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J. Phys. Chem. B 2006, 110, 13100-13106
Structures of 17,19-Hexatriacontadiyne Monolayers on Au(111) Studied by Infrared Reflection Absorption Spectroscopy and Scanning Tunneling Microscopy Osamu Endo,† Taro Furuta,† Hiroyuki Ozaki,*,† Masashi Sonoyama,‡ and Yasuhiro Mazaki§ Department of Organic and Polymer Materials Chemistry, Faculty of Technology, Tokyo UniVersity of Agriculture and Technology, Koganei, Tokyo 184-8588, Japan, Department of Applied Physics, Graduate School of Engineering, Nagoya UniVersity, Chikusa-ku, Nagoya 464-8603, Japan, and Department of Chemistry, School of Science, Kitasato UniVersity, Sagamihara, Kanagawa 228-8555, Japan ReceiVed: September 13, 2005; In Final Form: March 6, 2006
The aggregation and reaction of 17,19-hexatriacontadiyne molecules are studied on a Au(111) surface. The molecular orientation and arrangement are elucidated by infrared reflection absorption spectroscopy (IRAS) and scanning tunneling microscopy (STM). A vapor-deposited monolayer and a multilayered film formed by adsorption from the solution provide IRA spectra with bands due to the antisymmetric and symmetric stretching of methylenes in the gauche conformation. After the adsorbed film is rinsed with the solvent, however, the spectrum loses the gauche bands and is characterized by the enhanced C-Hdistal and C-Hproximal stretching bands, which means that all-trans molecules are laid flat. Only STM images for the rinsed film display columnar structures on the herringbones of the reconstructed Au(111) surface; the alkyl chain direction is found to be parallel to the Au atom row. The results indicate that an ordered monolayer is formed first at the liquid-solid interface, and then, disordered overlayers with the gauche conformation are grown but removed by a rinse. Upon exposure to UV light, thus obtained monomer columns are converted into oligomers with flexible backbones and an increased gauche population in the alkyl chains, which resemble red phase polydiacetylenes in LB films.
Introduction The self-assembly of chain hydrocarbons lying on metal surfaces is one of the intriguing phenomena in surface and interface chemistry. The very early study of low-energy electron diffraction showed that short n-alkanes lie flat to form a monolayer with columnar (or lamellar) structures on Pt(111) under an ultrahigh vacuum (UHV).1 Nowadays, the columnar structures are displayed more directly for n-alkanes and the related compounds with diverse lengths and functional groups by scanning tunneling microscopy (STM), mostly in monolayers at liquid-graphite (0001) and liquid-Au(111) interfaces.2-14 The elucidation of the monolayer structures enables us to expect the practical use of the inherent functionalities of the monolayers for applications including electronic devices, catalysts, and nanomaterials. We have planed such a development using an alkadiyne monolayer. A column of lying 17,19-hexatriacontadiyne (HTDY; C16H33C≡CC≡CC16H33) molecules self-assembled in a monolayer on a graphite (0001) surface is converted into a single sheet of sashlike polydiacetylene (PD) [atomic sash (AS)] upon exposure to UV light (Figure 1).15-18 PDs having onedimensional π electronic systems can be endowed with conductive properties and exhibit chromatic transition (blue to red or vice versa) induced by various stimuli including temperature, voltage, light, ligands, and pH.19-25 The chromatic transition is accompanied by changes in the electronic structure, effective conjugation length, and conductivity. For the AS molecules with * To whom correspondence should be addressed. E-mail: hiroyuki@ cc.tuat.ac.jp. †Tokyo University of Agriculture and Technology. ‡ Nagoya University. § Kitasato University.
Figure 1. Models for (a) the arrangement of HTDY molecules laid with their carbon zigzag planes parallel to the surface (flat-on orientation) in a monolayer and (b) the structure of a single sheet of a sashlike polydiacetylene (AS) formed by the intramonolayer photopolymerization of HTDY molecules in each column of part a.15-18 Note that part b is for one of the AS conformers (AS-II), in which all of the carbon atoms of the polydiacetylene chain and the alkyl chains are held in a common plane.18
lying PDs on graphite (0001), both the blue and the red phases have been detected by Raman spectroscopy.26 In addition, the phase transition of the AS between two structures with different contrasts in the STM images, which may also have some relation to the chromatic transition of the PDs, has been revealed recently.18 To understand the mechanism of polymerization in each HTDY column as well as that of the AS transition, we must make a thorough investigation on the structure of the HTDY monolayer. The orientation and arrangement of HTDY molecules, the internal structure of the AS, and changes in the electronic structures during the intramonolayer polymerization were confirmed by Penning ionization electron spectroscopy15,16 and
10.1021/jp055161+ CCC: $33.50 © 2006 American Chemical Society Published on Web 06/13/2006
Structures of 17,19-Hexatriacontadiyne Monolayers STM.13,17,18 It is further expected that the conformation and orientation of HTDY and AS molecules can be revealed directly by infrared reflection absorption spectroscopy (IRAS) because the wave numbers of the methylene stretching vibrational modes are very sensitive to the conformation on the surface and the relative intensities of the methylene stretching bands change dependent on the local orientation of the methylene units.27-36 However, IRAS cannot be easily applied to films on a graphite surface because of the low reflectivity, and hence, it is desirable that structural analyses for HTDY and AS monolayers by IRAS are performed on a metal surface with high reflectivity. The preparation of the AS on a metal surface is significant because it will be available as a good buffer layer for organic electronics (e.g., in electroluminescence devices and transistors) because the thickness of the monolayer is homogeneous and the conductivity and susceptibility can be modified by controlling the phase transition. In these devices, the behavior of an organic layer sandwiched with inorganic layers is critically important. Nevertheless, the AS monolayer has been constructed on graphite alone; no clean metal surface was used up to now as the substrate for AS formation. It is not even known whether a HTDY monolayer with the columnar structures as in Figure 1a can be obtained on a metal surface. The reactivity of HTDY molecules on a metal surface has not been examined either. In this paper, we study the aggregation of HTDY molecules on Au(111) by IRAS and STM; Au(111) was chosen as the substrate because it is inert and known to be suitable for the self-assembly of chain molecules.5-7,11 First of all, we search for a method to obtain a columnar structure similar to that in Figure 1a. We attempt a dry and a wet process for film preparation. Although we have used vapor deposition to form HTDY monolayers on graphite (0001),13,15-18 the method is not necessarily suitable to obtain a well-ordered structure of chain hydrocarbons on Au(111).37 Therefore, we also utilize adsorption from the solution to form a HTDY monolayer on Au(111). Because it is found that a film prepared by the wet process and rinsed with a pure solvent provides a columnar structure, the product obtained by exposing the HTDY monolayer to UV light is examined as well. 2. Experimental Section HTDY was purified by evaporation at 10-5 Pa.38 A Au film (100 nm thick) was grown on a mica (0001) surface held at 530 K in an UHV (10-8 Pa), and the formation and flatness of a Au(111) surface were confirmed with reflection high-energy electron diffraction. After the Au substrate was cooled to room temperature, 1 MLE (monolayer equivalence: the amount of sample necessary to form a closely packed monolayer with the flat-on orientation as in Figure 1a) of HTDY was deposited from a molecular beam source onto the Au(111) surface. The amount of deposited sample was controlled using a quartz oscillator calibrated in advance. For the wet process, the Au(111) surface exposed to the air was cleaned by flame-annealing and cooled in pure acetone immediately prior to use. The clean surface was immersed into the 0.2 mM acetone solution of HTDY for film growth. The adsorbed film was rinsed in pure acetone. A film prepared by this procedure with a rinse was irradiated with UV light from a deuterium lamp at room temperature in the air. The IRA spectra were obtained in the air at room temperature by an FTS-6000 spectrometer equipped with a RAS attachment (Varian, Inc.); p-polarized light produced by a KRS-5 polarizer was used at an angle of incidence of 76° relative to the surface normal. The incidence angle was determined so as to maximize the signal intensity. The reflected light was detected using an
J. Phys. Chem. B, Vol. 110, No. 26, 2006 13101
Figure 2. Methylene stretching vibration region of the IRA spectrum of a HTDY monolayer deposited on Au(111) under an UHV. Inset: schematic drawing for the explanation of the decoupled vibrational modes of methylenes with the flat-on orientation.
MCT detector with a resolution of 2 cm-1. The single-beam spectrum of a clean Au(111) surface was used as a reference, and 1024 scans were averaged for each spectrum. STM observation was carried out using a microscope as reported previously13 either under an UHV or in the air at room temperature; the images were obtained in the constant current mode with mechanically polished PtIr tips. 3. Results and Discussion 3.1. Comparison of Film Structures by IRA Spectra. Figure 2 shows the methylene stretching vibration region of an IRA spectrum obtained for a vapor-deposited HTDY monolayer on Au(111) under an UHV. The bands around 2925 cm-1 and at 2853 cm-1 in Figure 2 are assigned to the antisymmetric and symmetric stretching vibrational modes of methylene, respectively. The wave numbers 2925 and 2853 cm-1 indicate that the methylene groups are in the gauche conformation.39 The band centered at 2908 cm-1 is assigned to the C-Hdistal stretching of methylenes laid with the flat-on orientation (see the inset in Figure 2).30-35,40 The shoulder of this band at the higher wave number side is ascribable to the antisymmetric stretching mode of methylenes in the trans conformation. These spectral features suggest that the monolayer contains a considerable number of trans methylene sequences with the flat-on orientation, but there are disordered parts in the gauche conformation as well. In fact, we could not observe any ordered structures by STM for this monolayer. Figure 3a shows an IRA spectrum for a HTDY film on Au(111) prepared by the wet process without a rinse. The most prominent feature in this spectrum is the antisymmetric stretching band centered at 2925 cm-1 for methylene sequences in the gauche conformation. The symmetric stretching band for gauche methylene sequences and the C-Hdistal stretching band are also observed at 2855 and 2910 cm-1, respectively. These spectral features indicate that the unrinsed film consists of plenty of molecules having gauche conformations, although there exist some trans methylene sequences attached to the surface with the flat-on orientation. On the other hand, an IRA spectrum for the rinsed film shows plain features as depicted in Figure 3b: above 2900 cm-1, only the C-Hdistal mode can be observed, at 2909 cm-1. The broad band around 2816 cm-1 is assigned to the C-Hproximal stretching mode (the so-called soft mode30-35,40) of the flat-on methylenes (see the inset in Figure 2). These results imply that the rinsed film consists of flat-on molecules in the all-trans conformation, which is supported by the column structures observed by STM (see the next subsection). We consider that the adsorption-desorption equilibrium
13102 J. Phys. Chem. B, Vol. 110, No. 26, 2006
Figure 3. (a) IRA spectrum of an adsorbed HTDY multilayer grown on Au(111) from the acetone solution. (b) IRA spectrum of an adsorbed HTDY monolayer prepared by rinsing an adsorbed multilayer on Au(111) with pure acetone.
first forms an ordered monolayer at the liquid-solid interface. The surface diffusion of HTDY molecules, which is more or less inhibited by molecule-substrate interactions under an UHV, can be promoted by the solvent. However, disordered overlayers with gauche conformations grow on the ordered monolayer and yield the IRA spectrum in Figure 3a; the C-Hdistal stretching mode detected in Figure 3a is consistent with the underlying ordered monolayer. The selective enhancement of the C-Hdistal and C-Hproximal stretching bands in Figure 3b means that the disordered overlayers are effectively removed by the rinse. In this sense, we will refer to a film prepared by the wet process with/without rinse as an “adsorbed” mono-/multilayer for convenience. The softened stretching mode of conjugated CC triple bonds was observed in none of the spectra for HTDY on Au(111), unlike the case of acetylene adsorbed on Cu(111).41 We consider that the physisorbed nature of the HTDY molecule on Au(111) causes no changes in the hybridization of the diyne carbons, and hence, bond softening does not occur upon adsorption. This point will be discussed in the next subsection. 3.2. STM Images of an Adsorbed HTDY Monolayer. Figure 4 shows a typical STM image (100 nm × 100 nm) observed in the air for an adsorbed HTDY monolayer. Almost the same images were obtained by STM observation under an UHV. Bright lines are arrayed at intervals of 5.0 ( 0.2 nm on the herringbone pattern of the Au(111) reconstructed surface in Figure 4. The line spacing of 5.0 ( 0.2 nm is in good agreement with the molecular length along the alkyl chain direction (4.8 nm). The image indicates that HTDY forms a self-assembled column structure as n-alkanes at liquid-Au(111) interfaces.5-7,10,11 The lines can be related to the highest occupied molecular orbital with large electron distribution around the conjugated triple bonds at the middle of the HTDY molecule.13,17 Figure 5a depicts a magnified STM image (30 nm × 30 nm) for the adsorbed HTDY monolayer on Au(111). The stripes of
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Figure 4. STM image for an adsorbed HTDY monolayer on Au(111), observed in the air at room temperature. Image size 100 nm × 100 nm, sample bias voltage V ) -0.60 V, tunneling current I ) 60 pA. Besides bright lines due to the columns of HTDY molecules, the herringbone pattern of the Au(111) reconstructed surface is clearly shown.
the Au herringbone pattern run from the lower left to the upper right, making 60° angles with the HTDY column axes. Minute lines due to alkyl chains are clearly observed individually; they are at right angles to the thick bright lines of the column axes and at an angle of 30° to the herringbone stripes. A tentative packing model is shown in Figure 5b. Au atom rows in the top layer are compressed by 4.5% to one of the [110] directions ([01h1] in Figure 5b) perpendicular to the herringbone stripes ([21h1h] direction) on the reconstructed Au(111) surface. The compression leads to the changes of Au atom stacking in the top layer: the ridges of the herringbone stripes are formed by the Au atoms stacked on the bridge sites, while both sides of the ridges are Au atoms stacked on the fcc and hcp hollow sites.42 Now that the direction of the alkyl chains is inclined 30° against the stripes, it is parallel to another Au atom rows in the [11h0] direction. The interval of the alkyl chains of 0.48 ( 0.04 nm agrees with that for a closely packed HTDY column on graphite (0001) (0.47 ( 0.02 nm)13,15-17 and that for an n-alkane column on Au(111) (0.48 nm)10,11 within the experimental errors. It should be also mentioned that the value of 0.48 nm is equal to the reduced distance between the next nearest neighbor Au atoms in the [112h] direction on the reconstructed surface.10,11 We obtained other images (not shown), in which domains with the molecular column direction parallel to the herringbone stripes were observed as well, but the proportion of these domains is very small. This fact can be attributed to the corrugation of the reconstructed Au(111) surface6 and the disagreement of the distance between the next nearest neighbor Au atom rows in the [21h1h] direction of 0.50 nm with the interval of the alkyl chains in a closely packed column (0.48 nm).10,11 A further magnified STM image (14 nm × 14 nm, Figure 6a) shows the cranked shapes of the flat-on molecules in the all-trans conformation with some “straighter” (or less cranked) molecules in a column indicated by the arrow. The structural model for the “straighter” molecule is displayed in Figure 6b.
Structures of 17,19-Hexatriacontadiyne Monolayers
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Figure 5. (a) Magnified STM image for an adsorbed HTDY monolayer on Au(111), observed in the air at room temperature; 30 nm × 30 nm, V ) - 0.50 V, I ) 65 pA. (b) Structural model for the adsorbed HTDY monolayer in Figure 5a on the reconstructed Au(111) surface. The alkyl chains are parallel to the [11h0] direction and packed into the column with a spacing of 0.48 ( 0.04 nm. The column axes are along the [112h] direction, making a 60° angle with the Au herringbone stripes running in the [21h1h] direction.
The deformation is not ascribed to the chemisorption of the molecules but attributed to the addition of foreign species (presumably hydrogens introduced during the sample preparation by the wet process): the hybridization of carbon atoms numbered 1 and 4 is changed from sp to sp2 by the additives. The interactions of C≡C bonds with metal electrons through π donation and π* back-donation, which are detected for acetylene on fcc metal surfaces such as Cu(111) and Pt(111),43-45 do not occur in both the cranked and “straighter” molecules on Au(111). This is because the Au(111) surface is more noble than the copper and platinum surfaces.46 Moreover, interactions between the triple bonds of HTDY and the surface electrons are sterically hindered by the flat-on alkyl chains as follows.
The carbon zigzag plane is held at a distance of 0.33-0.39 nm from the top-layer atoms of the Au surface upon the flat-on adsorption of the alkyl chains.47,48 On the other hand, it was demonstrated by a total energy calculation for acetylene on Cu(111) that the chemisorption potential minimum for the carbon atoms appears at ca. 0.13 nm from the surface.45 One can deduce that the value 0.33-0.39 nm for the diacetylene C-Au distance is out of the range required for the formation of C-Au bonds even if a difference in the metal radius between Au (0.144 nm) and Cu (0.128 nm) is taken into account. From these results, it is concluded that the aggregation of the HTDY molecules is governed mainly by the alkyl chains and the conjugated triple bonds play a minor role.
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Figure 6. (a) Further magnified STM image for an adsorbed HTDY monolayer on Au(111), observed under an UHV at room temperature; 14 nm × 14 nm, V ) +2.00 V, I ) 100 pA. The arrow indicates a column composed of “straighter” (or less cranked) molecules. The image exhibits higher resolution than that obtained in the air (Figure 4 or 5) partly because of the absence of the contaminating species on the monolayer. (b) Structural model for the “straighter” molecule in Figure 6a.
3.3. Structural Changes of an Adsorbed HTDY Monolayer Induced by UV Irradiation. Figure 7 shows changes in the IRA spectra of an adsorbed HTDY monolayer on Au(111) caused by UV irradiation. The C-Hdistal stretching band indicating the formation of an ordered monolayer of flat-on HTDY molecules is observed at 2910 cm-1 in the spectrum before irradiation (Figure 7a). During UV irradiation, the intensity of this band gradually decreases and the antisymmetric (2928 cm-1) and symmetric stretching vibrational bands (2856 cm-1) for gauche methylene sequences emerge (see Figure 7b and c: IRA spectra after 20 and 80 s of UV irradiation, respectively). The increase of the gauche population in the alkyl chains suggests that some structural disorder occurs in the monolayer by UV irradiation. On the other hand, the STM image of an adsorbed monolayer after 860 s of UV irradiation shows that the columnar structures partly remain (see Figure 8a). However, the sequential images in Figure 8b-d display that the columnar structures are easily destructed during imaging; this is not the case for the monolayer before irradiation.
Figure 7. Changes in the IRA spectra of an adsorbed HTDY monolayer on Au(111), caused by UV irradiation in the air at room temperature. (a) Before irradiation. (b) After 20 s of UV irradiation. (c) After 80 s of UV irradiation.
Therefore, it is suggested that the chemical species existing in the irradiated monolayer are different from the monomer molecules.
Structures of 17,19-Hexatriacontadiyne Monolayers
J. Phys. Chem. B, Vol. 110, No. 26, 2006 13105 stretching bands at 2909 and 2816 cm-1 show that the columns consist of flat-on molecules in the all-trans conformation. The column width and intermolecular spacing revealed by STM observations are similar to those for a HTDY monolayer on graphite (0001). Upon UV irradiation, the columns are converted into oligomers. An HTDY monolayer prepared by vapor deposition has methylene sequences in both the gauche and trans conformations and provides no column structure. References and Notes
Figure 8. Sequential STM images for an adsorbed monolayer on Au(111) after 860 s of UV irradiation in the air at room temperature; observed at room temperature under an UHV at about 5 min intervals; 40 nm × 40 nm, V ) +1.00 V, I ) 50 pA.
We consider that the irradiated monolayer consists of oligomeric species. Because interactions between the Au(111) surface and alkyl chains are rather stronger than those between the graphite (0001) surface and alkyl chains, the mobilities of HTDY molecules are lower on Au(111) than on graphite (0001), and the intermolecular reactions taking place on graphite (0001) to afford long ASs are somewhat restrained on Au(111). Taking into account the fact that the alkyl chain direction is almost perpendicular to the column direction in the monomer monolayer (as in Figure 1a) but makes an angle of about 70° to the PD chain direction in the AS (as in Figure 1b), we also explain that the oligomerization of HTDY molecules more or less pinned to the surface brings about conformational disorder in each alkyl chain, and some methylene sequences become detached from the surface. Therefore, the alkyl chain-substrate interactions are reduced after oligomerization to facilitate molecular diffusion on the Au(111) surface, which makes it easier to destruct the columnar patterns. This process may be promoted by the tip as judged by the sequential images in Figure 8a-d. The IRA spectra indicating the presence of a large number of gauche methylene sequences are in line with the disordered alkyl chains included in the destructed “columns”. In addition, the oligomers or short ASs can be related to red-phase PDs because our recent results of X-ray absorption spectroscopy49 show that the peak corresponding to the C 1s f π* transition detected for an adsorbed monolayer after UV irradiation is quite similar to that reported for the red-phase PDs in the LB films.50,51 Thus, ASs with various lengths and properties will be prepared by controlling the mobility of the monomer molecules in the columnar structure on a proper substrate. 4. Conclusion The minute structures of HTDY films prepared by a wet and a dry process were thoroughly investigated by IRAS and STM. It was revealed that a HTDY monolayer with an ordered column structure is obtained by rinsing a multilayered film formed by adsorption from the acetone solution onto a Au(111) surface. The IRA spectra with the decoupled C-Hdistal and C-Hproximal
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