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Synthesis and investigation of π-conjugated azomethine self-assembled multilayers by layer-by-layer growth
Thin Solid Films 518 (2010) 5115–5120
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Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Synthesis and investigation of π-conjugated azomethine self-assembled multilayers by layer-by-layer growth Masakazu Kamura a,b,c,⁎, Yasutaka Kuzumoto a,b, Shigeru Aomori a,b, Hirohiko Houjou c, Masatoshi Kitamura b, Yasuhiko Arakawa b,c a b c
Advanced Materials & Energy Engineering Laboratories, Sharp Corporation, 273-1, Kashiwa, Kashiwa-shi, Chiba, 277-0005, Japan Institute for Nano Quantum Information Electronics (INQIE), the University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo, 153-8505, Japan Institute of Industrial Science, the University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo, 153-8505, Japan
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Article history: Received 9 November 2009 Received in revised form 15 February 2010 Accepted 3 March 2010 Available online 11 March 2010 Keywords: Layer-by-layer complexation Chemisorption Multilayers π-conjugation Azomethine
a b s t r a c t Layer-by-layer formation for π-conjugated azomethine multilayers bonded on substrates was investigated. The multilayers were synthesized using ethanol (EtOH) and dichloromethane (DCM) as reaction solvents. The multilayer characteristics were analyzed using UV–vis absorption spectroscopy, ellipsometric thickness, and atomic force microscopy. The absorption spectra and ellipsometric thicknesses of multilayers formed using EtOH and DCM were compared. The results indicate that EtOH is more suitable than DCM for such layer-by-layer formation. In addition, bandgaps estimated from the absorption edge of multilayers were investigated. The results indicate that the bandgap decreases as the number of benzene rings contained in the molecular chain of the multilayer increases. Also, a multilayer with four benzene rings bonded on a substrate had a bandgap close to that of a polymer with a similar chemical structure. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Self-assembled monolayers and multilayers have been studied, and the findings regarding their application to organic electronic devices have been interesting [1–8]. A self-assembled multilayer consists of selfassembled molecular chains; each chain has some units forming layers. Such a multilayer is formed by layer-by-layer chemisorption. Units are covalently-bonded with each other; a bottom unit is also covalentlybonded with a substrate. The covalent bond contributes to the electrical resistance in an electronic device using a multilayer film. Also, it is possible to vary the combination of units that form a multilayer film to target specific electrical and optical properties. Therefore, it is expected that self-assembled multilayer films will be used as carrier transport layers, insulating layers, and/or emitting layers in electronic devices such as transistors, solar cells, and organic light-emitting devices (LED). Self-assembled multilayer films have actually been used as semiconducting channel layers [5] and insulators [6,7] in transistors, and as hole transport layers in organic LEDs [8]. In addition, studies have been performed regarding the electric characteristics of selfassembled multilayers [9–12]. π-Conjugated orbitals in a multilayer play an important role in defining the electronic properties of the multilayer. In particular, a π⁎ Corresponding author. Advanced Materials & Energy Engineering Laboratories, Sharp Corporation, 273-1, Kashiwa, Kashiwa-shi, Chiba, 277-0005, Japan. Tel./fax: +81 3 5452 5711. E-mail address: [email protected] (M. Kamura). 0040-6090/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.03.006
conjugated orbital expanding to some units is desirable when the multilayer is used as a carrier transport layer of an electronic device. In order to fabricate a multilayer with such an orbital, it is important to appropriately select a reactant molecule for synthesis in layer-bylayer formation. A molecular structure having π orbitals such as a benzene ring or a thiophene ring is required to obtain π orbitals expanding in a molecular chain. On the other hand, two terminal groups in a reactant molecule determine the type of chemical bonds in the connecting units forming a molecular chain. Various chemical bonds have been used for self-assembled multilayers: imide [13,14], amide [13–17], urea [18], thioester [19], disulfide [20], ethylene and azomethine [10–12,21] bonds. The former five bonds provide σ bonds; ethylene and azomethine bonds provide π-conjugated bonds. Therefore, ethylene and azomethine bonds are suitable to achieve πconjugated orbitals expanding along a molecular chain. Meanwhile, to make multilayer films practical for use in electric devices, it is also important to form a multilayer structure with a high reaction yield. This is because a low reaction yield will lead to pinholes and voids in the film, which will result in a decline in electrical properties. In terms of reaction yield, an azomethine bond is preferable because the reaction mechanism is simple; no catalyst or condensation agent is required for an azomethine-forming reaction. Some studies about layer-by-layer formation using azomethine bonds have been reported [10–12,21]. In these reports, synthesis of azomethine multilayers from aromatic diamine and dialdehyde on a gold surface was described. Also, they investigated electrical properties of one or a few molecules using an atomic force microscope (AFM)
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[10] or a scanning tunneling microscope (STM) [11,12] in order to reveal a carrier transport mechanism along π orbitals inside the molecular chain. However, there have been few reports that discuss the characterization of π orbitals in conjugated multilayer films. In this paper, we describe π-conjugated azomethine multilayers formed on substrates terminated with hydroxyl groups (–OH) using water-soluble solvents. The multilayers were characterized by UV–vis absorption spectroscopy, ellipsometric thickness, and atomic force microscopy. In order to characterize the π-conjugated orbitals in the multilayers, we investigated the bandgaps of the multilayers in each deposition step with monomers having one or two benzene rings. 2. Experimental details 2.1. Materials p-Aminophenyltrimethoxysilane (APhS) and terephthalaldehyde (TPA) were purchased from Gelest and Tokyo Kasei, respectively. The reagents were used as received. The 4,4′-diaminostilbene (StDA) for multilayer formation was obtained using the following procedure. 4,4′Stilbenediamine dihydrochloride 300 mg (purchased from Aldrich) was dissolved in 150 ml water. After an aqueous sodium hydroxide (NaOH) solution (1 mol/l) was added dropwise to the aqueous solution of 4,4′stilbenediamine dihydrochloride until pH 10, the aqueous solution was poured onto 150 ml of dichloromethane and the two phases were separated. The aqueous layer was extracted with dichloromethane (100 ml twice) and the combined organic layers were dried over MgSO4. The solvent was removed under reduced pressure and the obtained pale yellow solid was purified by recrystallization from dichloromethane/ hexane solution. The obtained StDA was characterized by 1H nuclear magnetic resonance (1H NMR) and Fourier-transformed infrared spectroscopy (FT-IR) measurements. 1H NMR spectra were measured using a JEOL JNM AL-400 (400 MHz for 1H) spectrometer. 1H NMR data were recorded as follows: chemical shift in ppm from internal tetramethylsilance on the δ scale, multiplicity (s = singlet; d = doublet; m = multiplet), coupling constant (Hz), integration, and assignment. FT-IR spectra were recorded using a JEOL WINSPEC 100 FT-IR spectrometer. 1H NMR and FT-IR data of the StDA were as follows. 1H NMR (DMSO-d6): δ 5.14 (s, 4H, NH2), 6.52 (d, J=8.4 Hz 4H, ArH), 6.71 (s, 2H, –CH=), 7.16 (s, J =8.0 Hz 4H, ArH), and 8.44 (s, 1H, CH=N). FT-IR (KBr): 3383, 3356, 3298, 3195, 1607, 1264, 971, 830, and 537 cm−1. Dichloromethane anhydrous (DCM; water contents b0.001%), ethanol anhydrous (EtOH; water contents b0.005%), N,N-dimethylformamide anhydrous (DMF; water contents b0.005%) and tetrahydrofuran anhydrous (THF; water contents b0.002%), which were purchased from Kanto Kagaku, were used as received. 2.2. Substrates Polished single-crystal silicon (100) wafers and polished quartz slides were used as substrates. The silicon and quartz substrates have root-mean-square (rms) surface roughness levels of 0.11 nm and 0.34 nm, respectively. The silicon substrate was cleaned in acetone with an ultrasonic bath and then the native oxide on the silicon surface was removed using a hydrofluoric acid 1% aqueous solution. The procedure for handling hydrofluoric acid, which is extremely corrosive and a contact poison, was carefully performed with gloves and protective glasses in a well-ventilated fume hood. The silicon substrate was cleaned by UV/ozone treatment. The quartz substrate was cleaned in acetone with an ultrasonic bath and then cleaned with UV/ozone treatment. The surfaces were terminated with –OH by UV/ozone treatment. 2.3. Synthesis of the azomethine self-assembled multilayers Fig. 1 shows the schematic of layer-by-layer growth of azomethine self-assembled multilayers. For the first monolayer, a substrate is
placed with solid APhS in a Teflon container under dry nitrogen [22]. The container is heated at 100 °C for 3 h in an oven. Once vaporized, the APhS reacts with –OH groups of the substrate surface. The reaction results in an APhS monolayer formed on the surface as shown in Fig. 1(b). For the second layer, an aldehyde group of TPA reacts with an amine group on the substrate as reaction II in Fig. 1. For the third layer, an amine group of StDA reacts with the unreacted aldehyde group of the TPA formed on the substrate as reaction III in Fig. 1. For both the second and third layers, the reactions are performed using solutions of TPA and StDA, respectively. The substrate is immersed in 1 mmol/l EtOH or DCM solution at room temperature (20–25 °C) for 20 h. The solution is continuously stirred during the reaction. The reaction is performed under dry nitrogen ambient. After each layer is deposited, the substrate is cleaned in EtOH with an ultrasonic bath in order to remove monomers that have been physically adsorbed. The reaction of amine and aldehyde groups creates an azomethine π-conjugate bond. 2.4. Preparation of oligo- or poly-phenyleneazomethines We have synthesized oligo- and poly-phenyleneazomethines to compare their properties and the self-assembled multilayers formed on substrates. The chemical structures of the substances are shown in Fig. 2. The oligomers are benzylidene-aniline (dimer), N,N″-dibenzylidene-benzene-1,4-diamine (trimer), and N,N′-dibenzylidene-4, 4′diaminostilbene (tetramer). The polymer is poly(1,4-phenylenemethylidynenitrilo-1,4-phenylenenitrilomethylidyne) (polymer). The azomethine compounds were synthesized as follows. 2.4.1. General method The synthesis of oligo- and poly-azomethines was performed under a nitrogen atmosphere. Anhydrous dichloromethane was used as a reaction solvent. The oligo-azomethines were characterized by 1H NMR and FT-IR measurements. On the other hand, a formation of the poly-azomethine was confirmed by FT-IR measurement. We could not obtain 1H NMR of the polymer due to the low solubility in general solvents. In the FT-IR spectrum of the synthesized polymer, a specific peak was observed at 1613 cm−1, which is assigned to azomethine bonds. 2.4.2. Dimer Aniline (10.0 mmol, 0.913 ml) was added dropwise to a solution of benzaldehyde (10.0 mmol, 1.01 ml) in DCM (10 ml). The reaction mixture was stirred for 24 h at room temperature. After by-product water was removed by MgSO4 from the mixture, the dimer was purified by recrystallization from a dichloromethane/hexane solution. Yield: 0.813 g (4.49 mmol 45%) as a colorless solid. 1H NMR (CDCl3): δ 7.20–7.24 (m, 3H, ArH), 7.37–7.41 (m, 2H, ArH), 7.45–7.47 (m, 3H, ArH), 7.88–7.91 (m, 2H, ArH), and 8.44 (s, 1H, CH=N). FT-IR (KBr): 3061, 3028, 2889, 1625(C=N), 1590, 1578, 1484, 1451, 1194, 1173, 761, and 693 cm−1. 2.4.3. Trimer Aniline (24.0 mmol, 1.83 ml) was added dropwise to a solution of terephthalaldehyde (10.0 mmol, 1.34 g) in DCM (40 ml). The reaction mixture was stirred for 20 h at room temperature. The trimer was purified using the same method as the dimer. Yield: 1.15 g (4.03 mmol 40%) as yellow needles. 1H NMR (CDCl3): δ 7.24–7.26 (m, 6H, ArH), 7.40–7.44 (m, 4H, ArH), 8.02 (s, 4H, ArH), and 8.52 (s, 2H, CH=N). FT-IR (KBr): 3058, 2877, 1614(C=N), 1584, 1481, 1451, 1353, 1191, 971, 833, 760, and 695 cm−1. 2.4.4. Tetramer Benzaldehyde (0.67 mmol, 0.068 ml) was added dropwise to a solution of 4,4′-stilbenediamine (0.28 mmol, 59 mg) in DCM (20 ml). The reaction mixture was stirred for 20 h at room temperature. After
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Fig. 1. Schematic diagram of synthesis method of an azomethine self-assembled multilayer formed on a substrate.
by-product water was removed by MgSO4 from the mixture, the tetramer was purified by recrystallization from a dichloromethane/ ethyl acetate solution. Yield: 42 mg (0.11 mmol 39%) as a yellow solid. 1 H NMR (CDCl3): δ 7.14 (s, 2H, CH=CH), 7.25 (d, J = 7.8 Hz, 4H, ArH), 7.49–7.50 (m, 6H, ArH), 7.57 (d, J = 8.3 Hz, 4H, ArH), 7.91–7.94 (m, 4H, ArH), and 8.52 (s, 2H, CH=N). FT-IR (KBr): 3021, 2874, 1622 (C=N), 1574, 1502, 961, 845, 688, and 559 cm−1. 2.4.5. Polymer Terephthalaldehyde (12.3 mmol, 1.66 g) was slowly added to a solution of 1,4-phenylenediamine (12.3 mmol, 1.34 g) in DCM (50 ml). After the reaction mixture was stirred for 20 h, a yellow powder was produced. The yellow powder was purified by reprecipitation from dichloromethane into methanol (3 times). FT-IR (KBr): 3355, 2874, 1693, 1613(C=N), 1508, 1489, 1192, 849, and 563 cm−1. The FT-IR spectrum of the synthesized polymer matched with the reported values [23,24]. 2.5. Characterization of the self-assembled multilayer films UV–vis absorption spectra of films formed on quartz substrates were measured using a Shimadzu UV3600 UV–vis spectrometer. UV–
Fig. 2. Chemical structures of oligo- and poly-phenyleneazomethines. The oligomers are benzylidene-aniline (dimer), N,N″-dibenzylidene-benzene-1,4-diamine (trimer) and N,N′-dibenzylidene-4,4′-diaminostilbene (tetramer). The polymer is poly (1,4phenylenemethylidynenitrilo-1,4-phenylenenitrilomethylidyne) (polymer).
vis spectra of multilayers were obtained by subtracting the spectra for a bare quartz substrate from the spectra for a substrate with a multilayer. A J.A. Woolam M-2000U spectroscopic ellipsometer was used to perform film thickness measurements. The measurement wavelength ranges from 245 to 1000 nm, and the incident angle is 70°. Thickness measurements for organic layers were performed using the following procedure. First, the native oxide thickness on the silicon substrate was measured. After each monomer deposition, total film thickness including the organic layer was evaluated, assuming that the entire film on the silicon is a SiO2 layer (refractive index = 1.457 at 633 nm). The organic layer thickness was obtained by subtracting the native oxide thickness from the total thickness. Morphologies of films were characterized with a Veeco AFM in tapping mode. 3. Results and discussion 3.1. Estimation of the packing density for the first layer Packing density in the plane of the first monolayer affects subsequent multilayer formation. Thus we estimated the packing density based on the absorption spectra. The packing density was roughly calculated by comparing the maximum absorbance at 246 nm in the absorption spectrum of the first monolayer and a molar absorption coefficient measured for a dichloromethane solution of APhS [25]. The absorption intensity of the APhS layer increased with increasing reaction time during the early stage. Then the increase in absorbance became self-limiting after 2.5 h. The self-limiting behavior suggests that the substrate was fully covered with APhS molecules and that a selfassembled monolayer had been formed. As a result, we defined the reaction time of the first layer as 3 h. The absorbance of the APhS monolayer, which we used in the calculation, was 0.0116 ± 0.0027 on average. We obtained the average from about twenty samples that were allowed to react for 3 h. The molar absorption coefficient of the APhS solution was 1.41× 107 cm2/mol. From the absorbance of 0.0116 ± 0.0027 and the molar absorption coefficient of 1.41 × 107 cm2/mol, the packing density was calculated to be 4.1 × 10−10 mol/cm2. In addition, we roughly estimated the packing density for the first monolayer using a simple model, in which an APhS is assumed to be a cylindrical column with a diameter of 0.67 nm. The diameter of 0.67 nm corresponds to the sum of the distance between hydrogen atoms (0.43 nm) and the van der Waars radii of the hydrogen atoms (0.12 nm); each located in an ortho-position relative to the amino group on the benzene ring of an APhS. The distance was estimated from a molecular structure optimized with PM3 method of a semiempirical molecular orbital calculation. The cylindrical columns should have a hexagonal close-packed structure. The packing density was calculated to be 4.3 × 10−10 mol/cm2 in the above model. The
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packing density estimated for measurement is close to the hexagonal close-packed density. This indicates that the first monolayer formed has a high packing density. 3.2. The influence of solvent on an azomethine-forming reaction The reaction between amine and aldehyde is a dehydration reaction. Thus, the solvent used for the multilayer formation may affect the formation reaction. In order to investigate influence of solvent on the reaction, we measured the UV–vis absorption spectra of self-assembled multilayers synthesized in EtOH and DCM. Fig. 3(a) and (b) shows UV–vis the absorption spectra of self-assembled multilayers formed on quartz substrates in EtOH and DCM, respectively. The three spectra in each figure represent spectra for the first, second, and third deposited layers. For both the solvents, the absorption intensities increase with layer-by-layer growth. This implies that each reaction step yields chemisorption of reactants on
Fig. 3. UV–vis absorption spectra of multilayers vary with the number of layers; (a) formed in EtOH solution, (b) formed in DCM solution, and (c) UV–vis absorption spectra of multilayers after 2nd layer deposited in several solvents.
the substrate surface. However, the absorption increase is different for EtOH and DCM solvents. The multilayers synthesized in EtOH have a higher absorbance than those synthesized in DCM. This shows that EtOH is more suitable than DCM for the reactions in the formation of multilayers. The low absorption for DCM solvent probably relates to solubility of water in DCM. The behavior of DCM is considered as follows. DCM has a lower solubility of water than EtOH; DCM has an octanol–water partition coefficient (Log Pow) of 1.25, while EtOH has a Log Pow of −0.32. Thus, water produced in the reaction of azomethine on a substrate is less diffusible in DCM; the produced water remains close to the substrate. The water hydrolyzes the azomethine bonds. The hydrolysis results in a low percentage of reaction. In addition, DMF and THF were tested as reaction solvents for TPA deposition. DMF and THF, having respective Log Pow levels of −0.87 and 0.47, are water-soluble solvents. The absorption spectra of multilayers reacted in DMF and THF are similar to the spectrum of multilayers reacted in EtOH (Fig. 3(c)). Therefore, water-soluble solvents such as EtOH are more suitable for the azomethine reaction than DCM. The influence of solvent on the reaction was also evident in the film thickness of the multilayers. Fig. 4 shows the film thickness after each reaction versus number of the benzene rings in the molecular chain forming the multilayer. The film thickness was estimated by ellipsometric measurement. The calculated thickness corresponding to each film is also shown in Fig. 4. The thickness was calculated using the sum of the van der Waals radius of a terminal hydrogen atom of the amine or aldehyde group and the distance between the terminal atom and a plane formed by three oxygen atoms which are linked with a silicon atom in a model of the molecule that forms the multilayer. The structure of the model molecule was optimized with PM3 method. For a multilayer reacted in EtOH, the experimental thickness increases with an increase in the number of benzene rings and is close to the calculated thickness. The structure for the calculation is assumed to be perpendicular to the substrate. Therefore, the agreement of the measured values with the calculated values indicates that the multilayer is perpendicularly formed on a substrate and that the multilayer has few unreacted groups. On the other hand, for a multilayer reacted in DCM, the experimental thickness for each multilayer is less than the calculated thickness for a multilayer with the same number of benzene rings. The low increase in thickness indicates that unreacted groups exist to some extent. The difference in the increase in film thickness for EtOH and DCM is consistent with the difference in the absorption spectra shown in Fig. 3.
Fig. 4. Film thickness of layers with various numbers of benzene rings. Error bars represent a standard error for the three samples.
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3.3. Characterization of azomethine multilayers Since the results shown above indicate that EtOH is more suitable for the reaction of azomethine than DCM, multilayers formed in EtOH are discussed hereafter. Fig. 5(a) and (b) shows an AFM image and a cross section for a multilayer with four benzene rings. The surface has an rms surface roughness of 0.45 nm and no apparent structures such as domains or grains. The surface roughness of 0.45 nm is comparable to that for the amide-bond multilayer films reported in Ref. [13]. On the other hand, the maximum depth in a typical cross section is about 1.2 nm. The experimental thickness estimated from an ellipsometric measurement for the multilayer with four benzene rings is 2.8 nm as shown in Fig. 4. The maximum depth is less than the film thickness corresponding to the length of a molecular chain with four benzene rings. Therefore, this result suggests that the multilayer does not have large holes and defects as seen in the surface morphology in Ref. [3]. Furthermore, absorption spectra for monomers that were used as the reactants were measured in order to confirm that chemical linkages were formed after each layer deposition. Since a πconjugated chemical linkage between π-conjugated molecules yields a wave function expanding over the molecules, the formation of the chemical linkage results in a change of the absorption spectra. Thus, the absorption spectrum of the monomer was compared with the difference between spectra for multilayers. The comparison was discussed on the basis of an assumption that the absorption spectra for monomers and multilayers have no direction dependence [26]. Fig. 6(a) and (b) shows the UV–vis absorption spectra of TPA and StDA, respectively. The monomers were dissolved in EtOH for the absorption measurement. Also the difference spectra for second and third layers are shown by dotted lines in Fig. 6(a) and (b). The difference spectra for the second and third layers were simply obtained by subtracting the absorption for the first layer from that of the second layer and the absorption for the second layer from that of
Fig. 6. UV–vis absorption spectra of monomers and difference spectra of multilayers for (a) the second layer and (b) the third layer.
the third layer as shown in Fig. 3(a). The TPA absorption spectrum has narrow peaks at 245 and 310 nm. On the other hand, the difference spectrum for the second layer has a peak at 280 nm and a shoulder at around 310 nm. Also, the absorption spectrum of StDA has peaks at 320 and 347 nm. On the other hand, the difference spectrum of the third layer has a peak at 374 nm and a shoulder around 320 nm. It seems that the absorption spectra of the monomers (TPA and StDA) do not correspond to the difference spectra of the multilayer (second and third layer). Therefore this disagreement indicates that the monomers are not physically adsorbed on the substrates and that the change of the absorption spectra in each deposition step shown in Fig. 3(a) is attributed to the chemical formation of azomethine bonds. 3.4. Optical bandgaps of azomethine multilayers
Fig. 5. (a) Typical AFM image and (b) cross section of a multilayer with four benzene rings.
We estimated the optical bandgaps of multilayers to indirectly investigate the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels of the mutilayer. Also, we estimated the bandgaps of the oligomers and the polymer shown in Fig. 2 for the purpose of comparison to those of multilayers. Fig. 7 shows the bandgap of the azomethine multilayer and azomethine oligomers with various numbers of benzene rings. The bandgap was estimated from x-intercept of the line fitting to the absorption spectrum near the absorption edge. Bandgap values of three samples for the same multilayer were in the range of ±0.05 eV. The bandgaps of multilayers with two and four benzene rings are comparable to those of dimer and tetramer, respectively. The results suggest that the multilayer has similar wave functions for HOMO and LUMO levels as those of oligo-azometines with the same benzene rings. In addition, the bandgaps of the multilayer gradually decrease
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Acknowledgment This work was supported by Special Coordination Funds for Promoting Science and Technology.
References
Fig. 7. Energy bandgaps in the multilayers (filled rhombus) and oligomers (open circle) with the various numbers of benzene rings, and polymer (open triangle) as estimated based on the absorption spectra.
with the number of benzene rings toward the bandgap of the polymer. This indicates that formation of an azomethine bond in each deposition step results in a π-conjugated orbital expanding to neighbor benzene rings. The difference in the bandgaps of a multilayer with four benzene rings and a polymer is about 0.23 eV. Furthermore, the difference will reduce with an increase in benzene unit linked by azomethine bonds in the multilayer. 4. Conclusion We have formed π-conjugated azomethine multilayers on substrates using synthesis from solution. Use of EtOH rather than DCM as the solvent leads to better multilayer formation. Thus, we conclude that water-soluble anhydrous solvents such as anhydrous EtOH are suitable for such layer-by-layer formation. In addition, we investigated the optical bandgaps of multilayers and azomethine compounds with various numbers of benzene rings. The bandgap of the multilayer gradually decreases with an increase in the number of benzene rings, and approaches the bandgap of a polymer. From these results, we confirm that π-conjugation expands along a molecular chain in the azomethine multilayer. Based on these findings regarding π-conjugation in multilayer films, it can be inferred that azomethine multilayers could be used as carrier transport layers in electronic devices.
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Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Synthesis and investigation of π-conjugated azomethine self-assembled multilayers by layer-by-layer growth Masakazu Kamura a,b,c,⁎, Yasutaka Kuzumoto a,b, Shigeru Aomori a,b, Hirohiko Houjou c, Masatoshi Kitamura b, Yasuhiko Arakawa b,c a b c
Advanced Materials & Energy Engineering Laboratories, Sharp Corporation, 273-1, Kashiwa, Kashiwa-shi, Chiba, 277-0005, Japan Institute for Nano Quantum Information Electronics (INQIE), the University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo, 153-8505, Japan Institute of Industrial Science, the University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo, 153-8505, Japan
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Article history: Received 9 November 2009 Received in revised form 15 February 2010 Accepted 3 March 2010 Available online 11 March 2010 Keywords: Layer-by-layer complexation Chemisorption Multilayers π-conjugation Azomethine
a b s t r a c t Layer-by-layer formation for π-conjugated azomethine multilayers bonded on substrates was investigated. The multilayers were synthesized using ethanol (EtOH) and dichloromethane (DCM) as reaction solvents. The multilayer characteristics were analyzed using UV–vis absorption spectroscopy, ellipsometric thickness, and atomic force microscopy. The absorption spectra and ellipsometric thicknesses of multilayers formed using EtOH and DCM were compared. The results indicate that EtOH is more suitable than DCM for such layer-by-layer formation. In addition, bandgaps estimated from the absorption edge of multilayers were investigated. The results indicate that the bandgap decreases as the number of benzene rings contained in the molecular chain of the multilayer increases. Also, a multilayer with four benzene rings bonded on a substrate had a bandgap close to that of a polymer with a similar chemical structure. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Self-assembled monolayers and multilayers have been studied, and the findings regarding their application to organic electronic devices have been interesting [1–8]. A self-assembled multilayer consists of selfassembled molecular chains; each chain has some units forming layers. Such a multilayer is formed by layer-by-layer chemisorption. Units are covalently-bonded with each other; a bottom unit is also covalentlybonded with a substrate. The covalent bond contributes to the electrical resistance in an electronic device using a multilayer film. Also, it is possible to vary the combination of units that form a multilayer film to target specific electrical and optical properties. Therefore, it is expected that self-assembled multilayer films will be used as carrier transport layers, insulating layers, and/or emitting layers in electronic devices such as transistors, solar cells, and organic light-emitting devices (LED). Self-assembled multilayer films have actually been used as semiconducting channel layers [5] and insulators [6,7] in transistors, and as hole transport layers in organic LEDs [8]. In addition, studies have been performed regarding the electric characteristics of selfassembled multilayers [9–12]. π-Conjugated orbitals in a multilayer play an important role in defining the electronic properties of the multilayer. In particular, a π⁎ Corresponding author. Advanced Materials & Energy Engineering Laboratories, Sharp Corporation, 273-1, Kashiwa, Kashiwa-shi, Chiba, 277-0005, Japan. Tel./fax: +81 3 5452 5711. E-mail address: [email protected] (M. Kamura). 0040-6090/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.03.006
conjugated orbital expanding to some units is desirable when the multilayer is used as a carrier transport layer of an electronic device. In order to fabricate a multilayer with such an orbital, it is important to appropriately select a reactant molecule for synthesis in layer-bylayer formation. A molecular structure having π orbitals such as a benzene ring or a thiophene ring is required to obtain π orbitals expanding in a molecular chain. On the other hand, two terminal groups in a reactant molecule determine the type of chemical bonds in the connecting units forming a molecular chain. Various chemical bonds have been used for self-assembled multilayers: imide [13,14], amide [13–17], urea [18], thioester [19], disulfide [20], ethylene and azomethine [10–12,21] bonds. The former five bonds provide σ bonds; ethylene and azomethine bonds provide π-conjugated bonds. Therefore, ethylene and azomethine bonds are suitable to achieve πconjugated orbitals expanding along a molecular chain. Meanwhile, to make multilayer films practical for use in electric devices, it is also important to form a multilayer structure with a high reaction yield. This is because a low reaction yield will lead to pinholes and voids in the film, which will result in a decline in electrical properties. In terms of reaction yield, an azomethine bond is preferable because the reaction mechanism is simple; no catalyst or condensation agent is required for an azomethine-forming reaction. Some studies about layer-by-layer formation using azomethine bonds have been reported [10–12,21]. In these reports, synthesis of azomethine multilayers from aromatic diamine and dialdehyde on a gold surface was described. Also, they investigated electrical properties of one or a few molecules using an atomic force microscope (AFM)
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[10] or a scanning tunneling microscope (STM) [11,12] in order to reveal a carrier transport mechanism along π orbitals inside the molecular chain. However, there have been few reports that discuss the characterization of π orbitals in conjugated multilayer films. In this paper, we describe π-conjugated azomethine multilayers formed on substrates terminated with hydroxyl groups (–OH) using water-soluble solvents. The multilayers were characterized by UV–vis absorption spectroscopy, ellipsometric thickness, and atomic force microscopy. In order to characterize the π-conjugated orbitals in the multilayers, we investigated the bandgaps of the multilayers in each deposition step with monomers having one or two benzene rings. 2. Experimental details 2.1. Materials p-Aminophenyltrimethoxysilane (APhS) and terephthalaldehyde (TPA) were purchased from Gelest and Tokyo Kasei, respectively. The reagents were used as received. The 4,4′-diaminostilbene (StDA) for multilayer formation was obtained using the following procedure. 4,4′Stilbenediamine dihydrochloride 300 mg (purchased from Aldrich) was dissolved in 150 ml water. After an aqueous sodium hydroxide (NaOH) solution (1 mol/l) was added dropwise to the aqueous solution of 4,4′stilbenediamine dihydrochloride until pH 10, the aqueous solution was poured onto 150 ml of dichloromethane and the two phases were separated. The aqueous layer was extracted with dichloromethane (100 ml twice) and the combined organic layers were dried over MgSO4. The solvent was removed under reduced pressure and the obtained pale yellow solid was purified by recrystallization from dichloromethane/ hexane solution. The obtained StDA was characterized by 1H nuclear magnetic resonance (1H NMR) and Fourier-transformed infrared spectroscopy (FT-IR) measurements. 1H NMR spectra were measured using a JEOL JNM AL-400 (400 MHz for 1H) spectrometer. 1H NMR data were recorded as follows: chemical shift in ppm from internal tetramethylsilance on the δ scale, multiplicity (s = singlet; d = doublet; m = multiplet), coupling constant (Hz), integration, and assignment. FT-IR spectra were recorded using a JEOL WINSPEC 100 FT-IR spectrometer. 1H NMR and FT-IR data of the StDA were as follows. 1H NMR (DMSO-d6): δ 5.14 (s, 4H, NH2), 6.52 (d, J=8.4 Hz 4H, ArH), 6.71 (s, 2H, –CH=), 7.16 (s, J =8.0 Hz 4H, ArH), and 8.44 (s, 1H, CH=N). FT-IR (KBr): 3383, 3356, 3298, 3195, 1607, 1264, 971, 830, and 537 cm−1. Dichloromethane anhydrous (DCM; water contents b0.001%), ethanol anhydrous (EtOH; water contents b0.005%), N,N-dimethylformamide anhydrous (DMF; water contents b0.005%) and tetrahydrofuran anhydrous (THF; water contents b0.002%), which were purchased from Kanto Kagaku, were used as received. 2.2. Substrates Polished single-crystal silicon (100) wafers and polished quartz slides were used as substrates. The silicon and quartz substrates have root-mean-square (rms) surface roughness levels of 0.11 nm and 0.34 nm, respectively. The silicon substrate was cleaned in acetone with an ultrasonic bath and then the native oxide on the silicon surface was removed using a hydrofluoric acid 1% aqueous solution. The procedure for handling hydrofluoric acid, which is extremely corrosive and a contact poison, was carefully performed with gloves and protective glasses in a well-ventilated fume hood. The silicon substrate was cleaned by UV/ozone treatment. The quartz substrate was cleaned in acetone with an ultrasonic bath and then cleaned with UV/ozone treatment. The surfaces were terminated with –OH by UV/ozone treatment. 2.3. Synthesis of the azomethine self-assembled multilayers Fig. 1 shows the schematic of layer-by-layer growth of azomethine self-assembled multilayers. For the first monolayer, a substrate is
placed with solid APhS in a Teflon container under dry nitrogen [22]. The container is heated at 100 °C for 3 h in an oven. Once vaporized, the APhS reacts with –OH groups of the substrate surface. The reaction results in an APhS monolayer formed on the surface as shown in Fig. 1(b). For the second layer, an aldehyde group of TPA reacts with an amine group on the substrate as reaction II in Fig. 1. For the third layer, an amine group of StDA reacts with the unreacted aldehyde group of the TPA formed on the substrate as reaction III in Fig. 1. For both the second and third layers, the reactions are performed using solutions of TPA and StDA, respectively. The substrate is immersed in 1 mmol/l EtOH or DCM solution at room temperature (20–25 °C) for 20 h. The solution is continuously stirred during the reaction. The reaction is performed under dry nitrogen ambient. After each layer is deposited, the substrate is cleaned in EtOH with an ultrasonic bath in order to remove monomers that have been physically adsorbed. The reaction of amine and aldehyde groups creates an azomethine π-conjugate bond. 2.4. Preparation of oligo- or poly-phenyleneazomethines We have synthesized oligo- and poly-phenyleneazomethines to compare their properties and the self-assembled multilayers formed on substrates. The chemical structures of the substances are shown in Fig. 2. The oligomers are benzylidene-aniline (dimer), N,N″-dibenzylidene-benzene-1,4-diamine (trimer), and N,N′-dibenzylidene-4, 4′diaminostilbene (tetramer). The polymer is poly(1,4-phenylenemethylidynenitrilo-1,4-phenylenenitrilomethylidyne) (polymer). The azomethine compounds were synthesized as follows. 2.4.1. General method The synthesis of oligo- and poly-azomethines was performed under a nitrogen atmosphere. Anhydrous dichloromethane was used as a reaction solvent. The oligo-azomethines were characterized by 1H NMR and FT-IR measurements. On the other hand, a formation of the poly-azomethine was confirmed by FT-IR measurement. We could not obtain 1H NMR of the polymer due to the low solubility in general solvents. In the FT-IR spectrum of the synthesized polymer, a specific peak was observed at 1613 cm−1, which is assigned to azomethine bonds. 2.4.2. Dimer Aniline (10.0 mmol, 0.913 ml) was added dropwise to a solution of benzaldehyde (10.0 mmol, 1.01 ml) in DCM (10 ml). The reaction mixture was stirred for 24 h at room temperature. After by-product water was removed by MgSO4 from the mixture, the dimer was purified by recrystallization from a dichloromethane/hexane solution. Yield: 0.813 g (4.49 mmol 45%) as a colorless solid. 1H NMR (CDCl3): δ 7.20–7.24 (m, 3H, ArH), 7.37–7.41 (m, 2H, ArH), 7.45–7.47 (m, 3H, ArH), 7.88–7.91 (m, 2H, ArH), and 8.44 (s, 1H, CH=N). FT-IR (KBr): 3061, 3028, 2889, 1625(C=N), 1590, 1578, 1484, 1451, 1194, 1173, 761, and 693 cm−1. 2.4.3. Trimer Aniline (24.0 mmol, 1.83 ml) was added dropwise to a solution of terephthalaldehyde (10.0 mmol, 1.34 g) in DCM (40 ml). The reaction mixture was stirred for 20 h at room temperature. The trimer was purified using the same method as the dimer. Yield: 1.15 g (4.03 mmol 40%) as yellow needles. 1H NMR (CDCl3): δ 7.24–7.26 (m, 6H, ArH), 7.40–7.44 (m, 4H, ArH), 8.02 (s, 4H, ArH), and 8.52 (s, 2H, CH=N). FT-IR (KBr): 3058, 2877, 1614(C=N), 1584, 1481, 1451, 1353, 1191, 971, 833, 760, and 695 cm−1. 2.4.4. Tetramer Benzaldehyde (0.67 mmol, 0.068 ml) was added dropwise to a solution of 4,4′-stilbenediamine (0.28 mmol, 59 mg) in DCM (20 ml). The reaction mixture was stirred for 20 h at room temperature. After
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Fig. 1. Schematic diagram of synthesis method of an azomethine self-assembled multilayer formed on a substrate.
by-product water was removed by MgSO4 from the mixture, the tetramer was purified by recrystallization from a dichloromethane/ ethyl acetate solution. Yield: 42 mg (0.11 mmol 39%) as a yellow solid. 1 H NMR (CDCl3): δ 7.14 (s, 2H, CH=CH), 7.25 (d, J = 7.8 Hz, 4H, ArH), 7.49–7.50 (m, 6H, ArH), 7.57 (d, J = 8.3 Hz, 4H, ArH), 7.91–7.94 (m, 4H, ArH), and 8.52 (s, 2H, CH=N). FT-IR (KBr): 3021, 2874, 1622 (C=N), 1574, 1502, 961, 845, 688, and 559 cm−1. 2.4.5. Polymer Terephthalaldehyde (12.3 mmol, 1.66 g) was slowly added to a solution of 1,4-phenylenediamine (12.3 mmol, 1.34 g) in DCM (50 ml). After the reaction mixture was stirred for 20 h, a yellow powder was produced. The yellow powder was purified by reprecipitation from dichloromethane into methanol (3 times). FT-IR (KBr): 3355, 2874, 1693, 1613(C=N), 1508, 1489, 1192, 849, and 563 cm−1. The FT-IR spectrum of the synthesized polymer matched with the reported values [23,24]. 2.5. Characterization of the self-assembled multilayer films UV–vis absorption spectra of films formed on quartz substrates were measured using a Shimadzu UV3600 UV–vis spectrometer. UV–
Fig. 2. Chemical structures of oligo- and poly-phenyleneazomethines. The oligomers are benzylidene-aniline (dimer), N,N″-dibenzylidene-benzene-1,4-diamine (trimer) and N,N′-dibenzylidene-4,4′-diaminostilbene (tetramer). The polymer is poly (1,4phenylenemethylidynenitrilo-1,4-phenylenenitrilomethylidyne) (polymer).
vis spectra of multilayers were obtained by subtracting the spectra for a bare quartz substrate from the spectra for a substrate with a multilayer. A J.A. Woolam M-2000U spectroscopic ellipsometer was used to perform film thickness measurements. The measurement wavelength ranges from 245 to 1000 nm, and the incident angle is 70°. Thickness measurements for organic layers were performed using the following procedure. First, the native oxide thickness on the silicon substrate was measured. After each monomer deposition, total film thickness including the organic layer was evaluated, assuming that the entire film on the silicon is a SiO2 layer (refractive index = 1.457 at 633 nm). The organic layer thickness was obtained by subtracting the native oxide thickness from the total thickness. Morphologies of films were characterized with a Veeco AFM in tapping mode. 3. Results and discussion 3.1. Estimation of the packing density for the first layer Packing density in the plane of the first monolayer affects subsequent multilayer formation. Thus we estimated the packing density based on the absorption spectra. The packing density was roughly calculated by comparing the maximum absorbance at 246 nm in the absorption spectrum of the first monolayer and a molar absorption coefficient measured for a dichloromethane solution of APhS [25]. The absorption intensity of the APhS layer increased with increasing reaction time during the early stage. Then the increase in absorbance became self-limiting after 2.5 h. The self-limiting behavior suggests that the substrate was fully covered with APhS molecules and that a selfassembled monolayer had been formed. As a result, we defined the reaction time of the first layer as 3 h. The absorbance of the APhS monolayer, which we used in the calculation, was 0.0116 ± 0.0027 on average. We obtained the average from about twenty samples that were allowed to react for 3 h. The molar absorption coefficient of the APhS solution was 1.41× 107 cm2/mol. From the absorbance of 0.0116 ± 0.0027 and the molar absorption coefficient of 1.41 × 107 cm2/mol, the packing density was calculated to be 4.1 × 10−10 mol/cm2. In addition, we roughly estimated the packing density for the first monolayer using a simple model, in which an APhS is assumed to be a cylindrical column with a diameter of 0.67 nm. The diameter of 0.67 nm corresponds to the sum of the distance between hydrogen atoms (0.43 nm) and the van der Waars radii of the hydrogen atoms (0.12 nm); each located in an ortho-position relative to the amino group on the benzene ring of an APhS. The distance was estimated from a molecular structure optimized with PM3 method of a semiempirical molecular orbital calculation. The cylindrical columns should have a hexagonal close-packed structure. The packing density was calculated to be 4.3 × 10−10 mol/cm2 in the above model. The
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packing density estimated for measurement is close to the hexagonal close-packed density. This indicates that the first monolayer formed has a high packing density. 3.2. The influence of solvent on an azomethine-forming reaction The reaction between amine and aldehyde is a dehydration reaction. Thus, the solvent used for the multilayer formation may affect the formation reaction. In order to investigate influence of solvent on the reaction, we measured the UV–vis absorption spectra of self-assembled multilayers synthesized in EtOH and DCM. Fig. 3(a) and (b) shows UV–vis the absorption spectra of self-assembled multilayers formed on quartz substrates in EtOH and DCM, respectively. The three spectra in each figure represent spectra for the first, second, and third deposited layers. For both the solvents, the absorption intensities increase with layer-by-layer growth. This implies that each reaction step yields chemisorption of reactants on
Fig. 3. UV–vis absorption spectra of multilayers vary with the number of layers; (a) formed in EtOH solution, (b) formed in DCM solution, and (c) UV–vis absorption spectra of multilayers after 2nd layer deposited in several solvents.
the substrate surface. However, the absorption increase is different for EtOH and DCM solvents. The multilayers synthesized in EtOH have a higher absorbance than those synthesized in DCM. This shows that EtOH is more suitable than DCM for the reactions in the formation of multilayers. The low absorption for DCM solvent probably relates to solubility of water in DCM. The behavior of DCM is considered as follows. DCM has a lower solubility of water than EtOH; DCM has an octanol–water partition coefficient (Log Pow) of 1.25, while EtOH has a Log Pow of −0.32. Thus, water produced in the reaction of azomethine on a substrate is less diffusible in DCM; the produced water remains close to the substrate. The water hydrolyzes the azomethine bonds. The hydrolysis results in a low percentage of reaction. In addition, DMF and THF were tested as reaction solvents for TPA deposition. DMF and THF, having respective Log Pow levels of −0.87 and 0.47, are water-soluble solvents. The absorption spectra of multilayers reacted in DMF and THF are similar to the spectrum of multilayers reacted in EtOH (Fig. 3(c)). Therefore, water-soluble solvents such as EtOH are more suitable for the azomethine reaction than DCM. The influence of solvent on the reaction was also evident in the film thickness of the multilayers. Fig. 4 shows the film thickness after each reaction versus number of the benzene rings in the molecular chain forming the multilayer. The film thickness was estimated by ellipsometric measurement. The calculated thickness corresponding to each film is also shown in Fig. 4. The thickness was calculated using the sum of the van der Waals radius of a terminal hydrogen atom of the amine or aldehyde group and the distance between the terminal atom and a plane formed by three oxygen atoms which are linked with a silicon atom in a model of the molecule that forms the multilayer. The structure of the model molecule was optimized with PM3 method. For a multilayer reacted in EtOH, the experimental thickness increases with an increase in the number of benzene rings and is close to the calculated thickness. The structure for the calculation is assumed to be perpendicular to the substrate. Therefore, the agreement of the measured values with the calculated values indicates that the multilayer is perpendicularly formed on a substrate and that the multilayer has few unreacted groups. On the other hand, for a multilayer reacted in DCM, the experimental thickness for each multilayer is less than the calculated thickness for a multilayer with the same number of benzene rings. The low increase in thickness indicates that unreacted groups exist to some extent. The difference in the increase in film thickness for EtOH and DCM is consistent with the difference in the absorption spectra shown in Fig. 3.
Fig. 4. Film thickness of layers with various numbers of benzene rings. Error bars represent a standard error for the three samples.
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3.3. Characterization of azomethine multilayers Since the results shown above indicate that EtOH is more suitable for the reaction of azomethine than DCM, multilayers formed in EtOH are discussed hereafter. Fig. 5(a) and (b) shows an AFM image and a cross section for a multilayer with four benzene rings. The surface has an rms surface roughness of 0.45 nm and no apparent structures such as domains or grains. The surface roughness of 0.45 nm is comparable to that for the amide-bond multilayer films reported in Ref. [13]. On the other hand, the maximum depth in a typical cross section is about 1.2 nm. The experimental thickness estimated from an ellipsometric measurement for the multilayer with four benzene rings is 2.8 nm as shown in Fig. 4. The maximum depth is less than the film thickness corresponding to the length of a molecular chain with four benzene rings. Therefore, this result suggests that the multilayer does not have large holes and defects as seen in the surface morphology in Ref. [3]. Furthermore, absorption spectra for monomers that were used as the reactants were measured in order to confirm that chemical linkages were formed after each layer deposition. Since a πconjugated chemical linkage between π-conjugated molecules yields a wave function expanding over the molecules, the formation of the chemical linkage results in a change of the absorption spectra. Thus, the absorption spectrum of the monomer was compared with the difference between spectra for multilayers. The comparison was discussed on the basis of an assumption that the absorption spectra for monomers and multilayers have no direction dependence [26]. Fig. 6(a) and (b) shows the UV–vis absorption spectra of TPA and StDA, respectively. The monomers were dissolved in EtOH for the absorption measurement. Also the difference spectra for second and third layers are shown by dotted lines in Fig. 6(a) and (b). The difference spectra for the second and third layers were simply obtained by subtracting the absorption for the first layer from that of the second layer and the absorption for the second layer from that of
Fig. 6. UV–vis absorption spectra of monomers and difference spectra of multilayers for (a) the second layer and (b) the third layer.
the third layer as shown in Fig. 3(a). The TPA absorption spectrum has narrow peaks at 245 and 310 nm. On the other hand, the difference spectrum for the second layer has a peak at 280 nm and a shoulder at around 310 nm. Also, the absorption spectrum of StDA has peaks at 320 and 347 nm. On the other hand, the difference spectrum of the third layer has a peak at 374 nm and a shoulder around 320 nm. It seems that the absorption spectra of the monomers (TPA and StDA) do not correspond to the difference spectra of the multilayer (second and third layer). Therefore this disagreement indicates that the monomers are not physically adsorbed on the substrates and that the change of the absorption spectra in each deposition step shown in Fig. 3(a) is attributed to the chemical formation of azomethine bonds. 3.4. Optical bandgaps of azomethine multilayers
Fig. 5. (a) Typical AFM image and (b) cross section of a multilayer with four benzene rings.
We estimated the optical bandgaps of multilayers to indirectly investigate the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels of the mutilayer. Also, we estimated the bandgaps of the oligomers and the polymer shown in Fig. 2 for the purpose of comparison to those of multilayers. Fig. 7 shows the bandgap of the azomethine multilayer and azomethine oligomers with various numbers of benzene rings. The bandgap was estimated from x-intercept of the line fitting to the absorption spectrum near the absorption edge. Bandgap values of three samples for the same multilayer were in the range of ±0.05 eV. The bandgaps of multilayers with two and four benzene rings are comparable to those of dimer and tetramer, respectively. The results suggest that the multilayer has similar wave functions for HOMO and LUMO levels as those of oligo-azometines with the same benzene rings. In addition, the bandgaps of the multilayer gradually decrease
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Acknowledgment This work was supported by Special Coordination Funds for Promoting Science and Technology.
References
Fig. 7. Energy bandgaps in the multilayers (filled rhombus) and oligomers (open circle) with the various numbers of benzene rings, and polymer (open triangle) as estimated based on the absorption spectra.
with the number of benzene rings toward the bandgap of the polymer. This indicates that formation of an azomethine bond in each deposition step results in a π-conjugated orbital expanding to neighbor benzene rings. The difference in the bandgaps of a multilayer with four benzene rings and a polymer is about 0.23 eV. Furthermore, the difference will reduce with an increase in benzene unit linked by azomethine bonds in the multilayer. 4. Conclusion We have formed π-conjugated azomethine multilayers on substrates using synthesis from solution. Use of EtOH rather than DCM as the solvent leads to better multilayer formation. Thus, we conclude that water-soluble anhydrous solvents such as anhydrous EtOH are suitable for such layer-by-layer formation. In addition, we investigated the optical bandgaps of multilayers and azomethine compounds with various numbers of benzene rings. The bandgap of the multilayer gradually decreases with an increase in the number of benzene rings, and approaches the bandgap of a polymer. From these results, we confirm that π-conjugation expands along a molecular chain in the azomethine multilayer. Based on these findings regarding π-conjugation in multilayer films, it can be inferred that azomethine multilayers could be used as carrier transport layers in electronic devices.
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