Formation of Highly-Ordered Semiconducting Anthracene Monolayer

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Formation of Highly-Ordered Semiconducting Anthracene Monolayer Rigidly Connected to Insulating Alkanethiolate Thin Film Toyoaki Eguchi, Naoyuki Hirata, Masahiro Shibuta, Hironori Tsunoyama, and Atsushi Nakajima J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b08907 • Publication Date (Web): 22 Oct 2018 Downloaded from http://pubs.acs.org on October 23, 2018

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The Journal of Physical Chemistry

Formation of Highly-Ordered Semiconducting Anthracene Monolayer Rigidly Connected to Insulating Alkanethiolate Thin Film Toyoaki Eguchi,†,‡,|| Naoyuki Hirata,†,‡ Masahiro Shibuta,†,§ Hironori Tsunoyama,†,‡ and Atsushi Nakajima†,‡,§,* †

Nakajima Designer Nanocluster Assembly Project, ERATO, Japan Science and Technology Agency (JST), 3-2-1 Sakado, Takatsu-ku, Kawasaki 213-0012, Japan

‡

Department of Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan

§

Keio Institute of Pure and Applied Science (KiPAS), Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan

----------------------*Addresses correspondence to E-mail: [email protected]

Fax: +81-45-566-1697

Present Address: || Department of Physics, Graduate School of Science, Tohoku University, 6-3 Aramaki Aza-Aoba, Aoba-ku, Sendai 980-8578, Japan

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ABSTRACT The formation and molecular structure of self-assembled monolayer (SAM) of anthracene-substituted alkanethiol on Au(111) have been investigated by scanning tunneling microscopy and infrared reflection absorption spectroscopy. Clean and well-ordered SAM composed of densely-packed "standing-up" molecules is formed by a wet chemical process in air, followed by thermal annealing in vacuum. In the SAM, anthracene moieties are arranged in a bulk-like in-plane herringbone structure, and keep their crystalline ordering above room temperature, which is contrast to less ordering in Au nanoclusters ligated by the anthracene-alkanethiolates. In addition, hexagonal arrangement of anthracene dimer is found on the surface of the SAM. These structures remain after annealing at 400 K. Scanning tunneling spectroscopy performed on the surface shows an energy gap of ~3.6 eV, similar to the band gap of bulk anthracene. The above results demonstrate that the highly-ordered semiconducting anthracene monolayer is successfully formed on the insulating alkanethiolate thin film, and has a high thermal stability due to a strong chemical bonding across the interface.


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1. INTRODUCTION In recent decades, the fields of organic electronics have dynamically evolved owing to their strong possibilities as the next generation technology, such as flexible, large-area, and low-cost devices.1–5 Organic electronic devices consist of several thin film layers that promote various functionalities, e.g. injection, transport, and recombination/extraction of charges. Since organic electronic devices utilize thin films of organic semiconductors stacked between different materials, the construction of stacked structures using ultrathin organic thin films is vital for achieving very small, low-energy-consumption organic devices. To this end, self-assembled monolayers (SAMs) have attracted great interest as an integral part of organic electronic devices because of their versattility.6–9 SAMs are 2D crystalline films of organic molecules, which are chemically anchored to a substrate by a suitable head group. Spontaneous ordering of molecules is stabilized by non-covalent forces acting between tail groups of the host molecules, for example, van der Waals interactions, hydrogen bonds, – stacking, electrostatic and dipole–dipole interactions. By attaching functional molecules as an end group, a monolayer of functional molecules can be formed on a monolayer of host molecules having a strong covalent bond at its interface. The most widely used technique to prepare high-quality thin films is physical vapor deposition (PVD) in vacuum, where molecules are deposited to the substrate surface by physical adsorption.11,12 Since thin films of organic molecules grown by PVD do not contain a specific chemical interaction to the substrate, they can be formed on any kind of substrate, but usually show incomplete adhesion at the interface, which causes low thermal tolerance of organic devices. Here, we demonstrate the construction of a stacked structure of an organic molecular monolayer by preparing an anthracene-substituted alkanethiolate SAM. Alkanethiolate SAM formed on gold (Au) substrate is one of the most widely employed SAM systems owing to the easy preparation and relative simplicity of the molecules, which are highly stable, well 3 ACS Paragon Plus Environment


organized, and electrically insulating.6,8,10 Because alkanethiolate SAMs are good insulators, they are suitable substrates to separate photoexcited charged carriers13,14 and to sustain organic molecules without disturbing their intrinsic properties.15 On the other hand, the most promising classes of organic semiconductors for organic electronic devices are -conjugated oligoacenes such as anthracene, tetracene, and pentacene.16-22 Their planarity facilitates

crystal packing and improves the intermolecular overlap of the -system, which is favorable to obtain a high mobility of charge carriers. Among them, anthracene is the first organic semiconductor to be widely studied and is expected to be applied for organic light emitting diodes together with its derivatives.23–26 Since it is possible to introduce a methyl group into the C2 position in anthracene (2-methylanthracene) without disturbing the intermolecular overlap of the -system,27 anthracene-substituted alkanethiols are expected to enhance the intermolecular overlaps of the planar -system combining with the molecularly ordered alkanethiolate thin film. In this study, we apply 11-(anthracene-2-yl)undecane-1-thiol [(C14H9(CH2)11SH, hereafter denoted by Ant-C11, see Figure 1] to fabricate SAM on Au(111), and investigate its geometric structure, electronic properties, and thermal stability by using scanning tunneling

microscopy/spectroscopy (STM/S) and infrared reflection absorption spectroscopy (IRAS). Recently we have reported photoexcited state confinement in Ant-C11-SAM with two photon photoemission spectroscopy, in which their two-dimensionally well-packed structures have been briefly described based on STM observation.28 In this article, we will present the results on high thermal stability up to 400 K of Ant-C11-SAM with STM and IRAS measurements, focusing on (1) a crystalline ordering above room temperature of anthracene monolayer and (2) hexagonal arrangement of anthracene dimer formed through mechanochemical forces. In addition to Ant-C11-SAM, Au nanoclusters (NCs) protected by Ant-C11 thiolates (Au:Ant-C11) were synthesized and the structural ordering behaviors are compared between 4 ACS Paragon Plus Environment

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on the flat and on the spherical Au surface with IRAS. Furthermore, an energy gap of ~3.6 eV measured by STS is consistent with the band gap of bulk anthracene, exhibiting the formation well-ordered semiconducting molecular monolayer. Therefore, Ant-C11 SAM can be looked on as a single stacked pair of a semiconducting anthracene monolayer and an insulating undecane monolayer.

Figure 1. Schematic molecular skeleton of 11-(anthracene-2-yl) undecane-1-thiol [C14H9(CH2)11SH: Ant-C11]

2. EXPERIMENTAL METHODS SAM of Ant-C11 (purity > 99.5%, Tokyo Chemical Industry Co., Ltd.) was prepared by immersing the Au substrate into ethanolic solution of Ant-C11 at concentration of 0.2 mM for 36–40 hr. The substrate was placed horizontally in the solution, which was maintained at room temperature (RT, 20 °C). The Au(111) single crystal (orientation accuracy < 0.1°, MaTecK GmbH, Germany) was used as substrate, which was cleaned in ultra-high vacuum (UHV) by repeated cycles (at least 10 cycles) of Ar+ sputtering (0.6 keV) and annealing (720 K). The cleanness of the substrate surface was ensured by photoelectron spectroscopy measurements showing clear Shockley surface state located at 0.4 eV below the Fermi level and known work function of 5.5 eV. After the SAM formation, the substrate was washed with ethanol and dried in atmospheric condition. STM/S measurements were carried out in UHV

(< 1.0 × 10-8 Pa) at RT by using a commercial STM unit (VT-AFM-XA50/500, Omicron NanoTechnology GmbH). STM tips were prepared from a polycrystalline tungsten wire by electrochemical etching. STM image data were processed with WSxM software.29 IRAS was



carried out at a grazing incident angle of ~80° with respect to the surface normal. The spectra were recorded from 650 to 1800 cm-1 and 2700 to 3100 cm-1 at interval of 1 cm-1 using an FT-IR spectrometer (IFS66 vi, Bruker Optics). The IR optics and detector are mounted in a vacuum chamber (~10 Pa) in order to reduce spectral background components arising from atmospheric gases.

Au NCs ligated by Ant-C11 thiolates (Au:Ant-C11) were prepared by a ligand-exchange reaction using poly(N-vinyl-2-pyrrolidone) stabilized Au NCs (Au:PVP) as precursors. Au:PVP precursors with diameter of 0.9  0.2 nm were prepared by a chemical reduction method with a microfluidic mixer.30 Powder of Au:PVP (200 mg containing 43 mol of Au) was dispersed in 20 mL of ultra-pure water with ultra-sonication. Toluene solution (20 mL) of Ant-C11 (80 mg, 220 mol) was added onto the aqueous layer. The biphasic mixture was vigorously stirred for more than 24 h under ambient conditions. During the reaction, insoluble substances increase at an interface of aqueous and organic layers along with color change of both layers to colorless transparent. Insoluble substances were separated by centrifugation, and washed thoroughly with toluene and ethanol. Synthesized Au:Ant-C11 NCs are not soluble in dimethyl formamide, dimethyl sulfoxide, toluene, water, and alcohols. IR spectra of

Au:Ant-C11 were recorded with a Fourier transform infrared spectrometer (Bruker, Alpha-T) by potassium bromide (KBr) pellet method with resolution of 4 cm–1.

3. RESULTS AND DISCUSSION An STM image of an as-prepared sample is shown in Figure 2(a). The surface consists of a flat terrace having plural domains with a size of at most ~10 nm and depressed area with a single atomic depth of Au(111) (= 0.23 nm), also referred to as etch-pits. Small contaminants are sparsely distributed on the surface. It is found that the surface quality can be improved by thermal annealing under UHV. Figure 2(b) shows an STM image taken after annealing at 370 6 ACS Paragon Plus Environment

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K for several minutes. The surface becomes flatter without a depressed area, having large domains with size of at least 50 nm. The similar annihilation of etch pits from the surface by thermal annealing is also reported for alkanethiolate SAM formed on Au(111) having a "standing-up" structure.31–33 The large domain that was created shows a well-ordered molecular arrangement as seen in the zoomed image (Figure 2(c)). Several types of periodic

structures are found on the surface, which are discussed later in detail. The above results clearly indicate that a clean and ordered molecular film is formed on the surface.

Figure 2. STM image of Ant-C11-SAM; (a) as-prepared by immersing into an ethanolic solution (tip bias voltage Vt = +2.6 V, tunneling current It = 1 pA), (b) after annealing at 370 K in UHV (Vt = +2.5 V, It = 10 pA), and (c) its zoomed image (Vt = +2.5 V, It = 5 pA). All 7 ACS Paragon Plus Environment


images are represented with the same height scale. The scale bars in (a), (b), and (c) represent 20 nm, 20 nm and 5 nm, respectively.

Figure 3. IRAS spectra of Ant-C11-SAM recorded at (a) 120 K and (b) 300 K after annealing at 400 K in UHV. (c) IRAS spectrum of Au NCs ligated by Ant-C11 at 300 K. Schematic side views of (d) NC ligated by Ant-C11 and (e) Ant-C11-SAM. 8 ACS Paragon Plus Environment

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To characterize molecular species adsorbed on the surface and to analyze their chemical state, degree of order and orientation, IRAS measurements were performed for the above-prepared samples. Figure 3(a) shows a spectrum obtained at 120 K after annealing the sample at 400 K. The result corresponds well to the previously reported spectra for

anthracene-substituted alkanethiolate SAM;34,35 in addition to two intense absorption bands at 2921 and 2849 cm-1, which are characteristic of alkyl chains in alkanethiol moiety, the absorption signals originating from anthracene moiety are clearly observed in the spectral region of 800 to 1700 cm-1. For comparison, Figure 3(b) shows an IRAS spectrum for Ant-C11 SAM at 300 K, while Figure 3(c) shows an IR spectrum for Au NCs ligated by Ant-C11, the measuring methods of the latter are shown in the Supporting Information. With heating up the temperature from 120 K to 300 K, the peaks in the spectrum for the Ant-C11 SAM exhibit a little broadening of less than 1.5 times. However, the spectrum for Au NC ligated by Ant-C11, measured at 300 K, shows considerable broadening along with some enhanced vibrational modes. The broadening is attributed to disordering of the ligated molecules around Au NCs, and some enhanced peaks are ascribed to a randomized orientation of anthracene moieties. Furthermore, when the broadenings of the vibrational modes are compared between anthracene and alkyl chain, the peak widths assignable to anthracene framework vibrations (such as peaks 0 and 3) are 2–3 times broader than those for alkyl chain vibrations (peaks 1 and 2). This is seemingly because the spatial restrictions are more relaxed going outside the Au NCs; anthracene molecules are spatially freer than alkyl chains as shown in Figure 3(d).

Table 1 summarizes the frequencies of observed IRAS bands and their mode character, as assigned based on the previous reports.35,37 Due to the surface selection rule in IRAS, only the vibrations with a component of the transition dipole moment (TDM) aligned



perpendicular to the surface plane can interact with the incident IR light and contribute to the spectrum. Compared between Figures 3(a)(b) and Figure 3(c), the attenuation of the aromatic out-of-plane bands (peaks 6, 7, and 9) is apparent for Ant-C11-SAM, compared to the vibrational peaks for CH/CC vibrations of anthracene (see Supporting Information for calculated TDM for an anthracene molecule). The results reasonably indicate that the anthracene moieties of Ant-C11 molecules in the SAM are oriented almost perpendicularly to the substrate surface, as shown in Figure 3(e). Note that the CH/CC vibrations of anthracene are categorized into parallel TDM to the ab plane of the anthracene molecular plane (peaks 0, 3, 5, and 8). The IRAS data also provide information on the degree of order of the chemisorbed thiolate molecules, especially with respect to the alkyl chain. According to previous IR studies, the peak frequency positions the asymmetric and symmetric CH2 stretching modes vary sensitively with the extent of the lateral interactions between the alkyl chains in the assemblies;34,37 the asymmetric CH2 stretching mode appears at 2918 cm-1 for a crystalline alkyl chain appears at 2918 cm-1, which is 6 cm-1 lower than that for the liquid state (2924 cm-1), while the peak position of the symmetric CH2 stretching mode in the crystalline sample (2851 cm-1) is 4 cm-1 lower than in the liquid (2855 cm-1). As listed in Table 1, in the present study, the asymmetric CH2 stretching (band 1) and symmetric CH2 stretching modes (band 2) are observed at 2921 and 2849 cm-1, respectively. These peak positions correspond to those of the bulk crystalline phase, which indicates the formation of an ordered assembly of densely-packed alkyl chains. In fact, the crystalline behaviors of alkyl chains are enhanced with the anthracene

moieties. Figure 4 shows IRAS spectra for n-alkanethiolate C18-SAM and Ant-C11-SAM at 220 and 300 K. In the case of n-alkanethiolate SAM, the alkyl chains are mostly linear as all-trans conformation below 280 K, while gauche defects cause the rotator structure of alkyl


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chains at the higher temperatures. The phase transition from all-trans crystal phase to the rotator phase reflects the intensity of CH2 stretching at 2921 and 2849 cm-1.38 Previous works on n-alkanethiolate C18-SAM have shown that the tilting angle of alkyl chains relative to the surface normal decreases with temperature.38–40 It is possible that this contributes to our observation of a prominent decrease of CH2 stretching down to about one third as the temperature was varied from 220 to 300 K. For Ant-C11-SAM, however, the corresponding ratios decreased to three-fourths, which is predominantly caused by the change in the tilting angle of alkyl chains. The intensity and ratio are tabulated in Table 2 and the plots of the mode intensities are shown in the Supporting Information. The intensity smothering of CH2 stretching modes for Ant-C11-SAM shows that the phase transition induced by the gauche defects is suppressed by the crystalline packing of anthracene moieties.



Figure 4. IRAS spectra in the 650–1800 and 2700–3100 cm-1 regions for (a) C18-SAM at 220 K, (b) C18-SAM at 300 K, (c) Ant-C11-SAM at 220 K, and (d) Ant-C11-SAM at 300 K. Two peaks at 2921 and 2849 cm-1, labeled 1 and 2, respectively, are characterized as vibrational modes of alkyl chains, and their intensity changes are given in Table 2.

Based on the above results of STM and IRAS measurements, it is reasonable to conclude that highly-ordered monolayer film of Ant-C11 molecules, Ant-C11-SAM, are formed on the Au(111) substrate, where anthracene moieties stand in an upright position over the alkyl-layer of "standing-up" undecanethiol, as schematically illustrated in Figure 3(b). The structure of Ant-C11-SAM can be regarded as the stacking of semiconducting (anthracene monolayer), insulating (undecane thin film), and metallic layer (bulk Au substrate). Since these layers are interconnected via strong chemical bonding, high thermal stability can be expected and is 12 ACS Paragon Plus Environment

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confirmed by temperature programmed desorption (TPD) experiments; the desorption peak was observed from about 400 K and reaches maximum around 415 K, indicating a molecular desorption from Ant-C11-SAM itself.28,41 The results evidently show that the anthracene monolayer and undecane thin film are rigidly connected with a chemical bond stronger than the S–Au bond, as expected. Although it is known that alkanethiol molecules desorb as disulfied from a densely packed "standing-up" monolayer film,42 the recombinative desorption of Ant-C11 does not occur for Ant-C11-SAM in the present study. Molecular arrangements of Ant-C11-SAM have been investigated in detail based on highly resolved STM images. On the surface of Ant-C11-SAM, two types of periodic structures having rectangular or hexagonal unit cells are observed as shown in Figures 5(a) and 5(b), which near evenly share the surface area. The rectangular phase (Figure 5(a)), which has orthogonal unit vectors of |arect| = 0.75 nm and |brect| = 0.52 nm, contains two

nonequivalent protrusions in the unit cell; the protrusion located at the "center" of unit cell appears slightly darker than the "corner" one. The observed pattern appears like the molecular arrangement in the a-b plane of bulk anthracene crystal, where the anthracene molecules adopt a face-on-edge herringbone packing with a rectangular unit cell, as illustrated in Figure 5(c). Compared with bulk anthracene, which crystallizes in a monoclinic structure with |a| = 0.856 nm, |b| = 0.604 nm, |c| = 1.116 nm and  = 124.42º,43,44 the anthracene moieties in Ant-C11-SAM are arranged with higher density in a plane parallel to the surface. The higher lateral packing density in the SAM would result in an upright orientation with a smaller tilting angle , the angle between the long molecular axis and the direction normal to the a-b plane, than in the bulk anthracene crystal ( = 34.8º), because it is known that, in van der Waals bonded organic solids, the molecular tilt angle is directly related to the packing density within the monolayer.45–47 This is also supported by our IRAS result shown in Figure 3, although it is difficult to determine the correct tilting angle. The molecular arrangement in the rectangular

phase is not commensurate with the lattice period of Au(111); it aligns the diagonal of its unit _ cell along the direction of the Au(111) substrate. The diagonal axis depicted with a dashed-dotted line in Figure 5(a) corresponds to the nearest-neighbor direction of an anthracene moiety whose interline spacing (dL = 0.427 nm) closely matches three times the 13 ACS Paragon Plus Environment


_ distance between the Au atomic rows along direction, 0.144 × 3 = 0.432 nm. The findings indicate that the alkanethiolate layer lying underneath the anthracene monolayer has an ordered structure in at least one dimensionally and is chemically anchored on Au(111), which defines the orientation of the rectangular phase regarding to the Au(111) substrate.

Figure 5. High-resolution STM images of Ant-C11-SAM: (a) rectangular phase with arect = 0.75 nm and brect = 0.52 nm (Vt = +3.0 V, It = 5 pA), (b) hexagonal phase with ahex = 0.91 nm and bhex = 0.62 nm (Vt = +2.5 V, It = 3 pA). Crystallographic directions are determined by observing the bare surface of the Au(111) substrate. Schematic two-dimensional arrangements of anthracene moieties in Ant-C11-SAM for (c) rectangular and (d) hexagonal phases. A gray hexagonal mesh represents a (1×1) lattice of Au(111) surface. Dashed-dotted lines in (a) and (c) represent molecular rows along the nearest-neighbor direction of anthracene moiety 14 ACS Paragon Plus Environment

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having an interline spacing dL of 0.427 nm. The scale bars in (a) and (b) represent 1 nm.

The other is hexagonal phase (Figure 5(b)) having unit vectors of |ahex| = 0.91 nm and _ |bhex| = 0.62 nm, both of which align the direction on Au(111). It is well-known that alkanethiolate SAM on Au(111) in saturate coverage reveals the (√3×√3)R30º superstructure with |a√3| = |b√3| = 0.499 nm and its associated variations.6,10 The lattice parameters for the hexagonal phase, |ahex| and |bhex|, are close to the values of 11/6 |a√3| = 0.914 nm and 15/12 |b√3| = 0.623 nm, respectively, suggesting a long-period commensurate structure. In the hexagonal phase, each protrusion appears as an oval shape elongated along the bhex axis. By analogy with the molecular packing density of the bulk anthracene crystal (0.258

nm2/molecule) and that of the rectangular phase in Ant-C11-SAM (0.195 nm2/molecule), it is reasonable to consider that each oval-shaped protrusion observed in the STM images of hexagonal phase consist of a couple of anthracene moieties as illustrated in Figure 5(d), which leads to the packing density of 0.244 nm2/molecule. The STM results strongly suggest the formation of anthracene dimer in the hexagonal phase of Ant-C11-SAM. It is familiar that anthracene has the ability to dimerize with photoirradiation.48,49 In the present case, however, it can be ruled out because the photodimerization requires UV light at 365 nm. It is possible that the origin of dimerization is the compressive strain energy accumulated in the anthracene monolayer of Ant-C11-SAM given that there have been reports on the mechanochemical dimierization of anthracene under very high pressure.50–52 As described above, the lattice constants of the rectangular phase in Ant-C11-SAM is ~13 % smaller than that of bulk anthracene crystal in ambient conditions, and correspond to the bulk ones under high pressure above 5 GPa.53 Highly-compressed anthracene monolayer was achieved in the Ant-C11-SAM because the alkanethiolate moiety prefers the (√3×√3)R30º superstructure on Au(111) and has at packing density of 0.215 nm2/molecule. 15 ACS Paragon Plus Environment


It should be noted that a small topographic modulation with an amplitude of about 0.1 nm and a period of 3–4 nm exists on the surface of both phases, which can be seen in Figure 2(c). The modulation could be caused to relieve a strain induced by the lattice mismatch between the anthracene monolayer and the alkanethiolate film. The STM result also indicates that the underlying alkanethiolate film has a ordered periodic structure and rigidly connects the anthracene layer and the Au(111) substrate. The electronic structures of Ant-C11-SAM are evaluated on the basis of STS. Figure 6(a) shows a typical I-V curve on the Ant-C11-SAM obtained by averaging five curves taken at different points. Each curve was acquired during the STM imaging with tip bias voltage (Vtip) of +2.5 V and tunneling current (It) of 2 pA and then approaching toward the surface by 1 nm to get enough signal near Vt = 0 V. Simultaneously obtained STM image is illustrated as an inset in Figure 6(a) and shows the ordered molecular arrangement of the hexagonal phase,

which ensures successful I-V measurements without direct tip-sample contacts. The normalized conductivity (dI/dV)/(I/V) was numerically calculated from the I-V curve and is also shown in Figure 6(b). For negative tip bias, which probes filled states of sample, an onset voltage is about –2.4 V, whereas for positive sample biases probing empty states an onset voltage of +1.2 V is seen. The observed energy gap of about 3.6 eV is close to the value of 3.97 ± 0.22 eV experimentally determined for bulk anthracene crystal,54 but much smaller than the one for an anthracene molecule itself (6.91 eV; difference between ionization energy (7.44 eV)55 and electron affinity (0.53 eV)27) or the one for alkyl moiety (~9 eV).56 The result proves the successful formation of well-ordered semiconducting molecular monolayer in the Ant-C11-SAM.


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Figure 6. (a) Tunneling I-V curve acquired on Ant-C11-SAM and (b) normalized tunneling conductance, (dI/dV)/(I/V) numerically calculated from (a). Simultaneously obtained STM image (Vt = +2.5 V, It = 2 pA) is shown as an inset of (a).

4. CONCLUSIONS We have investigated the geometric structures, electronic states, and thermal stability of anthracene-substituted alkanethiolate SAM formed on Au(111) by using STM/S and IRAS methods. The Ant-C11 molecules were found to form a well-defined and highly-ordered SAM with the wet-chemical process followed by thermal annealing in UHV. The Ant-C11-SAM is composed of densely-packed anthracene-alkanethiol molecules arranged with standing-up configuration, which can be regarded as a stacked structure of a semiconducting anthracene monolayer, an insulating alkanethiolate layer, and a metallic Au substrate. In the anthracene monolayer, the molecules are arranged as a herringbone structure, similar to that in bulk crystal, but have a higher packing density. In some surface area, neighboring molecules make a pair suggesting a dimerization of anthracene moieties. The dimerization has been induced by the compressive strain energy accumulated in the anthracene monolayer of Ant-C11-SAM 17 ACS Paragon Plus Environment


due to the high packing density. The present results provide a new way to fabricate stacked structures of organic layers with molecular precision, which greatly contributes to further development of organic electronics.

Associated Content Supporting Information The Supporting Information is available free of charge on the ACS Publications website at XXX. Calculated transition dipole moments for vibrational modes of anthracene (Table S1) and the temperature dependence of integrated peak intensities of the IRAS spectra for C18- and Ant-C11-SAMs at 220 – 340 K (Figure S1).

Author Information Corresponding Author *E-mail: [email protected] Present Address ‖

Department of Physics, Graduate School of Science, Tohoku University, 6-3 Aramaki

Aza-Aoba, Aoba-ku, Sendai 980-8578, Japan. Notes The authors declare no competing financial interests.

Acknowledgements This work is partly supported by JSPS KAKENHI of Grant-in-Aids for Scientific Research (A) Grant Number 15H02002 and of Challenging Research (Pioneering) Grant Number 18 ACS Paragon Plus Environment

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17H06226.



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Application

of

Highly

Monodisperse

AuPd

Nanoclusters

Stabilized

by

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Table 1. Assignment of the Vibrational Peaks in the Ant-C11-SAM and in Au NCs Ligated by Ant-C11 ============================================================== peak position/cm-1 parta) mode assignments Ant-C11-SAM / -NCs ---------------------------------------------------------------------------------------------------------0 3055/3049 Ant (||b) symm. CH stretch. 1 2921/2920 Alkyl asymm. CH2 stretch. 2 2849/2850 Alkyl symm. CH2 stretch. 3 1633/1668 Ant (||a) CC stretch./CH bend. 4 1531/1532 Ant (||b) CC stretch./CH bend. 1461/1461 Alkyl/Ant (||b) CH2 scissor./CC stretch./CH bend. 5 6 --- /954 Ant () CH bend. out-plane (CH scissor.) 7 --- /890 Ant () CH bend. out-plane (CH wagg.) 8 804/805 Ant (||b) CC stretch./CH bend.(breath.) 9 --- /737 Ant () CH bend. out-plane (CH wagg.) 10 702/702 Alkyl CH2 rocking ============================================================== a) Ant: anthracene part with the direction of transition dipole moment in a parenthesis; the ab plane is the molecular plane. ||b; parallel to the b axis, ||a parallel to the a axis (perpendicular to the b axis), and ; perpendicular to the ab plane (parallel to the c axis) Alkyl: alkyl chain part

Table 2. Integrated Peak Intensities of IRAS Spectra Labelled 1 and 2 for C18- and Ant-C11-SAMs at 220 and 300 K, Denoted as I (220 K) and I (300 K ), Respectively. The ratio of integrated peak intensities, I (300 K) / I (220 K), is also shown. ================================================================== peak intensity intensity ratio peak sample I (220 K) I (300 K) I (220 K) / I (300 K) --------------------------------------------------------------------------------------------------------------1 C18-SAM 0.079 0.023 0.29 1 Ant-C11-SAM 0.066 0.052 0.79. 2 C18-SAM 0.043 0.016 0.37 2 Ant-C11-SAM 0.040 0.030 0.75 ==================================================================


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TOC GRAPHIC


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Formation of Highly-Ordered Semiconducting Anthracene Monolayer

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