Structural Disordering upon Formation of Molecular Heterointerfaces

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Structural Disordering upon Formation of Molecular Heterointerfaces Kouki Akaike, Akira Onishi, Yutaka Wakayama, and Kaname Kanai J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b01123 • Publication Date (Web): 24 Apr 2019 Downloaded from http://pubs.acs.org on April 24, 2019

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The Journal of Physical Chemistry

Structural Disordering upon Formation of Molecular Heterointerfaces Kouki Akaike,*,†,‡,§ Akira Onishi,†,‡ Yutaka Wakayama,*,¶ Kaname Kanai†

†

Department of Physics, Faculty of Science and Technology, Tokyo University of Science, 2641

Yamazaki, Noda, Chiba 278-8510, Japan ¶

International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for

Materials Science (NIMS), 1-1 Namiki, Tsukuba 305-0044, Japan

Corresponding Authors *Kouki Akaike, Central 5, 1-1-1, Higashi, Tsukuba, Ibaraki 305-8565, Japan, telephone number: +81-29-849-1221, e-mail: [email protected] *Yutaka Wakayama, 1-1 Namiki, Tsukuba 305-0044, Japan, telephone number: +81-29-859-4403, e-mail: [email protected]

ACS Paragon Plus Environment

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ABSTRACT. The formation of an organic heterointerface is an essential, albeit rather complex, process in the growth of a molecular heterostructure. Sequentially stacking dissimilar molecules occasionally leads to molecular rearrangement, which potentially involves structural perturbation of the first layer. Disorder on the growth process complicates the interface structure and energetic landscape. However, current understanding of this disorder on the molecular scale remains primitive, which hampers the realization of envisioned heterointerface designs. Herein, we used scanning tunneling microscopy to directly observe the disorder upon the formation of sexithiophene (6T, top)/perfluorinated copper phthalocyanine (F16CuPc, bottom) interfaces. Vacuum deposition of 6T onto an epitaxially grown F16CuPc monolayer completely perturbed the molecular arrangement of F16CuPc along the [1–21] direction of the Ag(111) surface. In the disordered structure, the 6T molecules adopted various bent shapes owing to cis–trans isomerization between the thiophene rings and were randomly mixed with F16CuPc. Comparative experiments demonstrated that (i) multiple C–F···H–C bonds between the F16CuPc and bent 6T molecules stabilized the disordered binary structure, and (ii) sparse molecular packing of the F16CuPc monolayer and the conformational variety of 6T led to substantial disorder. This realspace imaging of the disordered structure facilitates the molecular understanding of the structural perturbation and, furthermore, provides design rules for molecular heterointerfaces.


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INTRODUCTION Heterointerfaces of π-conjugated molecules can exhibit superior photophysical behavior to the pure molecules,1–3 tune the energy-level offsets for desired optoelectronic properties,4,5 and provide opportunities to investigate interfacial covalent reactions.6,7 Since the molecular contact is responsible for these phenomena, the interfacial geometry8–10 and energetics11–14 have been extensively investigated. Multi-stacking of π-conjugated molecules by either vacuum deposition or solution processing can afford a variety of interface morphologies. The formation of a sharp interface,15–18 the typically expected scenario, is a consequence of the sequential deposition of the molecules. Molecular exchange across a heterointerface,8,9 intermixing,19–22 and molecular reorientation23 may occur, depending on the deposition sequence9 and the molecular orientation of the bottom layer.24 These molecular rearrangements are expected to involve disordering on the heterointerface growth process, but little is currently known regarding the molecular arrangements in the disordered structures. The occurrence of structural perturbation, which we address in this paper, has been speculated based on analysis of the electronic structure at heterointerfaces.25,26 For instance, upon the deposition of sexithiophene (6T) onto a layer of perfluorinated copper phthalocyanine (F16CuPc), the tailing states of the frontier orbitals of the latter rationalize the “kink” of the electrostatic potential distributions across the interface.25 Since the energy distribution of the frontier orbitals originates from the fluctuation of the polarization energy, which varies inversely with the fourth power of the intermolecular distance,27 a broader distribution can be attributed to increased structural inhomogeneity. In fact, the Bragg peaks originating from a F16CuPc film disappeared upon the deposition of 6T.26 However, the disorder that occurs during the growth of heterointerfaces remains poorly understood on the molecular scale. This is because, in general,


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deliberately disordering the molecular order is challenging, and real-space imaging of the disorder is crucial for elucidating the molecular arrangement. Such a nanoscale investigation will disclose the nature of structural inhomogeneity, which is often correlated with the weak density of states appearing in the energy gap. In this study, we directly observed the disorder introduced into an F16CuPc monolayer upon the deposition of 6T using scanning tunneling microscopy (STM). The molecular arrangement of F16CuPc along the [1–21] direction of the underlying Ag(111) surface was completely perturbed. Concurrently, the 6T molecules became bent owing to the cis–trans isomerization of the thiophene rings. Consequently, they were found to be mixed randomly with F16CuPc. However, when 6T molecules were deposited onto an epitaxially grown CuPc monolayer, ordered domains of neat 6T and CuPc were formed. This indicates that the energetically unfavorable bent 6T molecules were stabilized through attractive interaction with F16CuPc. The absence of the disorder in the reversestacked structure suggests the importance of the structural stability of the first layer. A comparative experiment involving the p-sexiphenyl (6P, a rodlike molecule)/F16CuPc interface further demonstrated that the various molecular conformations of 6T due to cis–trans isomerization led to no short- or long-range molecular order. The results of this work shed light on the nature of structural perturbation on the molecular scale, providing an opportunity to control the molecular order and interface morphology of stacked molecular semiconductors.

EXPERIMENTAL SECTION STM: All experimental procedures, including substrate cleaning, molecular deposition, and STM observation, were performed under ultrahigh vacuum with a background pressure of 1 × 10−8 Pa. Single-crystal Ag(111) surfaces were used as the substrate. An atomically clean surface was


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obtained via cycles of Ar+ ion sputtering and thermal annealing at 600 K and confirmed by the clear observation of flat terraces on Ag(111). Molecular deposition was performed at room temperature. F16CuPc, CuPc, 6T, and 6P were deposited from Knudsen cells at a rate of 0.03–0.13 monolayer (ML)/min. Evaporation amounts required for pristine monolayers of these molecules were determined by the observation that well-ordered domains fully covered the silver surface. Nominal coverages of overlayers were calculated from deposition time. STM observations were conducted at room temperature using a tunneling current of 0.1 nA and applied bias voltages ranging from −0.2 to 1.3 V. UPS: UPS measurements of the monolayers of F16CuPc, CuPc, and 6T were performed under ultrahigh vacuum (base pressure < 5 × 10−8 Pa) at the end station of beamline BL-2B of UVSOR (Institute for Molecular Science, Japan) using an electron analyzer (Scienta R3000). A photon energy of 28 eV was used as the excitation source to acquire the UPS spectra. The photoelectron emission angle was 45°. The films were prepared via the same procedure as that used for the STM experiments. The sample preparation and measurements were performed at room temperature.

RESULTS AND DISCUSSION First, we investigated the molecular arrangement of neat F16CuPc and 6T monolayers deposited on Ag(111) surfaces. Figure 1a shows a large-scale STM image of the F16CuPc monolayer. The F16CuPc molecules were aligned in a wave-like manner along the [–101] direction of the Ag(111) surface. An interdigitated one-dimensional molecular arrangement was observed along the [1–21] direction, involving two in-plane molecular orientations (denoted A and B in Figure 1a). This observation is consistent with a previous report.28 The high-resolution STM image presented in Figure 1b demonstrates the formation of oblique cells with dimensions of a1 = 17.5  0.5 Å, b1 =


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13.5  0.5 Å, and 1 = 103  3°. The averaged intermolecular distance is ~15 Å, which is consistent with the value determined by low energy electron diffraction and STM.28 In contrast, the 6T molecules deposited on the Ag(111) surface adopted a side-by-side packing arrangement (Figure 1c), which is in accordance with the work of Zhang et al.29 The unit cell was almost rectangular with dimensions of a2 = 25.3  0.2 Å, b2 = 6.3  0.4 Å, and 2 = 97  1°. The value of a2 is close to the reported separation between 6T stripes.29 Hence, the deposition of F16CuPc or 6T directly onto a clean Ag(111) substrate formed monolayers with long-range order.

Figure 1. STM images of the monolayers of F16CuPc and 6T. (a), (c) the large-scale images of (a) F16CuPc and (c) 6T, with the molecular structures depicted in the insets. “A” and “B” in (a) denote the two in-plane molecular orientations of F16CuPc. The scale bars in (a) and (c) represent 5 nm.


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(b), (d) High-resolution STM images of monolayers of (b) F16CuPc and (d) 6T. The labels in (b) and (d) indicate the lattice parameters. The scale bars in (b) and (d) represent 2 nm. However, we found that the deposition of 6T onto the F16CuPc monolayer significantly disordered the molecular arrangement of the latter. Figure 2a shows an STM image after deposition of 6T onto the F16CuPc monolayer. Although the original arrangement of F16CuPc was retained in certain regions, as indicated by the dashed green circles at the bottom of Figure 2a, the majority of the F16CuPc molecules had become disordered. Upon increasing the 6T coverage (Figure 2b), the molecular order of F16CuPc was completely perturbed. On the other hand, 6T molecules were directly adsorbed on the metal surface. As a result, the number of F16CuPc molecules decreased upon the deposition of 6T, which may be attributable to desorption of a fraction of the F16CuPc molecules from the substrate surface. The coverage of F16CuPc molecules was found to decrease almost by half in the area corresponding to Figure 2b. The replacement phenomenon should be relevant to molecular exchange.8,9,30 Notably, the adsorbed 6T molecules had adopted various bent shapes and became randomly mixed with the F16CuPc molecules. The variety of molecular shapes observed for 6T is more apparent in Figure 2c. These molecular shapes were clearly distinct from those observed in the 6T monolayer on Ag(111) (Figure 1c, d), in which the 6T molecules displayed a straight rodlike geometry. The bent shapes originated from the cis–trans isomerization of the thiophene rings.


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Figure 2. STM images of the F16CuPc monolayer covered with 6T of (a) 0.13 and (b) 0.20 ML. The dashed green circles in (a) indicate the original arrangement of F16CuPc that remained after the initial deposition of 6T. The scales bars in (a) and (b) represent 5 nm. (c) Enlarged STM image of the disordered binary structure of F16CuPc and bent 6T molecules. The scale bar represents 2 nm. The diverse range of conformations observed for 6T results from variation of the number of cisconfigured neighboring thiophene rings. Figure 3a and b show high-magnification STM images of representative U- and S-shaped 6T molecules in the disordered structure, respectively, along with models of the corresponding molecular arrangements. The U-shaped 6T molecules should originate from cis–trans isomerization of all five of the C–C single bonds connecting the thiophene rings. In contrast, the S-shaped molecules must possess the cis configuration for all of the C–C bonds except the central bond. A previous study on bithiophene (2T) reported that 0.75 kJ/mol is required for the conformational change from the trans to the cis state.31 To stabilize the bent shapes on the solid surface, an energetic gain during the disordering is thus necessary. The STM images reveal that several F16CuPc molecules typically surrounded each bent 6T molecule, hinting at the presence of attractive interactions between F16CuPc and 6T in the disordered heteromolecular layer. Most likely, C–F···H–C bonding between the heteromolecules is at play. Previous studies showed that the hydrogen bonding between fluorinated and non-fluorinated molecules led to the


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formation of ordered binary structure in co-evaporated systems.21,32–34 Calculations for the dimer of trifluorobenzene indeed revealed an energy of 1–2 kJ/mol per hydrogen bond.35 Therefore, multiple C–F···H–C bonds between F16CuPc and 6T (indicated by the dashed blue lines in the models depicted in Figure 3a, b) may compensate for the increase in energy originating from the unfavorable conformations of the bent 6T molecules.

Figure 3. High-magnification STM images showing the disorder of the 6T/F16CuPc interface (left), along with models of the corresponding molecular arrangements (right). (a) A U-shaped 6T molecule is typically surrounded by 4–5 F16CuPc molecules. (b) S-shaped 6T molecules are intercalated between F16CuPc molecules aligned along the [1–21] direction of the Ag(111) surface. (c) Low-magnification STM image following the deposition of 0.13 ML of F16CuPc onto the 6T monolayer on Ag(111), showing the long-range molecular order. The scale bar represents 10 nm.

To examine the influence of the fluorine-mediated intermolecular interactions on the local structure, the molecular arrangement of a 6T/CuPc interface grown on Ag(111) was next


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investigated. As shown in Figure 4a, the CuPc monolayer on the Ag(111) surface formed an oblique cell with dimensions of a3 = 14.5  0.5 Å, b3 = 14.0  0.5 Å, and 3 = 102  2°. In stark contrast to the formation of the 6T/F16CuPc interface, the deposition of 0.13 ML of 6T onto the CuPc monolayer resulted in ordered domains of both CuPc and 6T (Figure 4b). Although minor disordered regions were observed (as indicated by the dashed green circle in Figure 4b), storing the specimen at room temperature promoted the nucleation of the ordered domains (see Figure S1). No tendency to form mixed structures was observed. These results indicate that the CuPc and 6T molecules preferentially remained phase separated. Oteyza et al. reported that weak lateral interaction between the heteromolecules led to structural disordering,32 but in the case of 6T/CuPc interface, thermal energy at room temperature is sufficient to render molecular arrangements of 6T and CuPc ordered. Thus, the disordered binary structure containing the bent 6T molecules that was observed upon the formation of the 6T/F16CuPc interface (Figures 2 and 3) was stabilized by attractive interactions between 6T and F16CuPc. It should be noted here that the side-by-side packing of the ordered 6T domains shown in Figures 4b and S1 is the same as the molecular arrangement observed for the 6T monolayer deposited on Ag(111). This result suggests that 6T molecules must diffuse into the CuPc layer through the grain boundaries and defects, thereby pushing a fraction of the CuPc away from the Ag surface.


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Figure 4. Formation of the 6T/CuPc interface. (a) STM image of a CuPc monolayer. The rectangle with the lattice parameters indicates the surface unit cell. The scale bar represents 2 nm. (b) Representative STM image showing the coexistence of ordered CuPc and 6T domains following the deposition of 0.13 ML of 6T. The scale bar represents 10 nm. The dashed green circle indicates a disordered region. The 6T molecules in this area are incorporated into the ordered 6T domain in the upper-right corner (see Figure S1 in the Supporting Information).

The above comparative experiment demonstrates the significance of the interaction between the heteromolecules in determining the local structure. Furthermore, when the deposition sequence was reversed, the formation of a disordered heteromolecular layer of F16CuPc and 6T was not observed. As shown in Figure 3c, the side-by-side packing of 6T was observed even after the deposition of F16CuPc. The long-range order of the 6T monolayer remained intact and no structural perturbation was observed in the imaging area. These results suggest that not only attractive interactions between F16CuPc and 6T (e.g., hydrogen bonds) but also the structural stability of the first layer are critical for the occurrence of the disordering on the growth of the heterointerfaces. A molecular monolayer adsorbed on a clean metal surface is stabilized by molecule–metal and molecule–molecule interactions. We first addressed the contribution of molecule–metal interactions by analyzing the UPS spectra of the respective monolayers on Ag(111) (Figure 5). The appearance of a new electronic state near the Fermi level was used as a measure of the interaction between the adsorbed molecules and the substrate.36 The formation of interface states should involve the electron transfer between the metal and molecule. In this case, stronger adsorption at the molecule–metal interface is typically expected. The F16CuPc monolayer formed interface states in the vicinity of the Fermi level, which likely originated from the filled lowest


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unoccupied molecular orbital (LUMO) of F16CuPc (bottom curve in Figure 5).36–39 The interface states were formed by the partial occupation of the LUMO upon adsorption. On the other hand, no states were observed in this energy region of the UPS spectrum for the 6T/Ag(111) interface (top curve in Figure 5). The absence of interface states is in accordance with the previous finding that no new components appeared in the S 2p and C 1s core-level spectra of a 6T monolayer on Ag(111).40 This comparison of the UPS spectra suggests that the molecule–metal interaction at the F16CuPc/Ag(111) interface is stronger than that at the 6T/Ag(111) interface. A similar conclusion was deduced for the CuPc monolayer. The interface states were clearly formed (middle curve in Figure 5), supporting the interaction of CuPc with the substrate. Nevertheless, as shown in Figure 4b, the ordered 6T domains were formed directly on Ag(111), which indicates the preferential formation of the ordered 6T monolayer on the metal surface. This is presumably attributable to the intermolecular interaction originating from the side-by-side packing of the 6T monolayer (Figure 1b, d). The dense molecular packing would increase the adsorption energy per area.30 Thereby, 6T molecules deposited onto a phthalocyanine layer on a Ag substrate cause the phthalocyanine domains to slide aside to gain the energy through lateral intermolecular interactions. The adsorption energy of F16CuPc and 6T monolayers will give a direct information to rationalize the inequivalent structures of 6T/F16CuPc and F16CuPc/6T interfaces. To address this point, further investigations, likely with thermal desorption spectroscopy (TDS),30 will be necessary.


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Figure 5. UPS spectra of monolayers of F16CuPc (blue), CuPc (green), and 6T (yellow), together with the spectrum of a cleaned Ag(111) surface (gray).

The above characterization results suggest that the lateral interaction in the first layer has additional significance for the occurrence of the disorder. Here, we again address the differences in the local structures of the 6T/F16CuPc and 6T/CuPc interfaces (random mixing vs. phase separation). Since intermolecular distance governs molecule–molecule interactions, we analyzed the interatomic distances in the F16CuPc and CuPc monolayers. Figure S2a in the Supporting Information depicts the parallel alignments of the F16CuPc molecules within the molecular arrangements with A and B orientations. The fluorine atoms of the neighboring molecules are separated by a minimum distance of 2.78 Å when the van der Waals radii of the atoms are considered (see Table S1). Furthermore, the molecules in arrangements A and B are separated by a minimum distance of 3.27 Å. In contrast, the CuPc molecules are arranged in a staggered manner and are densely packed with a minimum distance of < 1 Å (Figure S2b and Table S2 in the Supporting Information). This simple analysis suggests that the molecular arrangement of F16CuPc


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is rather sparse, leading to the weak lateral interaction and hence low structural stability of the F16CuPc monolayer. The denser packing in the CuPc monolayer should suppress the structural perturbation with random mixing. Therefore, in addition to the attractive interaction between the heteromolecules, the molecular packing in the first layer should affect the molecular order in the heterostructure formed by sequential stacking of the organic molecules. The preceding discussion related to the enthalpic contribution to the Gibbs free energy of the interface formation. Here, we further consider the entropic contribution. In general, the condensation of gaseous molecules on a solid surface decreases the enthalpy of a system, and it also decreases the entropy owing to the limited freedom of the adsorbates on the surface. When the enthalpic decrease overcomes the decrease of the entropic term of the Gibbs free energy, solidification of the molecules occurs. In this context, molecules of 6T vacuum deposited onto the F16CuPc monolayer possess a smaller entropy than those in the gas phase, but, in the present case, they can also adopt various molecular conformations (see Figure 2). Therefore, compared with the general case of molecular adsorption on a solid surface, the decrease in entropy upon the adsorption of 6T should be suppressed. Consequently, the solidification of bent 6T molecules becomes favorable. Concurrently, the entropic effect also promotes the disordering of the original arrangement of F16CuPc (Figure 1a, c). The aforementioned results further stimulated us to address a general question: Why does stacking a particular combination of π-conjugated molecules21 result in a binary superstructure upon the formation of their interface? To gain a comprehensive understanding of the formation of organic heterointerfaces, we finally investigated the para-sexiphenyl (6P)/F16CuPc interface. In contrast to 6T, full rotation of the benzene rings of 6P is prohibited owing to the steric repulsion between the C–H bonds of the neighboring units. Hence, the rodlike shape should remain


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unchanged even if structural perturbation occurs during interface growth. In addition to this conformational robustness, the molecular length of 6P is 28.5 Å, which is nearly twice that of F16CuPc (14.5 Å).41 We thus expected that the deposition of 6P onto the F16CuPc monolayer would afford an ordered arrangement involving multiple C–F···H–C bonds.

Figure 6. Formation of the 6P/F16CuPc interface. Nominal coverage of 6P is 0.18 ML. (a) Lowmagnification and (b) high-magnification STM images. The scale bars in (a) and (b) represent 5 nm and 1 nm, respectively. (c) Schematic model illustrating the molecular arrangement of F16CuPc and 6P corresponding to (b).

Figure 6a shows an STM image, where 6P molecules are deposited onto an ordered F16CuPc monolayer (Figure 1a, b). The molecular arrangement of F16CuPc completely disappeared upon the formation of the 6T/F16CuPc interface. Instead, a binary superstructure is formed. The molecularly resolved STM image (Figure 6b) revealed that pairs of F16CuPc molecules were interlinked by single rodlike 6P molecules. Both F16CuPc and 6P adopted a face-on orientation in the unit structure, although the molecular order was entirely distinct from those of neat F16CuPc (Figure 1a, b) and 6P monolayers (Figure S3a, b). The resulting arrangement was similar to that reported for a 6P:F16CuPc binary structure with a blend ratio of 1:2 prepared on a highly oriented


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pyrolytic graphite (HOPG) substrate.41 In this arrangement, 6P and F16CuPc can interact with each other via multiple C–F···H–C bonds (Figure 6c). This comparative experiment demonstrates that conformational flexibility influences the molecular order during heterointerface formation. The deposition of 6T, in which rotation of the thiophene rings is allowed, onto the F16CuPc monolayer leads to significant disorder, whereas the rodlike 6P forms an interlinked structure with F16CuPc. It should be noted that long-range order of the F16CuPc:6P binary structure is lacking. This state is also regarded as disorder. Similar to the results observed for the F16CuPc/6T interface (Figure S1b), upon reversed stacking, the binary structure was not formed (Figure S3c). As in the case of 6T on Ag(111), 6P molecules can reportedly be physisorbed on a Ag surface, as demonstrated from the analysis of photoelectron spectra.42 The structural tolerance of the 6P monolayer should again be attributable to a lateral interaction of the 6P monolayer. Figure 7 summarizes the findings in this study. For the structural disordering upon the formation of 6T/F16CuPc planar heterointerface at room temperature, the high conformational flexibility of 6T and stabilization through the hydrogen bonding21,32,41 between variously bent 6T and F16CuPc molecules are essential. Moreover, we found the following feature of the resulting molecular arrangement upon the formation of molecular heterointerfaces. 1.

When the interaction between the heteromolecules is weak, phase-separated binary structure is formed (as the case of 6T/CuPc interface). Most of molecular arrangement in respective domains are ordered, although disordered regions exist between the domains of different molecular components.

2.

The superstructure with local order is formed even by sequential deposition when a size of one molecule matches with that of another (as the case of 6P/F16CuPc interface).


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Figure 7. Summary of the results with the STM images of the three interfaces investigated in this study (30  30 nm2).

CONCLUSIONS We have used STM to clarify the molecular arrangement in the disordered structure formed on the growth of 6T/F16CuPc interfaces. The deposition of 6T completely perturbed the F16CuPc monolayer. Concurrently, the 6T molecules adopted various bent conformations and were randomly intermixed with the F16CuPc. As a result, no short- or long-range order was observed for either the pure (neat 6T and F16CuPc) phase or the binary phase. The disordered heteromolecular layer was stabilized by multiple C–F···H–C bonds between F16CuPc and bent 6T. The sparse molecular packing of the F16CuPc underlayer was also fundamental to the occurrence


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of the disorder. A comparative study of the 6P/F16CuPc interface highlighted the importance of the conformational flexibility of the deposited molecules. The cis–trans isomerization of 6T permits the formation of various molecular shapes, such as U- and S-shaped structures. The presence of these bent 6T molecules and their attractive interactions with F16CuPc lead to complete disorder during the growth of the 6T/F16CuPc interface. The results of the present study are expected to facilitate a deeper understanding of the formation mechanism of molecular heterointerfaces and ultimately provide ground rules for designing interfaces with various functionalities.

ASSOCIATED CONTENT Supporting Information. Supplementary STM images of the 6T/F16CuPc interfaces, neat 6P, and F16CuPc/6P interfaces, calculated interatomic distances in molecular arrangements of F16CuPc and CuPc monolayers. (PDF)

AUTHOR INFORMATION Present Addresses §

Nanomaterials Research Institute, National Institute of Advanced Industrial Science and

Technology, Central 5, 1-1-1, Higashi, Tsukuba, Ibaraki 305-8565, Japan Author Contributions ‡

K.A. and A.O. contributed equally to this work.


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ACKNOWLEDGMENT K.A., Y.W., and K.K. acknowledge JSPS for financial support (grant numbers: 18K04944, 15K13819, and 16K05956, respectively). Part of this work was supported by the Use-of-UVSOR Facility Program (BL2B, 2018, grant numbers: 29-805 and 30-505) of the Institute for Molecular Science.

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Structural Disordering upon Formation of Molecular Heterointerfaces

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