellar and closepacked phases (Figure 2d). While the resolution of the AFM at the solid/liquid interface is lower than that of the STM, we can clearly identify structurally identical lamellar domains with a periodicity of ca. 4.0 nm and 120° mutual orientation C
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The limited resolution of the ambient AFM does not resolve any molecular features within these domains; however, STM imaging of the same sample (in air, Figure 3e) reveals linear structures with a periodicity of 1.0 ± 0.1 nm, in agreement with the spacing of the close-packed structure at the solid/HOPG interface. AFM at smaller length scales shows periodic stripes with small height modulation (0.05−0.10 nm) between the close-packed domains (Figure 3c). The periodicity of these stripes (4.0 ± 0.5 nm) is similar to that of the lamellar domain of NDI-C16 at the solid/liquid interface. However, stripes of the same periodicity (4.2 nm, Figure S8) have also been observed to coexist with the shorter NDI-C8 molecules, thereby ruling out their attribution to the NDI lamellar phase. Similar stripes with periodicity of ∼4.2 nm have been previously reported at the graphene/water interface in the presence of nitrogen.49−51 These were attributed to the gas/water adsorption on a hydrophobic surface (vide inf ra). Additional AFM characterization of the same submonolayer coverage sample after it was held for 2 weeks in ambient reveals a substantial morphological change that can be described as a dewetting of the close-packed polymorph (Figure 3f). The apparent surface coverage decreases from ∼90 to ∼25%, and the step height of molecular islands increases from 0.3 to 3.3 nm. This is indicative of a transition from face-on to edge-on orientation of molecules. The ∼0.3 nm height variation of these new islands suggests that close-packed domains of face-on oriented molecules might be retained underneath some of them. At a moderate coverage (>1 ML) of NDI-C16 on HOPG, the AFM images show the coexistence of several layers with discrete, well-defined thicknesses (Figure 4a). Layer I is used to label the structure that has already appeared in the submonolayer film (Figure 3a). Layer II denotes newly emerged discrete molecular islands, which have a step height of 3.3 ± 0.2 nm (Figure 4b). This step height slightly exceeds the full van der Waals length of the molecule (3.1 nm). Within this layer, occasional depressions of 0.34 ± 0.07 nm step height are clearly discerned (Figure 4a, line 2). The presence of these step depressions correlates with the state of the surrounding surface, suggesting that the isolated molecular islands grow continuously above either the face-on monolayer (layer I, marked with blue line) or the bare HOPG substrate. Occasionally at this molecular coverage, layer III (Figure 4b) is observed with a double step height of 6.6 ± 0.2 nm. An adhesion channel of the peak-force AFM (Figure 4c, d) shows an identical contrast for layers II and III (as well as rarely observed layer V, 13.2 ± 0.2 nm), suggesting the same surface/ tip interaction. This implies the same chemical functionality on both surfaces, despite the asymmetric structure of the molecule (NH vs NC16H33 termination). The same observations were also made using phase contrast images of dissipation mode AFM (Figure S6). In both PeakForce and dissipation AFM images, a raised feature (“curb”) appears in the middle (Figure 4a, line 1) and/or along the edge (Figure S6) of some molecular islands in layer II. They appear as narrow lines of ∼0.3 nm height. The alignment of the curbs with the long edge of the islands corresponds to their fast-growth direction. Further extending the deposition time of NDI-C16 onto the HOPG substrate leads to a thicker film (Figure 5). The molecular islands coalesce at discrete angles of 30°, 60°, 90°, 120° and 150°, consistent with the epitaxial relation between the underlying HOPG lattice (unit cell angle 120°) and the 2D
(confirmed by 2D FFT), resembling those observed in STM. The brighter contrast islands show a ∼0.20−0.25 nm higher topography than the surrounding lamellar domains (Figure 2d inset). This is smaller than the expected interplanar spacing of NDI cores (i.e., 0.34 nm in the single crystal of unsubstituted NDI48). The edges of these bright islands are coaligned with the surrounding lamellar domains, and we attribute them to the close-packed domain, as mentioned above. The height difference between the two polymorphs is therefore attributed to the “effective” thickness of the disordered alkyl chains in solution. Self-assembly at the gas/HOPG interface. To investigate the transition of molecular assembly from adsorbed monolayers to ultrathin films (the device-relevant form), we performed ex situ AFM studies of vacuum-deposited NDI-R on an HOPG surface. At submonolayer coverage (ca. 0.9 ML, based on AFM analysis), NDI-C16 formed flat domains with a step height of 0.30 ± 0.07 nm (Figure 3a, c).
Figure 3. (a) PeakForce mode AFM topography image (1 × 1 μm2) and (b) adhesion map of an NDI-C16 thin film on HOPG at submonolayer coverage (0.9 ML). (c) Zoom-in area (200 × 200 nm2) of the NDI-C16 monolayer on HOPG and (d) height profile corresponding to the black line in (c). (e) STM image (24 × 24 nm2) of the same NDI-C16 and a corresponding 2D FFT image showing a periodicity of 1.0 ± 0.1 nm. Vb = 1000 mV, It = 0.10 nA. (f) AFM topography image (1 × 1 μm2) of the NDI-C16 submonolayer film sample shown in (a−c) after storage in ambient atmosphere for 2 weeks. D
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crystal lattice of molecular layers (unit cell angle 90°, vide inf ra) (Figure 5, Figure S10 and Scheme S1). NDI-C8 thin films on HOPG also form a flat lying closepacked monolayer (0.30 ± 0.05 nm) (Figure S8), and isolated islands with a first-layer height of 2.5 ± 0.2 nm (also larger than the full van der Waals length of the molecule: 1.97 nm according to X-ray analysis, 2.1 nm according to DFT) and a second-layer height of 4.8 ± 0.2 nm (Figure S9a-c). Self-assembly at gas/SiO2 interface. Substrate effects were explored by growing ultrathin NDI-C16 and NDI-C8 films on a SiO2/Si substrate, a typical gate dielectric used in fieldeffect transistors. The hydrophilic SiO2 surface does not interact as strongly with the molecules as HOPG and phases comprising face-on oriented molecules were not observed on SiO2/Si by either PeakForce or dissipation mode AFM.52 Instead, both NDI-C16 (Figure 6 and S7) and NDI-C8 (Figure S9d−f) grow in an edge-on orientation with the same height as that measured on HOPG for layers II and III (Table 1). No step depressions associated with face-on a “sublayer” are observed on SiO2, yet the same “curb” features appear ∼0.3 nm above the islands, with a similar frequency as on HOPG (Figure 6b and S7c). The molecular islands grown on SiO2/Si do not display any preferred mutual orientation (Figure S10d), and their edges are much less sharp than those observed on HOPG, highlighting the epitaxial role of HOPG in controlling the growth of these molecular films. Self-assembly in 3D crystals. The supramolecular structure of NDI-C8 in spatially unrestricted (3D) single crystals was investigated by X-ray diffraction. Grown from octanoic acid solution, NDI-C8 forms monoclinic (but nearly orthorhombic) crystals with the space group P21/c. There are four identically structured but differently oriented molecules in the primitive unit cell (a = 4.77 Å, b = 7.71 Å, c = 48.85 Å, α = 90°, β = 91.6°, γ = 90°) (Figure 7). The molecules form H bonded pairs with the same R22(8) binding motif observed on surfaces. These pairs are linked together in continuous aromatic lamellae via weaker CH···O interactions of NDI bay hydrogens and imide oxygens. The geometry of these contacts defines the displacement (D) of adjacent NDI pairs along the long molecular axes. The value of displacement in the bulk structure (Figure 7b, D = 0.42 nm) is similar to that in the lamellar 2D phase (Figure 2c, D = 0.42 nm) yet different from the close-packed 2D polymorph (Figure 1b, D = 0.28 nm). Although these CH···O interactions are
Figure 4. (a) PeakForce mode AFM topography image (1 × 1 μm2) of an NDI-C16 film on HOPG at moderate coverage. The dotted light blue line defines a borderline between two graphene sublayers. (b) The height profile corresponding to the lines shown in (a). (c) PeakForce mode AFM topographic image and (d) adhesion channel image (1 × 1 μm2) of an NDI-C16 thin film on HOPG at a moderate coverage (>1 ML).
Figure 5. AFM topography image (3 × 3 μm2) of the NDI-C16 thin film at a high coverage on HOPG obtained in tapping mode, and the histogram of the angles between coalesced islands. E
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Figure 7. Different views of molecular packing in NDI-C8 single crystal highlighting: (a) unit cell; (b) aromatic lamella formed by lateral interaction of H bonded NDI pairs (D denotes the displacement of the neighboring NDI pairs along the long molecular axis); (c) π-interactions showing quasi-two-dimensional brickwall-type packing. Figure 6. (a) PeakForce mode AFM topography (4.1 × 4.1 μm2) of a NDI-C16 thin film with a nominal thickness of 1.0 nm on SiO2/Si. (b) Topography image of zoom-in area (1 × 1 μm2), and (c) height profiles along the black and red lines in (b), which respectively cross the islands and curbs in the middle of the islands.
is typically observed for alkylated aromatic molecules (Figure 1).55−58 As the solution concentration increases, the alkyl chains start to desorb from the surface leading to a close-packed lattice of face-on aromatic cores (Figure 2). As discussed above, this process is a result of a delicate balance between molecular strain (∼7 kcal/mol in a fully coplanar conformation) and adsorption energy gained by placing the alkyl chain on the surface. Reducing the latter term by shortening the alkyl chain makes the close-packed polymorph the preferred structure for NDI-C8 (Figure 1). The same close-packed polymorph is also observed in NDIC16 submonolayer films vacuum-deposited on the conducting HOPG surface (Figure 4). However, in contrast to the liquid/ HOPG interface, the close-packed polymorph (layer I) at the air/HOPG interface is metastable: aging the films (Figure 3f) or increasing molecular coverage (Figure 4) leads to dewetting of the molecules, which form islands (layer II) of edge-on orientated molecules. This implies that the solvent provides an additional stabilization energy solvating one face of the adsorbed molecules in layer I. During the dewetting process, the face-on monolayer I is preserved under the edge-on layer II although the latter can also continue its growth over the bare HOPG surface, as suggested by occasional 0.3 nm depressions on layer II (Figure 5). We speculate that the alkyl chains of the face-on layer I interdigitate in the layer II, acting as an anchor for the growth of the latter (Scheme 1). This interaction resembles that observed in the bulk 3D crystal structure of NDI-C8 and the AFM measured thickness of layer II (2.5 nm) is in agreement with the corresponding distance L (2.5 nm) in the crystal structure (Scheme 1). This model also explains the epitaxial relationship between the edge-on layers and HOPG, which is apparent from the discrete mutual orientation angles of the molecular islands (Figure 5). The interdigitation of the alkyl chains also likely takes place between layer II and the subsequently grown edge-on layers (III, etc.), as in the bulk
Table 1. Summary of the Heights of Hierarchical Layer Structures in Ultrathin Films of NDI-R Height (nm) Film/substrate
Layer I
NDI-C16@HOPG NDI-C16@SiO2/Si NDI-C8@HOPG NDI-C8@SiO2/Si
0.30 ± 0.05 not observed 0.30 ± 0.05 not observed
Layer II
Layer III
± ± ± ±
6.6 ± 0.2 6.7 ± 0.2 4.8 ± 0.2 not observed
3.3 3.3 2.5 2.5
0.2 0.2 0.2 0.2
much weaker than the R22(8) synthons,53 they are nevertheless consistently observed in most NDI derivatives.54
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DISCUSSION Our STM/AFM/X-ray diffraction studies provide a clear picture of the mechanisms of self-assembly of monoalkylated NDI in different environments−from monolayers at a liquid− solid interface to vapor-grown thin films and bulk 3D crystals (Scheme 1). Octanoic acid was selected as a solvent due to its ability to form strong H bonding (15.6 kcal/mol as estimated by B3LYP, even larger than 12.5 kcal/mol for self-association of NDI-R, Table S1) with the imide moiety that prevents aggregation of NDI-R in solution. All films grown on HOPG from the NDI-R solutions (presumably monomeric) are characterized by face-on orientation of the dimerized NDI cores, which engage in π−π interactions with HOPG, thereby driving the assembly at the liquid−solid interface. At low concentrations (10−6...10−5 M), the alkyl groups of the longer chain derivative (NDI-C16) are also adsorbed on the surface forming characteristic lamellae, as F
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Scheme 1. Different Modes of NDI-R Self-assembly at Solid-Liquid (a) and Solid-Gas (b) Interfaces, and in 3D Crystals (c)
and in single crystal (0.08 cm2V−1s−1,39) are comparable with those of dialkylated NDIs without H bonding.54,62 Also, very similar mobilities were measured on bare SiO2/Si and alkylsilanyzed (hydrophobic) dielectric substrates. We conclude that electron transfer in this system does not lead to protonation/deprotonation events, likely because the most acidic (NH) proton is stabilized by strong H bonding. Finally, we discuss the unusual periodic ∼4 nm strip features visualized by PeakForce AFM on the HOPG surface between the molecular domains. As already mentioned, this nanopattern is unlikely to originate from the deposited NDI-R structures since its periodicity (4.0 ± 0.5 nm) is larger than the molecular length and is identical for both short- (NDI-C8) and long-chain (NDI-C16) derivatives. Additional PeakForce adhesion mapping experiments indicate that hydrophilic silicon nitride AFM tips interact more strongly with these stripy regions than with the NDI-C16 face-on domains (Figure S5). Recently, several AFM (PeakForce and frequency-modulation) studies have revealed similar ∼4.2 nm pitch strip domains formed at the HOPG/water49 or graphene/air51,63 interface. These are typically attributed to nanostructured gas molecules selfassembled at the HOPG/water interface, which relates to a more general phenomenon of interfacial nanobubbles (INB) on hydrophobic surfaces.64−66 When scanning at the HOPG/air interface the water phase could be created by adsorption of ambient humidity; an alternative explanation of surface water crystallization63 was also proposed. The two hypotheses are not mutually exclusive, but neither one, to our knowledge, predicts the specific persistent periodicity of ∼4.2 nm for the observed nanostripes. These nanostripes were highly reproducible at the HOPG/ air interface in the presence of a submonolayer of NDI-C8 or NDI-C16, but were never observed on pristine HOPG in these conditions. This could be explained by either a confinement49b or seed63 effect of the molecular layers. Related to the applications of the studied materials as n-type semiconductors, the presence of water at the interface creates a well-known problem of trapping negative charge carriers.67,68 Our studies highlight that even hydrophobic surfaces may contain an adsorbed ultrathin layer of water when exposed to ambient conditions.
of the NDI-R crystal. The 0.3 nm curbs frequently observed on top of layer II likely represent the seeds for the crystal growth of the upper layers. No face-on layer I was observed for films vacuum-deposited on the insulating SiO2 surface (Figure 6, Figures S7, S9d,e), which can be attributed to the loss of the π−π interactions with the surface and also to its higher roughness. The edge-on layer II grows directly on the substrate and has the same thickness as on HOPG, but in contrast to the latter, the resulting molecular islands are randomly oriented on the surface. Previous studies of a structurally related OSC without alkyl chains (PTCDI) on NaCl insulating substrate have reported both the face-on and edge-on molecular orientation.5,6 Similar to our observation for NDI-NDI-R on HOPG, aging of PTCDI/NaCl films over several days leads to dewetting of the face-on layers into edgeon islands. On both HOPG and SiO2 surfaces the edge-on layers (II, III, etc.) form molecular islands with uniform height and sharp edges, indicating their high crystallinity. Both PeakForce tapping and dissipation mode AFM show that the islands have adhesive properties that are similar to one another, and that the measured adhesion on the islands is distinctly different from that of layer I (Figure 4c,d, Figures S6, 7). This, along with the fact that the thickness of these layers (h = 2.5 ± 0.2 nm for NDI-C8; 3.3 ± 0.2 nm for NDI-C16) slightly exceeds the length of the molecules in the most extended conformation (2.1 nm for NDI-C8; 3.1 nm for NDI-C16 according to DFT calculations), suggests that each edge-on layer consists of H bonded pairs of NDI-R. In both bulk crystals and thin films the NDI-C8 molecules πstack into a quasi two-dimensional brickwork structure (Figure 7c). Such cofacial packing is also observed in many highperformance dialkylated NDI semiconductors, although the displacement of the NDI core with respect to the neighboring molecules (and therefore the degree of π-overlap) varies with the substituents.54,59 DFT calculations of charge transfer integrals suggest that electron hopping occurs preferentially along pathway A, which corresponds to the largest intermolecular π-overlap (Figure 7c).60 A recent study on diketopyrrolopyrrole semiconductors has proposed H bonds as a possible charge hopping pathway.61 While an experimental verification of this hypothesis would require measurements of anisotropic charge mobility, it is clear that intermolecular H bonding between amide/imide groups is not detrimental for electron transport. The field-effect electron mobilities of NDIC8 and NDI-C16 in thin films (0.02−0.06 cm2V−1s−1, Table S2)
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CONCLUSIONS AND PERSPECTIVES We have unraveled the packing of monoalkylated NDIs in both on-surface self-assembly and bulk crystals by using a combination of STM, AFM, and X-ray diffraction analyses. G
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Article
Chemistry of Materials
Inc., Novato, CA). The topographies were acquired either in PeakForce mode (BrukerScanAsyst mode and Nanoscope 8.15r3 software) on the Nanoscope Multimode 8 or in AIST-NT Dissipation mode70 (IAPro-3.2.1 software) on the SmartSPM. In PeakForce mode, silicon nitride cantilevers (ScanAsyst-Air Bruker) with a nominal spring constant of 0.4 N/m, a nominal resonance frequency of 50−90 kHz, and a nominal tip radius of 2 nm were used for imaging in air, and silicon nitride cantilevers (ScanAsyst-Fluid Bruker) with a nominal spring constant of 0.7 N/m, a nominal resonance frequency of 150 kHz, and a nominal tip radius of 20 nm were used for imaging in liquid. In dissipation mode, HQ-NSC14-AlBS probes (Mikromasch probes) with a spring constant of 5 N/m, a resonance frequency of 160 kHz, and a nominal radius of 7 nm were used for imaging in air. To prepare a liquid sample for AFM imaging, NDI-C16 solution in octanoic acid (5 × 10−5 M) was introduced into a liquid cell (∼30 μL) and the cantilever was completely submerged in the solution. Scanning was performed immediately and molecular patches were observed after 45 min. AFM image analysis was performed using WSxM,69 Nanoscope Analysis1.4 and IAPro-3.2.1 software. The height of each molecular layer on AFM topographical images, together with its standard deviation, was determined statistically from numerous height profiles extracted from different locations of the images and from repeated image scans. Molecular modeling. The molecular assemblies were geometrically optimized by a combination of molecular mechanics (using HyperChem 7.5271) and DFT calculations (using Gaussian 0972). All single molecule calculations were carried out by DFT at the B3LYP/631G(d) level. A supramolecular cluster (containing 4 pairs of H bonded NDI-R model molecules) was placed on a (larger) graphene cluster and optimized by MM+ force field to a rms deviation of energy gradient smaller than 0.01 kcal/(Å·mol). During the optimization process, the graphene layer was kept frozen. For the close-packed structure calculations, an ethyl-substituted NDI-C2 model was used. As the MM+ force field overestimates the length of H bonds (O···H 0.28 nm), a pair of H bonded NDI-Me molecules was optimized by DFT at B3LYP/6-31G(d) and the obtained geometry (O···H distance 0.19 nm, similar to that found by X-ray diffraction of NDI-C8 single crystal) was used to adjust the MM+ optimized molecular cluster.
STM in conjunction with AFM reveals the dynamic selfassembly of the NDI-R monolayer at both liquid/solid and gas/ solid interfaces. H bonding clearly dominates the self-assembly of these molecules, which are found as H bonded dimers in all cases. AFM reveals the hierarchical molecular ordering between the layers of the growing films. XRD elucidates the molecular ordering of monoalkylated NDI in bulk. By integrating the results from all three analyses, we provide a comprehensive view of the thin film structure of NDI-R beyond the first monolayer. Both NDI-R molecules form a flat-lying monolayer on HOPG with the H-bonded aromatic cores parallel to the surface, and the alkyl chain adsorbed/desorbed from the surface depending on the chain length and the environment (solution concentration; solid/liquid or solid/air interface). In the subsequent film growth on HOPG or SiO2/Si substrate, the molecular model deduced from the single crystal structure indicates an edge-on orientation of NDI cores on HOPG after the first flat-lying monolayer (Stranski−Krastanov-like growth) or directly on SiO2/Si substrate (Volmer−Weber growth). The use of H bonding interactions is already emerging as a reliable approach to control the assembly of organic semiconductors, yet its impact on molecular ordering in thin films (that is, in the intermediate regime between the face-on monolayers and bulk 3D crystals) has not been previously explored. Our work is an attempt to establish connections between the vast literature on STM exploration of 2D supramolecular assemblies of polycyclic aromatic molecules and the structural information required to understand the semiconducting properties of these compounds. Future challenges lie in applying the knowledge of the surface-confined self-assembly to control the properties of organic semiconducting films and optoelectronic devices thereof.
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EXPERIMENTAL SECTION
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Synthesis. NDI-Rs were synthesized from naphthalene-1,4,5,8tetracarboxylic dianhydride (NTCDA) by sequential reactions with corresponding alkylamine and ammonia, following the literature procedure for NDI-C8.39 Sample preparation. Thin films of NDI-C16 and NDI-C8 were prepared by evaporation at 415 and 400 K, respectively, onto a clean highly oriented pyrolytic graphite (HOPG) or SiO2/Si surface held at room temperature in a glass chamber evacuated to 10−6 mbar. The typical deposition rate was 0.01 nm/s and the nominal film thickness was calculated according to the deposition time. The HOPG (grade 2, SPI Supplies) was cleaned by exfoliating with adhesive tape. The SiO2/ Si wafer (100-oriented, 1−5 nm nominal thickness of oxide layer, ptype and degeneratively doped to
