Scanning Tunneling Microscopy Study of Solvent-Dependent

Nov 12, 2011 - Shern-Long Lee, Yi-Chen Chu, Hung-Jen Wu, and Chun-hsien Chen* ... In contrast, the formation of 1D or 2D C60 arrays with long-...
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Template-Assisted Assembly: Scanning Tunneling Microscopy Study of Solvent-Dependent Adlattices of Alkyl-Derivatized Tetrathiafulvalene Shern-Long Lee, Yi-Chen Chu, Hung-Jen Wu, and Chun-hsien Chen* Department of Chemistry, Center for Emerging Material and Advanced Devices, National Taiwan University, Taipei, Taiwan 10617

bS Supporting Information ABSTRACT: The self-assembly of an adsorbate as a function of the strength of solvent substrate adsorption is an important yet relatively unexplored subject. In this study, how the strength of solvent substrate adsorption and solvent solvent attraction affects the assembly of tetrakis(octadecylthio)tetrathiafulvalene (1) is scrutinized by scanning tunneling microscopy (STM). For solvents with strong intermolecular interactions and adsorption onto graphite, such as long n-alkanes (CnH2n+2, n g 13), STM reveals that the solvent molecules form lamellae which become a template to direct the assembly of 1 into one-dimensional arrays. The lengths of one of the unit cell vectors for the assemblies are increased and well correlated with the solvent sizes. In situ STM monitoring of 1 introduced onto graphite with preadsorbed n-tetradecane adlattices shows that the developed assemblies of 1 have striped features aligned parallel to the underlying template. In contrast, for solvents with weak adsorption, such as short n-alkanes (CnH2n+2, n e 12), toluene, and 1,2,4-trichlorobenzene, the adlattice structures of 1 are solvent-independent.

’ INTRODUCTION Since the emergence of nanotechnology in the 1980s, precise control of desired spatial arrangements of functional molecules has been a subject of extensive research interest.1 10 Among the methods reported for molecular patterning,1,4 6,11 15 self-assembly5,6 is considered a relatively effortless way to tailor molecular building blocks with nanometer-scale precision. The processes are sensitive to a delicate balance among intermolecular, intramolecular, and molecule substrate interactions,16,17 usually weak and reversible forces such as hydrogen-bonding,14,18 22 van der Waals,23 26 electrostatic,27 29 dipolar,30 35 and π π23,36 41 attraction. Through the interplay among these interactions, a wide variety of self-assembled systems have been developed. The control of molecular assembly involving two or multiple components has recently garnered much attention.42 50 Various phases of multicomponent architectures have been tailored by taking advantage of specific intermolecular interactions, such as metal ligand bonding1 or noncovalent attraction,5 to configure ordered patterns of target molecules which alone would otherwise exhibit ill-defined assemblies on substrates. However, multicomponent molecular systems usually result in phase separation or randomly mixed phases with the domain sizes on the nanometer scale. To pattern molecules on surfaces, an alternative is template-assisted assembly in which preadsorbed molecules serve as the template to direct the arrangement of subsequently deposited molecules in a controlled fashion.51 56 Molecules that easily form uniform and stable monolayers are likely suitable template molecules. The templates, stabilized via intermolecular r 2011 American Chemical Society

interactions, are typically developed into stripe51,52,57,58 and porous54,59 nanostructures which readily host subsequently deposited molecules that match the nanotemplates in shape, size, and symmetry.51 53 For example, the adlayer structures of fullerene (C60) on solid surfaces are dependent on the substrates and experimental conditions.60 64 It is in general very difficult to obtain high-resolution STM (scanning tunneling microscopy) images of C60 at room temperature, ascribed to weak C60 substrate interactions. In contrast, the formation of 1D or 2D C60 arrays with longrange order takes place with the assistance of adsorption sites or a nanoporous framework formed by template molecules, such as αsexithiophene,51,65 F-sexiphenyl,52 porphyrin,66 phthalocyanine,67 1,3,5-tris(10-carboxydecyloxy)benzene,54 and acridine-9-carboxylic acid.59 The adsorption positions of C60 are so stable that some examples of STM images can even differentiate the details of C60, such as facets of the aromatic rings to identify the adsorption orientations.51 Tetrathiafulvalene and its derivatives (TTFs) featuring strong electron-donating ability are important components in electronic devices.18,68 74 The performance of TTF devices (e.g., electron transport) strongly depends on the packing structures of TTFs between electrodes. Therefore, much effort has been contributed to unraveling factors that govern the packing behaviors of TTFs on surfaces as well as in the crystalline phase. In particular, the Received: August 12, 2011 Revised: November 11, 2011 Published: November 12, 2011 382

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Scheme 1. Structure of Tetrakis(octadecylthio)tetrathiafulvalene (1)

solvent effect is found to significantly affect the intermolecular interactions of TTFs. The use of specific solvents can lead to the controlled motifs of TTFs. For example, bicomponent assemblies comprising prototypical TTF and n-tetradecane molecules were recently investigated by Zhao and co-workers, who found periodic separated phases of alternate TTF and n-tetradecane on graphite.69,70 In view of reports demonstrating the tendency of n-tetradecane toward the development of lamellar structures on graphite,69,70 n-tetradecane molecules may thus play a dual role in which they compete with adsorbate molecules for the adsorption sites yet concomitantly serve as a template to host the adsorbate molecules. Therefore, it is important to examine how the competition of adsorption sites between solvent and adsorbate molecules leads to a hierarchical structure via template-assisted assembly.75 Along this line, solvent molecules with relatively strong and weak adsorption onto graphite are utilized to study the self-assembled features of an alkylated TTF (1, tetrakis(octadecylthio)tetrathiafulvalene; see Scheme 1) by STM. The former, such as long n-alkanes (CnH2n+2, n g 13), is expected to adsorb first and then become the template to facilitate the subsequent assemblies of 1 in which the lattice spacings may be well correlated with the lengths of the solvent molecules. For the latter, such as short n-alkanes (CnH2n+2, n e 12), toluene, and 1,2,4-trichlorobenzene, the adlattice structures of 1 are expected to be solvent-independent. The process of template-assisted assembly for 1 on preadsorbed n-tetradecane lamellae is demonstrated via in situ STM monitoring. Scanning tunneling spectroscopy measurements on the templated assemblies of 1 are also carried out.

Figure 1. STM images of (a, c) 5 μg/mL and (b, d) 50 μg/mL 1 selfassembled in 1-phenyloctane on graphite. (e) is a possible arrangement of 1, suggesting the intercalation of the alkyl chains. Unit cell parameters |a B|, and α: (a) 1.9 ( 0.2 nm, 3.1 ( 0.3 nm, 77° ( 3°; (b) 2.0 ( B|, |b 0.2 nm, 2.9 ( 0.2 nm, 78° ( 2°. Image sizes: (a, b) 13 nm  13 nm; (c, d) 45 nm  45 nm. Imaging conditions for Ebias and itunneling: (a, c) 0.52 V, 28 pA; (b, d) 0.80 V, 80 pA. (f) is the section profile corresponding to the green line shown in the lower left part of panel d.

’ EXPERIMENTAL SECTION Materials and Sample Preparation. All chemicals were ACS grade (TCI, Japan) and were used without further purification, including compound 1 and the solvents 1-phenyloctane, 1-phenyltetradecane, 1,2,4-trichlorobenzene, toluene, and normal alkanes (CnH2n+2, n = 10, 12 16). The samples were prepared by depositing, with a pipet, a 5 μL droplet of the sample solution onto HOPG (highly orientated pyrolytic graphite; ZYH grade, Advanced Ceramics Corp.). The sample concentrations of 1 ranged from 1.0 μg/mL to 1.0 mg/mL.

1-phenyloctane has been studied by STM in the literature, and the images are resolved down to the molecular level.18,74 In addition, 1-phenyloctane exhibits weak adsorption, which makes it a good starting point to explore the solvent effect on the adlayer structures of 1. In the following, the adlattices of 1 will be examined in 1-phenyloctane first to make a direct comparison of our findings with literature reports. Subsequently, the adlattices of 1 formed in solvents of 1-phenyltetradecane, normal alkanes (CnH2n+2, n = 10, 12 16), toluene, and 1,2,4-trichlorobenzene will be resolved and discussed in terms of their tendency to adsorb on graphite. Assemblies of 1 in 1-Phenyloctane. Displayed in Figure 1 are typical STM images of 1 assembled on graphite in 1-phenyloctane. Panels a and b are obtained from samples containing, respectively, 5 and 50 μg/mL 1, exemplifying relatively low and high concentration levels. The corresponding large-area images are displayed in panels c and d. The lengths of the unit cell vectors and the cell angles appear indistinguishable (see the caption of Figure 1 for details). However, the molecules at low concentrations are relatively mobile and result in ill-defined adlattices (e.g., the left-hand side of panel a and the lower right quadrant of panel c), whereas at high concentrations the molecules of 1 form

Scanning Tunneling Microscopy/Spectroscopy (STM/STS). STM experiments were carried out with a PicoScan controller (4500, Agilent Technologies, Chandler, AZ) at room temperature (ca. 25 °C). STM tips were commercially available Pt/Ir (PT, Nanotips, Veeco Metrology Group/Digital Instruments). The images were obtained in constantcurrent mode under ambient conditions. The bias voltage and tunneling current were typically 0.10 1.50 V and 10 500 pA, respectively. Note that the imaging features were not influenced by the experimental conditions of the bias voltage (∼0.30 1.00 V) and tunneling current. STS measurements were acquired with the feedback loop deactivated, and each of the presented traces was averaged from more than 10 samples.

’ RESULTS AND DISCUSSION The goal of this study is to manifest the role that solvent molecules can play in the development of solute adlayers, specifically template-assisted assembly. The structure of 1 in 383

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of 1-phenyltetradecane adsorption at high concentration levels of 1 is insignificant because the molecules of 1 appear to be in contact with HOPG. At concentrations around 100 μg/mL, boundaries of the striped and close-packed domains can be observed (Figure 2g). To focus on the solvent-assisted assemblies of 1, in the following sections we examine samples prepared from diluted 1. As presented in Figure 2a, the striped features develop only at relatively low concentration levels (e5 μg/mL). The domain size of the striped features usually extends over several hundred nanometers (see Figure S1 in the Supporting Information). To correlate the stripes with the solvent molecules, a typical image of pure 1-phenyltetradecane dropcast on HOPG is displayed in Figure 2e. The molecules of 1-phenyltetradecane adopt a structure of lamellae with an averaged width, L (∼2.6 nm), approximately half the length of the lattice vector |b B| (5.2 ( 0.3 nm), indicative of a commensurate adlattice of 1 over the 1-phenyltetradecane underlayer. In addition, the section profile across the stripes (Figure 2f) manifests the presence of likely two layers of 1, consistent with those observed at high concentrations of 1 in 1-phenyloctane and 1-phenyltetradecane. The tendency toward the formation of two or multiple layers of 1 indicates significant intermolecular attractions between TTFs.74 Assemblies of 1 in Strongly and Weakly Adsorbed Solvents. In this section, long-chain n-alkanes (CnH2n+2, n g 13) are found exhibiting lamellar assemblies on HOPG and thus are grouped into the category of strongly adsorbed solvents. Their images of striped bilayers of 1 are presented in panels a d of Figure 3. The weak ones examined herein are shown in panels e h, including relatively short n-alkanes (CnH2n+2, n = 10, 12), toluene, and 1,2,4-trichlorobenzene. All the sample concentrations in Figure 3 are 1 μg/mL because at high concentrations the assemblies of 1 are solvent-independent and have the same close-packed structure as those discussed previously in Figures 1b and 2b. The unit cell parameters, |a B|, and α, are listed in the caption of B|, |b Figure 3. It is worth noting that the lattice vector |b B| in panels a d increases from 4.0 ( 0.2 nm for n-tridecane to 4.7 ( 0.2 nm for n-hexadecane, slightly larger than 2 times the length of the solvents used. Vector |a B| = (1.3 1.4) ( 0.3 nm in the long alkanes (CnH2n+2, n = 13 16) is slightly shorter than that of 1.8 ( 0.2 nm found in phenyltetradecane (Figure 2). Although the difference could be due to the benzene moiety of phenyltetradecane, the measured lengths might be the same after consideration of the margin of error. For panels e h of Figure 3, the similarity in the close-packed structures suggests again the solvent-independent assembly in weakly adsorbed solvents. The comparison between assemblies of panels a (striped) and f (close-packed), formed respectively from n-tridecane and n-dodecane, makes it interesting to note that the dramatically different adlattices originated from the difference of only one methylene unit of the two solvents. The results are well correlated with literature reports that n-tridecane forms lamellae on graphite, whereas n-dodecane adsorbs in a disorderly fashion.76 Thus, the tendency of solvent adsorption or, specifically, whether the solvent is capable of forming lamellae on graphite is an important factor leading to the template-assisted assemblies of 1. In addition, it appears there are fewer defects of TTF adlattices in hexadecane than in tridecane, tetradecane, and pentadecane. The width L of the solvent lamellae to accommodate the van der Waals interactions with octadecyl side chains of 1 might be responsible for the discrepancy. Moreover, for the weakly adsorbed solvents (panels e h of Figure 2), assemblies of 1 in chlorobenzene and toluene show

Figure 2. Assembly of 1 in 1-phenyltetradecane on HOPG. STM images of samples prepared from (a, c) 5 μg/mL 1, (b, d) 1.0 mg/mL 1, (e) pure 1-phenyltetradecane, and (g) 100 μg/mL 1. (f) Cutaway view of the green line shown in panel a. Image sizes: (a, b, g) 42 nm  42 nm; (c) 9 nm  15 nm; (d) 15 nm  15 nm; (e) 5 nm  15 nm. Imaging conditions for Ebias and itunneling: (a, c) 1.00 V, 28 pA; (b, d, g) 0.98 V, 80 pA; (e) 0.10 V, 200 pA. Unit cell parameters |a B|, and α: (a) B|, |b 1.8 ( 0.2 nm, 5.2 ( 0.3 nm, 80° ( 3°; (b) 2.2 ( 0.3 nm, 2.9 ( 0.2 nm, 77° ( 2°. L (nominal lamellar width, panel e) = 2.6 ( 0.2 nm.

stable motifs on HOPG. Intriguingly, the image contrast for panels a and b of Figure 1 are quite different. Hence, presented in Figure 1d is an image exhibiting a few defects that disclose two types of brightness in image contrast. Section profiles of the images are made along the directions of the lattice vector |b B|. The difference in height is approximately 0.25 nm (Figure 1f). Thus, the relatively brighter and darker spots in Figure 1d are ascribed, respectively, to the second and first adlayers of 1. The results are consistent with the literature report71,74 that the adsorption of 1 is weak at the level of monolayers or submonolayers, but bilayers of 1 are relatively stable. Assemblies of 1 in 1-Phenyltetradecane. Panels a and b of Figure 2 show the concentration-dependent assemblies of 1 respectively prepared from diluted (5 μg/mL) and high (1.0 mg/mL) solute concentrations in 1-phenyltetradecane. The former exhibits previously unreported striped features of 1, distinctly different from the close-packed structure of the latter. Panels c and d are the corresponding high-resolution images for panels a and b. After careful assessment of the lattice parameters and section profiles for Figure 2b, we conclude that the closepacked structure is the same as that observed in weakly adsorbed 1-phenyloctane (Figure 1b). The results indicate that the effect 384

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Figure 3. Assemblies of 1 μg/mL 1 in strongly and weakly adsorbed solvents. Experiments were conducted in (a) n-tridecane, (b) n-tetradecane, (c) n-pentadecane, (d) n-hexadecane, (e) n-decane, (f) n-dodecane, (g) toluene, and (h) 1,2,4-trichlorobenzene. Image size: 30 nm  30 nm. Imaging conditions for Ebias and itunneling: (a) 0.60 V, 100 pA; (b) 1.00 V, 200 pA; (c) 0.50 V, 100 pA; (d) 1.00 V, 58 pA; (e) 0.70 V, 80 pA; (f) 0.70 V, 62 pA; (g) 0.60 V, 53 pA; (h) 0.68 V, 53 pA. Unit cell parameters |a B|, and α: (a) 1.4 ( 0.2 nm, 4.0 ( 0.2 nm, 84° ( 3°; (b) 1.4 ( 0.3 nm, 4.3 ( 0.2 nm, 84° ( 2°; B|, |b (c) 1.3 ( 0.2 nm, 4.5 ( 0.3 nm, 80° ( 3°; (d) 1.3 ( 0.2 nm, 4.7 ( 0.2 nm, 83° ( 3°; (e) 1.5 ( 0.2 nm, 2.6 ( 0.2 nm, 80° ( 2°; (f) 1.6 ( 0.2 nm, 2.6 ( 0.3 nm, 87° ( 3°; (g) 1.6 ( 0.2 nm, 2.6 ( 0.3 nm, 86° ( 3°; (h) 1.6 ( 0.2 nm, 2.5 ( 0.3 nm, 86° ( 3°. For panels a d, the lengths of |Ba| are well correlated with the dimensions of the alkanes. For panels e h, the unit cells are solvent-independent.

chains of 1 which align nearly perpendicular to the lattice vector |a B|. To further explore the structural correlation between 1 and the underlying hexadecane template, a sudden drop of Ebias from 0.7 to 0.1 V was applied. Note that the condition was extreme and the tip likely penetrated into the film. The direction of hexadecane appeared parallel to the alkyl chains of 1 (Figure S3, Supporting Information). Accordingly, a model is proposed and superimposed on the image. In Situ Monitoring of the Growth of 1 on Preadsorbed nTetradecane Lamellae. To further confirm that the formation of the one-dimensional arrays of 1 is assisted by the underlying template molecules, in situ monitoring of the growth of the striped assemblies on preadsorbed n-tetradecane lamellae was performed at the liquid solid interface. Figure 5a is a typical STM image of an n-tetradecane monolayer, obtained ca. 1 h after introduction of a 5 μL aliquot of n-tetradecane on graphite. To achieve real-time monitoring, upon applying a drop of 1-phenyloctane containing 1 μg/mL 1, imaging was performed immediately and panels b e of Figure 5 were acquired consecutively. The striped structures of 1 formed rapidly, indicative of fairly strong interactions of 1 with the underlying n-tetradecane template. In addition, two features in the images are noticed. First, the stripe direction of the n-tetradecane monolayer and the assembly of 1 are the same (cf. panels a and b), implying that 1 grows along the direction of the n-tetradecane lamellae. Second, the spacing between neighboring stripes in panels b e of Figure 5 corresponds to approximately 2 times the width of the n-tetradecane lamellae, consistent with the finding reported in panels a d of Figure 3. The simple integer relationship indicates that 1 adopts preferential adsorption sites on the n-tetradecane template and forms a commensurate adlattice. It is also worth noting that the formation of the striped structures

Figure 4. High-resolution STM image and proposed model of 1 in n-hexadecane. Solvent molecules, hexadecane, are illustrated in black. For clarity, the second layers of 1 were not drawn. Image size: 10 nm  10 nm. Imaging conditions: Ebias = 0.70 V; itunneling = 189 pA.

better uniformity and fewer defects than in decane and dodecane. This might be attributed to the solubility of alkylated 1 in that a lower dissolving power of the solvent leads to a stronger tendency for 1 residing at the liquid solid interface. To propose a possible packing structure of the assemblies of 1 at the graphite/n-hexadecane interface, presented in Figure 4 is a high-quality STM image that emphasizes the contrast of the alkyl 385

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Figure 5. In situ monitoring of the assembly formation of 1. (a) STM image of preadsorbed n-tetradecane on graphite and (b e) four consecutive STM images captured immediately after introduction of 1 onto the sample. Image size: 100 nm  100 nm. Image conditions: acquisition rate, 120 s/frame; Ebias = 0.70 V; itunneling = 55 pA. The arrows in panel a indicate the 3-fold symmetry of the HOPG substrate. The blurred images of panels b e are due to the relatively fast scan rate.

facile tunneling through the HOMO of TTFs than through the LUMO.8,41,72,77 82

Figure 6. (a) STM image and (b) STS traces of the assemblies of 1 in n-tetradecane. Notations indicate where STS traces were taken: A, aromatic moiety of 1, red trace; B, alkyl chains of 1, green trace; C, n-tetradecane, black trace. Image size: 14 nm  18 nm. Imaging conditions for Ebias and itunneling: 1.00 V, 200 pA. The I V traces were averaged from more than 10 samples. STS conditions: Ebias = 0.85 V; itunneling = 15 pA.

of 1 has been observed at the STM tip bias of 0.7 0.4 V (see Figure S2 in the Supporting Information). Although the possibility of tip-induced assemblies cannot be ruled out, the control experiments indicate that Ebias is not a crucial factor.37 Tunneling Spectroscopy of the Assemblies of 1 on nTetradecane. In the previous sections, bilayers of 1 on an n-alkane template have been elucidated by STM images. The electronic properties of alkylated TTF have been examined in literature studies by STS for isolated and close-packed bilayers.71 STS spectra for one-dimensionally arranged TTFs have not been explored. To characterize the electronic properties of the bilayers, a surface area, such as in Figure 6a, consisting of bilayers of 1, its alkyl chains, and the underlying template was found for the subsequent STS measurements. It has been reported by the group of De Schryver and De Feyter that, after being subjected to STS experiments, molecules of isolated 1 or its monolayer might detach from a graphite substrate.71,74 In the present study, the templated assemblies of 1 are more stable than its monolayers or submonolayers such that the STS measurements on 1 are reasonably reproducible (Figure S4, Supporting Information). Figure 6b shows STS traces acquired at locations A (aromatic moieties of 1), B (alkyl chains of 1), and C (the underlying template) of Figure 6a. The averaged I V trace obtained at A (red curve) exhibits a pronounced asymmetry, whereas the asymmetry of the averaged I V traces for B and C is insignificant (green and black curves). The remarkably asymmetric I V traces for aromatic TTF moieties have been attributed to a more

’ CONCLUSION In conclusion, we have demonstrated that solvents adsorbed strongly on a substrate can serve as templates to engineer nanostructures and patterns for molecules of interest. For solvents exhibiting lamellae on graphite, such as 1-phenyltetradecane and long-chain n-alkanes, the assembly of 1 is developed into onedimensional arrays. The length of one of the unit cell vectors is well correlated with the solvent size. In situ STM imaging upon the assembly process unveils that the direction of the striped feature is parallel to that of the preadsorbed template. In weakly adsorbed solvents and for samples containing high concentrations of 1, the adlattice structures are close-packed and solventindependent. The templated assemblies of 1 are quite stable so that the STS measurements on 1 are reproducible. The discovery of the competitive adsorption between the solvent and adsorbate molecules opens a new strategy of template-assisted assembly for the patterning of functional molecules. ’ ASSOCIATED CONTENT

bS

Supporting Information. STM images of 1 under high and diluted concentrations (5 μg/mL and 1.0 mg/mL, Figure S1) and under Ebias = 0.4 0.7 V (Figure S2), proposed packing structure (Figure S3), and STS spectra (Figure S4). This material is available free of charge via the Internet at http://pubs.acs.org/.

’ AUTHOR INFORMATION Corresponding Author

*Phone: +886 2 3366 4191. Fax: +886 2 2363 6359. E-mail: [email protected].

’ ACKNOWLEDGMENT We thank the National Science Council (Taiwan) and National Taiwan University (Center for Emerging Material and Advanced Devices) for financial support. ’ REFERENCES (1) Barth, J. V. Annu. Rev. Phys. Chem. 2007, 58, 375. (2) M€ullen, K.; Rabe, J. P. Acc. Chem. Res. 2008, 41, 511. (3) Ulgut, B.; Abru~ na, H. D. Chem. Rev. 2008, 108, 2721. 386

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