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Site-Selective Adsorption of Benzoic Acid Using an Assembly of Tridodecylamine as the Molecular Template Sheng-Bin Lei, Chen Wang,* Xiao-Lin Fan, Li-Jun Wan,* and Chun-Li Bai Key Laboratory of Molecular Nanostructure and Nanotechnology, Center of Molecular Sciences, Institute of Chemistry, The Chinese Academy of Sciences, Beijing 100080, People’s Republic of China Received June 8, 2003. In Final Form: September 12, 2003
A well-ordered assembly of tridodecylamine (TDA) molecules at the liquid-solid interface has been studied using scanning tunneling microscopy (STM). A significant effect of the applied tunneling bias voltage on the contrast of the amino group was observed in the STM images. In addition, benzoic acid molecules were found exclusively adsorbed onto the TDA assembly at the sites of the amino groups. The observation reveals the feasibility of template-directed adsorption of functional organic molecules with the aid of molecular assemblies.
Introduction The adsorption and assembling of organic molecules at surfaces and interfaces have attracted extensive interest for many years. The investigations on this subject could benefit our understanding of the microscopic behavior of adsorption processes and adsorbate structures.1-5 Since the emergence in the 1980s, scanning tunneling microscopy (STM) has become a powerful tool to study the surface adsorption and assembly of molecules. STM images of the adsorbed overlayers, obtained at a submolecular resolution, could provide not only spatial information on the electronic coupling matrix elements that are associated with the heterogeneous and homogeneous electrontransfer processes but also information such as the binding sites of individual molecules with respect to the substrate lattices and conformational states of individual molecules, as well as defects and domains that are prevalent in the ordered molecular structures.1-6 Although STM has been used to investigate a rapidly growing list of organic materials at surfaces and interfaces, the imaging mechanisms of insulating organic material are still being pursued.1,6 It is interesting, both theoretically and experimentally, to understand the factors that control the spatial image contrast in the molecularly resolved STM image. In another research front, controlled formation of twodimensional molecular patterns is an important topic in self-assembly and molecular electronics.7 To achieve the ordering of molecules in two-dimensional monolayers, * To whom correspondence should be addressed. E-mail:
[email protected] (C.W.);
[email protected] (L-J.W.). Fax: 86 10 6255 7908. (1) Rabe, J. P.; Buchholz, S. Science 1991, 253, 424. (2) Claypool, C. L.; Faglioni, F.; Goddard, W. A., III; Gray, H. B.; Lewis, N. S.; Marcus, R. A. J. Phys. Chem. B 1997, 101, 5978. (3) Stawasz, M. E.; Sampson, D. L.; Parkinson, B. A. Langmuir 2000, 16, 2327. (4) Baker, R. T.; Mougous, J. D.; Brackley, A.; Patrick, D. L. Langmuir 1999, 15, 4884. (5) Cyr, D. M.; Venkataraman, B.; Flynn, G. W.; Black, A.; Whitesides, G. M. J. Phys. Chem. 1996, 100, 13747. (6) Cyr, D. M.; Venkataraman, B.; Flynn, G. W. Chem. Mater. 1996, 8, 1600. (7) Gomar-Nadal, E.; Abdel-Mottaleb, M. M. S.; De Feyter, S.; Veciana, J.; Rovira, C.; Amabilino, D. B.; De Schryver, F. C. Chem. Commun. 2003, 906.
noncovalent interactions, such as hydrogen bonding, are of great importance.8-10 It has been suggested that controlled heterogeneous two-dimensional patterns could be formed via the coadsorption of two or more molecular species.11-13 The formation of such heterogeneous assemblies is directly associated with the intermolecular interactions. It is well-known that alkane derivatives physisorb on highly ordered pyrolytic graphite (HOPG) as well-ordered two-dimensional lamella structure.1,5 Such assemblies could be considered as a venue to introduce various functional groups in the two-dimensional lamella structures. Considering the variety of polarities and chemical activities of the functional groups of alkane derivatives, the assembled lamella structures could be exploited as possibly templates to direct the assembling and ordering of other organic and inorganic materials.14-15 In this report, the assembly and contrast variation of the amino group of tridodecylamine (TDA) as well as the TDA template-induced assembling of benzoic acid were studied on the surface of HOPG. The contrast variation of amino group under different biases was obtained and is attributed to the induced dipole moment variation perpendicular to surface under the influence of the applied bias. Experimental Section TDA and benzoic acid were purchased from Acros Co. and used as received. The assembly of TDA was prepared by directly depositing a drop of TDA onto the graphite surface. The adsorption structure of benzoic acid on the TDA template was achieved by depositing a drop of TDA containing 1:1 (molar ratio) (8) Gottarelli, G.; Masiero, S.; Mezzina, E.; Pieraccini, S.; Rabe, J. P.; Samori, P.; Spada, G. P. Chem. Eur. J. 2000, 6, 3242. (9) Lei, S. B.; Wang, C.; Yin, S. X.; Wang, H. N.; Xi, F.; Liu, H. W.; Xu, B.; Wan, L. J.; Bai, C. L. J. Phys. Chem. B 2001, 105, 10841. (10) De Feyter, S.; Gesquiere, A.; Abdel-Mottaleb, M. M.; Grim, P. C. M.; De Schryver, F. C.; Meiners, C.; Sieffert, M.; Valiyaveettil, S.; Mullen, K. Acc. Chem. Res. 2000, 33, 520. (11) Lei, S. B.; Wang, C.; Yin, S. X.; Bai, C. L. J. Phys. Chem. B, 2001, 105, 12272. (12) Lei, S. B.; Yin, S. X.; Wang, C.; Wan, L. J.; Bai, C. L. Chem. Mater. 2002, 14, 2837. (13) Hipps, K. W.; Scudiero, L.; Barlow, D. E.; Cooke, M. P. J. Am. Chem. Soc. 2002, 124, 2126. (14) Hoeppener, S.; Wonnemann, J.; Chi, L. F.; Erker, G.; Fuchs, H. ChemPhysChem 2003, 4, 490. (15) Hoeppener, S.; Chi, L. F.; Wonnemann, J.; Erker, G.; Fuchs, H. Surf. Sci. 2001, 487, 9.
10.1021/la035003e CCC: $25.00 © 2003 American Chemical Society Published on Web 10/14/2003
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Figure 1. Large-scale view of the adsorption structure of TDA molecules. The 120° interdomain rotation angle indicates the registry of alkane chains with respect to the graphite lattice. benzoic acid on the bare surface of graphite or depositing a drop of TDA containing 1:1 (molar ratio) benzoic acid on a graphite surface already covered with a TDA monolayer. No difference has been observed for these two cases. STM experiments were carried out on the liquid/graphite interface, by immersing the tip in the solvent liquid under room temperature, with a Nanoscope IIIA SPM system (Digital Instruments, Santa Barbara, CA). The tip used was mechanically formed Pt/Ir wires (90:10). The bias-dependent imaging of the amino group was performed using different tips (more than five tips), and the same results were obtained. The contrast of the amino group under each bias was measured from images obtained with different tips and different sites of each image.
Results and Discussion The conformation of nitrogen atom in the amino group is tetrahedral, in which a nitrogen atom sits on one acme of this tetrahedron. Figure 1 shows a STM image obtained on an assembled monolayer of TDA on the graphite surface. Well-ordered lamella structures can be identified on the surface. Several domains can also be seen in this image. The 120° relative rotation angle between adjacent domains is indicative of a registry effect of the alkane chains with the graphite lattice. A high-resolution STM image in Figure 2b reveals that TDA molecules adsorb with their alkyl chains parallel to the basal plane of graphite with a 90° angle to the lamella axis. To obtain a close-packing structure on the surface, the symmetry of the TDA molecules is observed to be reduced from threefold to a linear conformation, in which one carbon chain is positioned on one side of the lamella axis while the other two chains are on the opposite side (Figure 2b). Due to the conformational restriction, the TDA molecule does not adsorb with its carbon backbone completely parallel or perpendicular with respect to the graphite surface but bears an intermediate conformation in the assembly, as indicated in the model in Figure 2b. The electric-field effect on the contrast of the amine groups was studied by sequentially changing the bias from negative to positive as well as from positive to negative during the imaging process. It was observed that the amino group appears with a higher contrast than the alkane part at the positive tip bias and darker contrast than the alkane parts at a negative bias (Figure 3a,b). The contrast variation is also revealed in the accompanying cross-
Figure 2. (a) High-resolution STM image of the TDA lamella structure and (b) proposed packing model of the lamella structure.
sectional profiles in Figure 3c,d. The measured height of the amino group relative to the carbon backbone versus the bias voltage is given in Figure 4. The contrast of the amino group under each bias is measured from different images and different sites of each image; each data point here is an average of more than 20 measurements. As mentioned previously, the nitrogen atom in the amino group is on one acme of the tetrahedron, and because the C-N bond is dipolar, the amino group will have a net dipole moment in which nitrogen is partially negatively charged. Thus, when TDA molecules adsorb onto an inert surface of graphite, the dipole moment of the amino group should be directed nearly perpendicular to the surface. In addition, the electric field underneath the tip would also interact with the amino group, as will be discussed in the following. As a result, the contrast of nitrogen could be enhanced or weakened depending on the polarity of the applied bias. As one of the mechanisms proposed for the STM observation of organic molecules, the contrast of functional groups could be associated with the modification of the local work function by the molecular dipole moment.16 (16) Spong, J. K.; Mizes, H. A.; LaComb, L. J.; Dovek, M. M., Jr.; Frommer, J. E.; Foster, J. S. Nature 1989, 338, 137.
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Figure 3. Effect of applied bias polarity on the contrast of amino groups: (a) +459 mV, 626 pA and (b) -405 mV, 626 pA. Parts c and d are the corresponding cross-sectional profiles of the above STM images.
the tip and the sample. Therefore, it can be seen that the induced dipole moment is dependent on the direction of the external field. The work function for different bias polarities could be expressed in a simplified form:
φ+ ) φ0 - eµ0/0 + R(Edp + Eext)/0
(3)
φ- ) φ0 - eµ0/0 + R(Edp - Eext)/0
(4)
The difference in the work function will be reflected in the tunneling current I:
I ∝ V exp(-Aφ1/2d) (d is the separation between the tip and the sample) (5) Figure 4. Dependence of the contrast of the amino group on the tip bias voltage. All the images were acquired under the same tunneling current of 626 pA with different tip biases.
The local work function with an adsorbed molecule of dipole moment µ could be expressed as follows:16
φ ) φ0 - eµ/0
(1)
where φ0 is the work function of the bare substrate. The dipole moment of the molecule is
B dp + E B ext) b µ)b µ 0 - R(E
(2)
Here, µ0 is the permanent dipole moment, R is the polarizability of the molecule, and e is the electron charge. E B dp is the depolarization field associated with the adjacent B ext is the external field between dipole molecules17 and E (17) Weber, R. E.; Peria, W. T. Surf. Sci. 1969, 14, 13.
According to eqs 3 and 4, the reversal of the direction of the external field will have an effect opposite to that of the local work function and, subsequently, the tunneling current at the functional group. As a result, the contrast of the functional group will be enhanced or reduced in comparison to the level where little external field effect is involved (in this case the alkanes). It is plausible that the difference in the work function is more pronounced when the molecular dipole is aligned with the field direction (such as in the TDA system) than perpendicular to the field direction (in the case of flatlying molecules on the surface). As the result, the reversal of the bias polarity could lead to an appreciable change in the apparent contrast of the functional group with the dipole moment perpendicular to the surface, as in this study. The bias dependence of the image contrast of the amino group with a “flat” conformation, where the molecular dipole is aligned perpendicular to the field
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Figure 6. STM images showing the effect of the STM tip scanning on the adsorbed benzoic acid molecules. (a) Acquired immediately as the tip engaged. (b) Acquired after 7 min of scanning. It can be seen that most of the benzoic acid was removed from the TDA template. Tunneling conditions 1.04 V, 14.6 pA. Figure 5. (a) STM image of the adsorption structure of benzoic acid adsorbed on the lamellae of TDA; tunneling conditions 1.04 V, 14.6 pA. (b) Cross-sectional profile corresponding to the site marked in part a. (c) The schematic models illustrating the side and top views of the binding of a benzoic acid molecule to the TDA template.
direction, has been systematically studied previously.5,18 The amino group of amine derivatives showed a relatively high contrast under both positive and negative bias with respect to alkane parts, and no contrast reversal in contrast has been observed when the bias polarity is changed. This result is considered consistent with the current work where TDA molecules adsorb with the molecular dipole parallel with the external field direction. It should be acknowledged that a number of studies on the contrast mechanisms of molecules have suggested that the molecular front orbitals (highest occupied molecular orbitals and lowest unoccupied molecular orbitals) are the main origin for a number of systems, such as planar molecules of phthalocyanines, porphyrins, and a number of liquid crystal molecules.19,20 The above discussions could (18) Giancarlo, L.; Cyr, D.; Muyskens, K.; Flynn, G. W. Langmuir 1998, 14, 1465.
provide complementary evidence to the effect of the molecular dipole moment on the observed contrast. It is conceived that a rigorous interpretation of the STM images should take into account all the contributing factors associated with the electronic nature of the functional groups. Because the nitrogen atom bears a pair of unbonded electrons, it can serve as an electron donor to form the hydrogen bond. Therefore, it is possible to bind other small organic molecules on this site through hydrogen bonding. On the basis of this assumption, we carried out the experiments on the selective bonding of benzoic acid on the monolayers of TDA. During the experiment, two ways of benzoic acid deposition were tested. In the first approach, a drop of TDA containing 1:1 (molar ratio) benzoic acid was deposited on the freshly cleaved graphite surface, and in the second approach, a drop of TDA containing 1:1 (molar ratio) benzoic acid was deposited on a graphite surface already covered with a TDA monolayer. No difference has been observed for these two preparation (19) Wiesendanger, R., Gu¨ntherodt, H.-J., Eds.; Scanning Tunneling Microscopy III; Springer-Verlag: Berlin, 1993. (20) Sautet, P. Chem. Rev. 1997, 97, 1097.
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procedures. The results presented here were obtained using the first method. Figure 5 shows the assembling structure obtained from the mixed solution of benzoic acid and TDA on the graphite surface. It could be seen that there are some large bright spots in the center of the TDA lamellae. The diameter of these bright spots is 0.60 ( 0.05 nm, in agreement with the dimension of single benzene rings. The measured height is around 0.04-0.06 nm. Compared with the observed lamellae in the uncovered area, these bright spots could be attributed to benzoic acid molecules over the sites of the amino groups in the TDA lamellae, possibly through the O-H-N hydrogen bond. It is well-known that the STM contrast does not reflect the real height of adsorbed atoms or molecules, and compared with the much smaller contrast of the alkane and amine groups (
