Complexity in the Self-Assembly of Bifunctional Molecules on HOPG

Mar 1, 2007 - Anna Llanes-Pallas , Carlos-Andres Palma , Luc Piot , Abdelhalim Belbakra , Andrea Listorti , Maurizio Prato , Paolo Samorì , Nicola Ar...
0 downloads 0 Views 2MB Size
Download PDF
Langmuir 2007, 23, 3513-3522

3513

Articles Complexity in the Self-Assembly of Bifunctional Molecules on HOPG: The Influence of Solvent Functionality on Self-Assembled Structures Feng Tao and Steven L. Bernasek* Department of Chemistry, Princeton UniVersity, Princeton, New Jersey 08544-1009 ReceiVed May 14, 2006. In Final Form: December 18, 2006 Self-assembled monolayers of bifunctional molecules HOOC(CH2)nCOOH (n ) 20, 18, 16, 14, 12, 10), HOOC(CH2)nCH2OH (n ) 13, 14), and HOCH2(CH2)14CH2OH dissolved in octanoic acid were investigated using scanning tunneling microscopy, to understand the self-assembly of bifunctional molecules and the influence of a carboxylic acid solvent on the formation of self-assembled structures on HOPG. In the series of di-acids (HOOC(CH2)nCOOH), only HOOC(CH2)20COOH forms stable coadsorption structures with the solvent octanoic acid. The remaining di-acids form stable single-component monolayers and do not coadsorb with solvent octanoic acid. Coadsorption structures involving mixtures of di-acids were observed. This result suggests that coadsorption with acid solvent or with other di-acids occurs to maximize hydrogen-bond density in the overlayer. A quantitative model based on this concept is proposed. For hetero-bifunctional molecules HOOC(CH2)nCH2OH (n ) 13, 14), the coadsorption of HOOC(CH2)14CH2OH and octanoic acid at the molecular level produces a microscopic mesh made of homogeneously arranged openings with a dimension of ∼12.5 Å × ∼5.0 Å × ∼1.8 Å. For the hetero-bifunctional molecule HOOC(CH2)13CH2OH, hydroxyl groups of two adjacent lamellae assemble to form a herringbone geometry, and the two carboxylic acid groups assemble with a straight head-to-head configuration. In addition, a new mixed hydrogen-bonding network of COOH‚‚‚O-H was observed in another self-assembled structure of this molecule. The bifunctional molecule HOCH2(CH2)14CH2OH exhibits multiple packing patterns on HOPG via different hydrogen-bonding networks. HOCH2(CH2)14CH2OH self-assembles using the H-O‚‚‚O-H network typical of the n-alcohol herringbone structure, forming an asymmetric adsorbate on HOPG. It also forms domains with another hydrogen-bonding network, in which molecules in adjacent lamellae are parallel to each other. This investigation demonstrates the complexity and diversity of self-assembled structures formed from bifunctional molecules on solid surfaces. It also indicates that a solvent with the same functional group as the solute can significantly impact the formation of the self-assembled structures of these bifunctional molecules.

Introduction The growth of monolayer organic thin films on solid surfaces has attracted considerable interest due to the widespread applications of these films in various technological areas, including microelectronics, organic light-emitting devices, corrosion inhibitor layers, and chirally selective heterogeneous catalysis.1-3 The adsorption of organic molecules on graphite provides an excellent model system to study the growth mechanisms of organic thin films and their self-assembled structures on crystalline solid surfaces. These adsorption systems have been studied since the early 1970s.4-6 In these studies, the molecular arrangement of the adlayer could only be deduced from microcalorimetry, adsorption isotherms, and adsorption isobar measurements since no visualization techniques with molecular resolution were available. In the early 1980s, scanning tunneling microscopy (STM)7 offered a direct observation of * Corresponding author. E-mail: [email protected]. (1) Frank, C. W. Organic Thin Films: Structure and Application; American Chemical Society: Washington, DC, 1998. (2) Tickle, A. C. Thin-Film Transistor: A New Approach to Micrelectronics; Wiley: New York, 1999. (3) Elshabini-Riad, A. A. A.; Barlow, F. D. Thin-Film Technology Handbook; McGraw-Hill: New York, 1998. (4) Groszek, A. J. Proc. R. Soc. London, Ser. A 1970, 314, 473. (5) Findenegg, G. H. J. Chem. Soc., Faraday Trans. 1 1972, 68, 1799. (6) Findenegg, G. H. J. Chem. Soc., Faraday Trans. 1 1973, 69, 1069. (7) Binnig, G.; Rohrer, H.; Gerber, C.; Weibel, E. Phys. ReV. Lett. 1982, 49, 571.

adsorbed molecules in self-assembled monolayers at the atomic scale, significantly facilitating the study of self-assembled organic layers on solid surfaces. Recent systematic investigations in this field have mainly focused on the self-assembled monolayers of single-functional molecules and the self-assembled systems of a single component.8-10 It is well-documented that both molecule-substrate and molecule-molecule interactions impact the formation of self-assembled monolayers and that the competition and balance between these interactions governs the molecular arrangement in these monolayers. The main difference from molecule to molecule is the molecule-molecule interaction, which includes four weak interactions (i.e., van der Waals forces existent for every molecular system, the possibility of hydrogen-bonding networks, electrosatic interactions between molecules, and dipolar interactions). The hydrogen-bonding network plays a major role in determining a self-assembled structure for n-carboxylic acids and n-alcohols. For example, in the self-assembled monolayer of n-carboxylic acid, the formation of a straight head-to-head (8) (a) Cyr, D. M.; Venkataraman, B.; Flynn, G. W. Chem. Mater. 1996, 8, 1600. (b) Giancarlo, L. C.; Flynn, G. W. Annu. ReV. Phys. Chem. 1998, 49, 297. (9) (a) De Feyter, S.; Gesquiere, A.; Abdel-Mottaleb, M. M.; Grim, P. C. M.; De Schryver, F. C.; Meiners, C.; Sieffert, M.; Valiyaveetttil, S.; Mullen, K. Acc. Chem. Res. 2000, 33, 520. (b) De Feyter, S.; De Schryver, F. C. Chem. Soc. ReV. 2003, 32, 139. (10) (a) Plass, K. E.; Kim, K.; Matzger, A. J. J. Am. Chem. Soc. 2004, 126, 9042. (b) Kim, K.; Plass, K. E.; Matzger, A. J. J. Am. Chem. Soc. 2003, 19, 7149.

10.1021/la0613631 CCC: $37.00 © 2007 American Chemical Society Published on Web 03/01/2007

3514 Langmuir, Vol. 23, No. 7, 2007

Tao and Bernasek

Figure 1. (a) Large-scale image of the self-assembled monolayer of HOOCCH2(CH2)12CH2COOH on HOPG. 240 Å × 240 Å. Vb ) 0.89 V and It ) 0.79 nA. (b) High-resolution image of HOOCCH2(CH2)12CH2COOH on HOPG. 60 Å × 60 Å. Vb ) 0.88 V and It ) 0.75 nA. The short white lines are used to differentiate adjacent molecules in a lamella. The numbers 1, 2, 3, etc. label each molecule. The green and blue dashed lines show the left and right half of the same molecule along the molecular long axis. A half of a molecule is defined here as all the hydrogen atoms on one side of the all-trans-carbon axis. Two long white lines are the molecular axes of the two closest molecules from two adjacent lamellae such as molecules 3 and 4. They are off-set by ∼2.1 Å along the direction perpendicular to the molecular chain. In panels a and b, the dim area between two adjacent lamellae corresponds to the hydrogen-bonded carboxylic acid groups. (c) Molecular packing model of HOOC(CH2)12COOH. Each molecule has a trans-rotamer (I) shown in panel c. The trans-rotamer is defined as two hydroxyl groups arranged at different sides of an all-trans-carbon skeleton. (d) Packing model on HOPG for a molecule with another trans-rotamer (II) shown in panel d. (e) Packing model on HOPG for a molecule with a cis-rotamer shown in panel e. The cis-rotamer is defined as two carbonyl groups arranged on the same side of an all-trans-carbon skeleton. HOOC(CH2)8COOH is used to represent HOOC(CH2)12COOH for a clearer presentation.

hydrogen-bonding network between two adjacent lamellae makes molecules alternately pack with opposite orientations in a lamella.11 Hydrogen bonding between OH groups of n-alcohols results in the familiar herringbone structures observed for these molecules.12 The formation of self-assembled structures from bifunctional molecules can offer the multi-functionality of organic materials and the flexibility of fine-tuning the chemical, physical, and mechanical properties of the thin film for desirable applications in a wide spectrum of technologies. For the formation of selfassembled structures from a bifunctional molecule, however, molecule-molecule interactions become much more complicated due to the additional interactions between hetero-functional groups in the self-assembled structure. The formation of self-assembled structures from bifunctional molecules results from the combination and balance of various weak interactions including possible interactions with solvent. It is of fundamental interest to investigate the competitive and cooperative interactions of these different functional groups systematically. To understand self-assembly processes of bifunctional molecules, the self-assembled monolayers formed from bifunctional di-acids HOOC(CH2)nCOOH (11) Hibino, M.; Sumi, A.; Tsuchiya, H.; Hatta, I. J. Phys. Chem. B 1998, 102, 4544. (12) Claypool, C. L.; Faglioni, F.; Goddard, W. A.; Gray, W. B.; Lewis, N. S.; Marcus, R. A. J. Phys. Chem. B 1997, 101, 5978.

(n ) 20, 18, 14, 12, 10), acid-alcohols HOOC(CH2)nCH2OH (n ) 13, 14), and di-ol HOCH2(CH2)14CH2OH dissolved in octanoic acid were examined. Experimental Procedures All experiments were performed with a laboratory-built ultrahigh vacuum (UHV) compatible variable-temperature STM. The microscope was mounted in an UHV chamber on an eddy current damped spring suspension stage. The single-tube scanner and tip were horizontally mounted and used for scanning a vertically mounted sample.13 The tip was made by mechanically cutting 0.25 mm platinum/iridium wire (Pt/Ir ) 90:10) (Goodfellow). HOOC(CH2)20COOH (97%), HOOC(CH2)14COOH (98%), HOOC(CH2)12COOH (98%), HOOC(CH2)10COOH (94%), HOOC(CH2)14CH2OH (98%), HOOC(CH2)13CH2OH (99%), HO(CH2)16OH (98%), and HOOC(CH2)6CH3 (99%) were purchased from Aldrich, and HOOC(CH2)16COOH (97%) and HOOC(CH2)18COOH (97%) were purchased from TCI. All the chemicals were used without further purification. Highly oriented pyrolytic graphite (HOPG) substrates of ZYA grade were obtained from Advanced Ceramics Corporation. All the solutions used for fabricating the self-assembled monolayers were prepared by dissolving the molecules into octanoic acid. The concentrations of solutions used to form these self-assembled monolayers of the three categories of bifunctional molecules were 0.005-0.02 mol/L. For each bifunctional molecule, different (13) Tao, F.; Goswami, J.; Bernasek, S. L. J. Phys. Chem. B 2006, 110, 4199.

Self-Assembly of Bifunctional Molecules on HOPG

Langmuir, Vol. 23, No. 7, 2007 3515

Figure 2. (a) Image of HOOC(CH2)20COOH in solution of octanoic acid. 302 Å × 302 Å. Vb ) 0.71 V and It ) 0.76 nA. (b) Image of HOOC(CH2)20COOH in solution of octanoic acid. 200 Å × 200 Å. Vb ) 0.74 V and It ) 0.87 nA. (c) Image of HOOC(CH2)20COOH in a solution of octanoic acid. 187 Å × 187 Å. Vb ) 0.71 V and It ) 0.78 nA. (d) Image of HOOC(CH2)20COOH in a solution of octanoic acid. 150 Å × 150 Å. Vb ) 0.80 V and It ) 0.78 nA. (e) Model of lamellae without coadsobed octanoic acid (I). In panels a-d, the dim area between two adjacent lamellae corresponds to the hydrogen-bonded carboxylic acid groups. (f) Model of lamellae with one coadsorbed octanoic acid molecule (II). (g) Model of lamellae with two coadsorbed octanoic acid molecules (III). HOOC(CH2)10COOH and HOOC(CH2)4CH3 represents HOOC(CH2)20COOH and HOOC(CH2)6CH3, respectively, for a clearer presentation. concentrations gave very similar STM images. A self-assembled monolayer was prepared by gently depositing 2 µL of the prepared solution onto a freshly cleaved basal plane of HOPG with a dimension of 6 mm × 6 mm and leaving it enough time for crystallization. The liquid-solid interface was scanned at room temperature. Samples were positively biased. Images were obtained in constant current mode. STM images of these monolayers were collected under various tunneling conditions (Vb ) 0.6-1.0 V and It ) 0.6-1.0 nA). Experiments were repeated with different tips and samples to ensure that the images were not influenced by tip and sample artifacts. On the basis of the experimental STM images, several structural models for the self-assembled molecules on HOPG were constructed here for aiding the interpretation of molecular packing on this substrate. The method used for constructing this kind of structural model has been described in detail previously.13

Results and Discussion Adsorption Chemistry of Di-acids HOOC(CH2)nCOOH/ HOOC(CH2)6CH3 (n ) 20, 18, 16, 14, 12, 10). Figure 1 shows an STM image of the self-assembled monolayer of HOOC(CH2)12COOH. Figure 1a is a large-scale image, in which all lamellae

have the same width of 18.0 ( 0.4 Å. The measured clockwise chain-to-trough angle is ∼90°. The lamella width is consistent with the molecular length of HOOC(CH2)12COOH. Figure 1b is a high-resolution image of this molecular overlayer. Clearly, molecular axes (long white lines) of two neighboring molecules such as 3 and 4 from two adjacent lamellae are offset by ∼2.1 Å (marked with two white arrow lines). The left half hydrogen atoms (green dashed lines) of molecule 1 (or 2, 3, 4, etc.) are on the same line with the right half hydrogen atoms (blue dashed lines) of molecule 2 (or 3, 4, 5, etc.), respectively. On the basis of these packing features, a molecular arrangement model is proposed in Figure 1c. In this model, the two hydroxyl groups of each di-acid molecule are arranged in a trans configuration to form a trans-rotamer (I). All the molecules of this model are trans-rotamer (I). This trans configuration results in an offset packing of the two neighboring molecules such as 3 and 4 in the two adjacent lamellae. The self-assembly of this molecule does not follow the packing model of Figure 1d, in which a chainto-trough angle of 60° is expected, and no offset between the two neighboring molecules of two adjacent lamellae is seen because of the arrangement of the two hydroxyl groups in the trans-

3516 Langmuir, Vol. 23, No. 7, 2007

Tao and Bernasek

Figure 3. (a) Image of a mixture of HOOC(CH2)20COOH (50% molar ratio) and HOOC(CH2)12COOH (50% molar ratio) dissolved in octanoic acid. 492 Å × 492 Å. Vb ) 0.88 V and It ) 0.81 nA. (b) Image of a mixture of HOOC(CH2)20COOH (50% molar ratio) and HOOC(CH2)12COOH (50% molar ratio) dissolved in octanoic acid. 300 Å × 300 Å. Vb ) 0.70 V and It ) 0.76 nA. (c) Image of a mixture of HOOC(CH2)20COOH (50% molar ratio) and HOOC(CH2)12COOH (50% molar ratio) dissolved in octanoic acid. 250 Å × 250 Å. Vb ) 0.88 V and It ) 0.81 nA. In panels a-c, the dim area between two adjacent lamellae corresponds to the hydrogen-bonded carboxylic acid groups. (d) Molecular packing model of the coadsorption system of HOOC(CH2)20COOH and HOOC(CH2)12COOH.

rotamer (II). In addition, no structures from the model of Figure 1e were observed. On the other hand, no structure resulting from coadsorption with octanoic acid was observed. The reason for this will be discussed next. Similar self-assembled structures, showing uniform lamella widths, arrangement using trans-rotamer (I), and no solvent coadsorption, were observed for di-acids HOOC(CH2)nCOOH (n ) 10, 14, 16). Figure 2a-d shows four representative images of selfassembled monolayers of HOOC(CH2)20COOH in octanoic acid. Distinctly different from the images in Figure 1, the lamellae in all the images of Figure 2 have three different widths of 28.5 ( 0.6 Å (I), 42.4 ( 0.9 Å (II), and 56.0 ( 1.2 Å (III). The lamellae with a width of 28.5 ( 0.6 Å correspond to HOOC(CH2)20COOH (docosanedioic acid, D) without coadsorption of octanoic acid (O) (I). The spacing of 42.4 ( 0.9 Å corresponds to coadsorption with one octanoic acid molecule (DO) (II), and spacing of 56.0 ( 1.2 Å corresponds to coadsorption with two octanoic acid molecules (ODO) (III). The chain-to-trough angle of all the lamellae is 90 ( 3°, indicating the selection for transrotamer I, as with the shorter di-acid structures. On the basis of these features, the models of the three lamellae (I, II, and III) are proposed in Figure 2e-g, respectively. Notably, the three lamellae are randomly distributed. This suggests that the coadsorption is not at the molecular level as each component of the coadsorption system is not homogeneously distributed on the substrate and the two components do not form a twodimensional organic lattice in the self-assembled monolayer.

Their arrangements are random even for the self-assembled monolayers prepared from very dilute solutions, indicating that the distribution of the different lamellae does not strongly depend on the concentration of the di-acid HOOC(CH2)20COOH and that the structures are energetically very similar. Notably, the internal hydrogen-bonded carboxylic acid groups in the coadsorption lamellae (see Figure 2e-g) do not appear as dark features, as in the lamella of type I. This is an interesting point that should be explored further. Figure 3 shows three images of mixed di-acid monolayers formed from HOOC(CH2)12COOH (50% molar ratio) and HOOC(CH2)20COOH (50% molar ratio) dissolved in octanoic acid. Clearly, the chain-to-trough angle is ∼90 ( 3°, suggesting that all the molecules pack on HOPG with a trans-rotamer (I) configuration as shown in Figure 3d. There are two different lamellae in each image. Their distribution is not homogeneous. Their widths are 18.0 ( 0.4 and 28.5 ( 0.6 Å, corresponding to the molecular length of HOOC(CH2)12COOH and HOOC(CH2)20COOH, respectively. Thus, di-acid HOOC(CH2)20COOH can coadsorb with di-acid HOOC(CH2)12COOH but not with octanoic acid. This suggests that HOOC(CH2)20COOH prefers to coadsorb with HOOC(CH2)12COOH rather than octanoic acid, as was seen for pure HOOC(CH2)20COOH dissolved in octanoic acid discussed previously. The fact that the coadsorption of HOOC(CH2)20COOH and octanoic acid occurs in the pure solution of HOOC(CH2)20COOH dissolved in octanoic acid, butnot in the mixed solution of HOOC(CH2)12COOH (50% molar

Self-Assembly of Bifunctional Molecules on HOPG

Figure 4. (a) Packing model of HOOC(CH2)20COOH molecules with a trans-rotamer (I) without coadsorption. The inset shows the unit cell of the graphite lattice. (b) Packing model of HOOC(CH2)12COOH molecules with a trans-rotamer (I) without coadsorption. (c) Packing model of HOOC(CH2)6CH3 with a trans-rotamer.

ratio) and HOOC(CH2)20COOH (50% molar ratio) dissolved in octanoic acid, suggests that the stability of the coadsorption structures mainly depends on the hydrogen-bond density in the resulting overlayer. Figure 4a-c shows models of the self-assembled monolayers of HOOC(CH2)20COOH, HOOC(CH2)12COOH without any coadsorption, and HOOC(CH2)6CH3, respectively. The hydrogenbond densities of the self-assembled monolayers of HOOC(CH2)20COOH, HOOC(CH2)12COOH, and HOOC(CH2)6CH3 can be estimated as 4/12.5 (four hydrogen bonds per 12.5 graphite unit cells, where a graphite unit cell is as defined in the inset of Figure 4a), 4/8.5, and 2/5.5, for the three molecules, respectively. The hydrogen-bond density of HOOC(CH2)6CH3 is smaller than HOOC(CH2)12COOH but larger than HOOC(CH2)20COOH, in accordance with the result that HOOC(CH2)6CH3 coadsorbs with HOOC(CH2)20COOH but not with HOOC(CH2)12COOH and consistent with the fact that HOOC(CH2)12COOH coadsorbs with HOOC(CH2)20COOH but not with HOOC(CH2)6CH3. This result suggests that the molecular self-assembly forms a structure that maximizes the overall hydrogen-bond density in the overlayer. This is consistent with the influence of different packing structures of the self-assembled alkyl dicarbamates on the hydrogen-bond density of the self-assembled monolayers on HOPG,14 as previously suggested by Matzger and co-workers.

Langmuir, Vol. 23, No. 7, 2007 3517

Diverse Adsorption Structures of Mixed Bifunctional Molecules HOOC(CH2)nCH2OH (n ) 13, 14). HOOC(CH2)13CH2COH. Figure 5a is an image of the HOOC(CH2)13CH2OH monolayer at atomic resolution. This image is made up of alternate bright herringbone bands and dark troughs. On the two sides of each dark trough, molecules are parallel to each other due to the configuration of hydrogen bonds between two adjacent carboxylic acid groups. The two adjacent molecules from two adjacent bands are off-set in the direction perpendicular to the molecular chain by one graphite unit cell, forming an interdigitated packing structure. Each herringbone band has two lamellae. The width of each lamella is consistent with the molecular length of HOOC(CH2)13CH2OH. Figure 5c is a molecular model of this image. Carboxylic acid groups in two adjacent bands assemble together via the formation of two hydrogen bonds. The OH groups in two adjacent lamellae pack into a herringbone structure through a H-O‚‚‚H-O hydrogenbonding network, forming a chain-to-chain angle of 120 ( 5°. In each lamella, all molecules use the same face to pack in the structure. However, two adjacent lamellae such as A1 and A2 of a herringbone band (A) have opposite chiralities, producing a racemic band. Two adjacent bands (A and B) have opposite orientations. Figure 5a is made up of alternately arranged bands A and B. The homogeneous self-assembled structure is racemic. Figure 5b is another image of HOOC(CH2)13CH2OH observed in the same monolayer as Figure 5a. The alternate packing of bands A and B is terminated at the section marked with a green dashed line box. Close examination shows that there must be a new hydrogen-bonding network of COOH‚‚‚OH at the border (marked with white boxes in Figure 5b) between lamellae 1 and 2 and between lamellae 3 and 4. Figure 5d is a packing model of this structure. The configuration of this new hydrogen bond COOH‚‚‚OH is clearly presented in Figure 5e. The hydrogen atom of the COOH from the right molecule and the oxygen atom of the OH from the left molecule act as donor and acceptor, forming a strong H-O‚‚‚H-O hydrogen bond. The hydrogen atom of the OH from the left molecule interacts with the carbonyl oxygen of the COOH group from the right molecule, producing another hydrogen bond. Notably, the region at the right and left sides of the green dashed line box of Figure 5b are racemic domains identical to Figure 5a. However, the formation of two lamellae through the new COOH‚‚‚OH hydrogen-bonding network (white boxes in Figure 5b,d) changes its chirality. In the area marked with green dashed lines, lamellae 3 cannot be superimposed on molecules of its adjacent lamellae 2. Considering this image, the section at the left of the white dashed line (Figure 5b,d) cannot be superimposed on the molecules of the right section of the white dashed line. However, the mirror image of the molecules of the left section can be superimposed on the molecules of the right section. This phenomena fits the definition of chirality. Therefore, the two adjacent sections could be called an enantiomeric chiral domain. All the molecules on the two sides of the white dashed line are the same species and are not chiral by themselves. However, they use opposite faces to pack on an achiral substrate, forming a chiral structure. This results from a novel hydrogen-bond network of COOH‚‚‚OH and the registry of the molecular chains with the underlying substrate lattice. It is another example of 2-D chiral structure formation in the adsorption of achiral molecules.13,15,16 More importantly, it demonstrates that subtle changes of molecular packing can induce the occurrence of chirality in these organic overlayers. (14) Kim, K.; Plass, K. E.; Matzger, A. J. J. Am. Chem. Soc. 2005, 127, 4879. (15) Cai, Y.; Bernasek, S. L. J. Am. Chem. Soc. 2004, 126, 14234.

3518 Langmuir, Vol. 23, No. 7, 2007

Tao and Bernasek

Figure 5. (a) Image of HOOC(CH2)13CH2OH. 106 Å × 106 Å. Vb ) 0.71 V and It ) 0.80 nA. A1, A2, B1, B2, etc. label the lamellae in this domain. The six arrows at the lower part of this panel show the interdigitated packing of COOH groups of two adjacent lamellae. The molecular packing model of structure I is presented in Figure 5c. (b) Image of HOOC(CH2)13CH2OH. Green box of dashed lines shows four lamellae, in which molecules are packed with a new hydrogen-bonding network (Figure 5e). 152 Å × 152 Å. Vb ) 0.75 V and It ) 0.80 nA. The molecular packing model of this image is presented in Figure 5d. In panels a and b, the dim area between two adjacent herringbone lamellae corresponds to the hydrogen-bonded carboxylic acid groups. The hydrogen-bonded OH groups in the middle of a herringbone lamella cannot be clearly identified, possibly due to their relatively close packing in contrast to the hydrogen-bonded carboxylic acid groups. (c) Molecular packing model for the image in Figure 5a. (d) Molecular packing model for image of Figure 5b. The COOH‚‚‚OH hydrogen-bonding networks are marked with white boxes. (e) An enlargement of the model of the new hydrogen-bonding network between -OH and -COOH groups. HOOC(CH2)5CH2OH represents HOOC(CH2)13CH2OH for a clearer presentation.

The COOH group of the carboxylic acid has two possible conformations, rotamers I and II.17 They have almost the same energy.13 The three-dimensional crystal of carboxylic acid may select either one of them18 or both of them.19 For rotamer I, at least two kinds of domains with a chain-to-trough angle of 60 or 90° can be expected (Figure 1c,d). Rotamer II only packs into a domain with a chain-trough angle of 60° (Figure 1e). Since the chain-to-trough angle is 60° and the COOH groups of two adjacent lamellae such as B2 and A1 arrange via an interdigitated pattern (16) (a) Tao, F.; Bernasek, S. L. J. Phys. Chem. B 2005, 109, 6233. (b) Meier, C.; Ziener, U.; Landfester, K.; Weihrich, P. J. Phys. Chem. B 2005, 109, 21015. (c) De Feyter, S.; De Schryver, F. C. J. Phys. Chem. B 2005, 109, 4290. (d) Yablon, D. G.; Giancarlo, L. C.; Flynn, G. W. J. Phys. Chem. B 2000, 104, 7627. (e) Fang, H. B.; Giancarlo, L. C.; Flynn, G. W. J. Phys. Chem. B 1998, 102, 7311. (f) Li, C. J.; Zeng, Q. D.; Wang, C.; Wan, L. J.; Xu, S. L.; Wang, C. R.; Bai, C. L. J. Phys. Chem. B 2003, 107, 747. (17) Rotamer I is defined as the conformation in which the carbonyl group is on the same side as the C2-H bond (see Figure S1a). Rotamer II is defined as the other conformation in which the carbonyl group is on the same side as the C3-H bond (see Figure S1b). (18) (a) Kay, M. I.; Katz, L. Acta Crystallogr. 1958, 11, 289. (b) Abrahamsson, S.; Ryderstedt-Nahringbauer, I. Acta Crystallogr. 1962, 15, 1261. (c) Strieter, F. J.; Templeton, D. H. Acta Crystallogr. 1962, 15, 1240. (d) Housty, J.; Hospital, M. Acta Crystallogr. 1996, 21. (e) Jeffrey, G. A.; Sax, M. Acta Crystallogr. 1963, 16, 1196. (f) Scheuerman, R. F.; Sass, R. L. Acta Crystallogr. 1962, 15, 1244. (19) Lomer, T. R. Acta Crystallogr. 1963, 16, 984.

(shown in Figure 5a) in the herringbone structure of HOOC(CH2)13-CH2OH, the COOH groups select rotamer I in this case. It has been previously observed that HOOC(CH2)13CH2OH dissolved in the solvent 1-hexanol forms a monolayer made of domains with a structure like that of Figure 5c. In this case, no domain with the novel hydrogen-bonding network seen here (Figure 5d,e) was observed.20 HOOC(CH2)14CH2OH. HOOC(CH2)14CH2OH displays multiple adsorption structures. Its structures are distinctly different from those of HOOC(CH2)13CH2OH described previously. Figure 6 shows one self-assembled structure of this molecule. It is composed of alternately bright bands and dark hole bands. The area between a bright band and its adjacent dark hole band is marked with green dashed lines in Figure 6a. Examining the image from the direction of the molecular long axis, it is made of two alternately packed molecular chains (marked with white and turquoise lines). Each white line is made of almost continuous long molecular chains. However, each turquoise line includes alternate short molecular chains and dark holes. Because each molecular chain along the white line is longer than that indicated (20) Wintgens, D.; Yablon, D. G. Flynn, G. W. J. Phys. Chem. B 2003, 107, 173.

Self-Assembly of Bifunctional Molecules on HOPG

Figure 6. (a) Image of HOOC(CH2)14CH2OH dissolved in octanoic acid. 140 Å × 140 Å. Vb ) 0.80 V and It ) 0.81 nA. The area between two adjacent green dashed lines (marked BB) is a bright area that is filled by alternating HOOC(CH2)14CH2OH and octanoic acid. The area between two adjacent green dashed lines (marked with BHB) consists of an alternate unoccupied hole and partial alkyl chain of a HOOC(CH2)14CH2OH molecule. (b) Image of HOOC(CH2)14CH2OH dissolved in octanoic acid. 64 Å × 64 Å. Vb ) 0.82 V and It ) 0.81 nA. The white and turquoise lines show the extension directions of long and short molecular chains, respectively. The green dashed line shows the width of the bright band and the dark hole band. In panels a and b, the green arrows are superimposed onto two holes of the open mesh structure. (c) Molecular packing model of this coadsorption system. A mesh made of evenly arranged nanometer-sized openings is formed. The white boxes show the formed openings.

by the turquoise line, evenly arranged holes are formed in the rows of turquoise lines. Two green arrows mark two of the formed holes in Figure 6a,b. Line-profile measurements show that the

Langmuir, Vol. 23, No. 7, 2007 3519

average dimension of one opening in this structure is 12.5 ( 0.6 Å × 5.0 ( 0.4 Å × 1.8 ( 0.3 Å. The lengths of the long chain and short chain are 20 ( 0.5 and 10 ( 0.3 Å, in accordance with the molecular length of HOOC(CH2)14CH2OH and octanoic acid, respectively. This suggests that this image is made of alternate rows of HOOC(CH2)14CH2OH (white line) and octanoic acid molecules (turquoise line). Figure 6c models the molecular arrangement of this coadsorption system on HOPG. Each HOOC(CH2)14CH2OH molecule coadsorbs with one octanoic acid molecule via the formation of two hydrogen bonds. Notably, to form this mixed self-assembled structure, HOOC(CH2)14CH2OH uses a specific rotamer different from that seen for the HOOC(CH2)13CH2OH structures of Figure 5. As seen in Figure 6c, both the COOH groups of HOOC(CH2)14CH2OH and the COOH groups of octanoic acid choose rotamer II17 to form this hydrogenbond linked molecular network of two components. Thus, this molecule displays significantly different self-assembled structures even though it differs by only one CH2 group from HOOC(CH2)13CH2OH. Moreover, octanoic acid coadsorbs with a rotamer similar to HOOC(CH2)14CH2OH in this case. The two molecules can form two hydrogen bonds in different rows, therefore producing openings in the row of octanoic acid molecules as has been previously observed in the coadsorption of octanoic acid and isophthalic acid.21 This is a new 2-D structure made of evenly distributed openings with a size of 12.5 ( 0.6 Å × 5.0 ( 0.4 Å, formed via the coadsorption of HOOC(CH2)14CH2OH and HOOC(CH2)6CH3 at the molecular level. Notably, the proposed structure for this coadsorption domain exhibiting holes has a relatively low stability due to low molecular coverage on the graphite surface and a weak hydrogen-bonding network. It is probably not the thermodynamically most favorable structure, indicated by the low resolution of this image. Definitely, it is not a major phase, although it has been observed in each selfassembled monolayer of HOOC(CH2)14CH2OH. In addition, some domains without the coadsorption of octanoic acid were observed for this self-assembled monolayer. The structure of these domains is the same as the uniform monolayer domains observed for HOOC(CH2)14CH2OH dissolved in 1-hexanol.20 As compared to HOOC(CH2)13CH2OH, the acid-alcohol HOOC(CH2)14CH2OH does not form the normal herringbone structure because the added CH2 group adjacent to the terminal OH creates an unfavorable conformation for the formation of the H-O‚‚‚H-O hydrogen-bonding network. Alternatively, it packs with a distinctly different model, in which every four molecules with even-numbered carbon atoms form a tetramer structural unit via a head-to-tail arrangement where two COOH groups and two OH groups assemble together through hydrogen bonds.20 Multiple Packing Structures of Di-ol HOCH2(CH2)14CH2OH. Lattice Match-Induced Distortion of Conformation and Chirality. Three packing structures were observed in the self-assembled monolayer of the di-ol HOCH2(CH2)14CH2OH. Figure 7a shows a homogeneous herringbone structure. Lamellae 2, 4, 6, and 8 have a chain-to-trough angle of 60 ( 3° along the clockwise direction. Correspondingly, lamellae 1, 3, 5, and 7 have a clockwise chain-to-trough angle of 120 ( 5°. The two supplementary angles result from two opposite packing faces of this di-ol molecule on HOPG. Figure 7b presents a model of the hydrogen-bonding networks at each trough between the two adjacent lamellae. Notably, lamellae 1, 3, 5, and 7 are brighter than lamellae 2, 4, 6, and 8 in Figure 7a. Further close examination indicates that, on average, the two ends of lamellae 2, 4, 6, and 8 have a higher contrast than the center parts of these lamellae. Line-profile analyses in Figure 7c show that lamellae (21) Tao, F.; Bernasek, S. L. J. Am. Chem. Soc. 2005, 127, 12750.

3520 Langmuir, Vol. 23, No. 7, 2007

Tao and Bernasek

Figure 7. (a) Image of HOCH2(CH2)14CH2OH dissolved in octanoic acid corresponding to structure I. 140 Å × 140 Å. Vb ) 0.81 V and It ) 0.78 nA. The dim area between two adjacent lamellae corresponds to hydrogen-bonded OH groups. (b) Schematic presentation for the herringbone hydrogen-bonding network of structure I. (c) The average height profiles of 20 sections in terms of 20 molecules as marked with red dashed lines along the chain direction of HOCH2(CH2)14CH2OH in lamellae 3. The starting point A and ending point B of line-profile analyses are on the left and right white lines of panel a. The line-profile section from A to B is also marked in panel d. (d) Scheme for the starting and ending points of line-profile analyses of panel c. HOCH2(CH2)6CH2OH represents HOCH2(CH2)14CH2OH for a clearer presentation.

1, 3, 5, and 7 are obviously higher than lamellae 2, 4, 6, and 8. Similar line-profile analyses for the sections from the right end of lamella 2 (or 4) to the left end of lamella 4 (or 6) were carried out. These analyses suggest that the variation of contrast possibly results from a distortion of the alkyl chains upon adsorption on the surface due to the slight mismatch of the HOCH2(CH2)14CH2OH molecule with the substrate lattice. In general, one possible driving force for the alternate bending up and down of the alkyl chains is to have a good match between the molecular alkyl chain and the graphite surface lattice for the maximization of molecule-substrate interactions. However, there is still a 2.5% lateral mismatch as the graphite lattice unit and the H-H distance of two adjacent methylene groups in the hydrogen chain are 2.46 and 2.52 Å, respectively. To maintain good registry with the substrate lattice and maximize van der Waals interactions between molecules and substrate lattice, a compression of 2.5% of the chain length is advantageous. This compression will slightly distort the molecular chain. The distortion of the molecular chain does cost energy, while the reduction of the chain length lowers the overall energy due to a better adsorbate-substrate interaction. How much the chain is compressed depends on the balance of these two opposite energetic effects. The observed alternate change of contrast along the column propagation direction might result from preservation of the planar geometry of H-O‚‚‚H-EnDashO hydrogen bonds between two adjacent lamellae of HOCH2(CH2)14CH2OH. The alternate up and down distortion of the alkyl chain of the di-ol is similar to the adsorbate-induced asymmetric assembly of 1-octadecanol on HOPG.15 In a pair of 1-octadecanol molecules hydrogen

bonded across the chevron, one molecule bends up and the other one bends down, resulting in an asymmetric distortion of the pair. Another possible explanation for the difference in contrast between two adjacent lamellae of HOCH2(CH2)14CH2OH is that two adjacent lamellae overlap the graphite substrate lattice differently. The different offset of molecular alignment along the graphite surface lattice does not make the same angle with the column propagation direction. The offset of the molecular alkyl chain along the graphite surface lattice is supported by the observed non-30° angle between the molecular chain and the column propagation direction in the self-assembled monolayers of decanol, dodecanol, and tetradecanol.22 Another example is an unusually large offset of alignment between the molecular chain and the graphite surface lattice revealed in the selfassembled monolayer of 4-heptyl-4′-cyanobiphenyl on HOPG.23 Hydrogen-Bonding Network of Alcohol and Di-ol Molecules. To understand the hydrogen-bonding network of di-ol molecules on HOPG, the self-assembled structure of n-alcohols should be clarified. Figure 8a is the packing model of a typical n-alcohol on HOPG. The zigzag red line shows the hydrogenbonding network formed between two adjacent lamellae of n-alcohols. To form this network, molecules in two adjacent lamellae approach in an interdigitated arrangement. The alcohol molecules from two adjacent lamellae use opposite faces to pack on the HOPG, resulting in opposite two-dimensional chirality in terms of forming a racemic pair in each herringbone. Face 1 (or (22) 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. (23) Schulze, J.; Stevens, F.; Beebe, T. P. J. Phys. Chem. B 1998, 102, 5298.

Self-Assembly of Bifunctional Molecules on HOPG

Langmuir, Vol. 23, No. 7, 2007 3521

Figure 8. (a) Herringbone packing models of n-alcohol molecules on HOPG and the identification of the molecular face adsorbed on HOPG. (b) Scheme for the definition of face 1 and face 2 of n-alcohol and di-ol. (c) Parallel packing models of n-alcohol molecules on HOPG, in which all molecules are parallel to each other. The red box shows the weak hydrogen bond between two adjacent molecules in a lamella. (d) Image of HOCH2(CH2)14CH2OH. 165 Å × 165 Å. Vb ) 0.81 V and It ) 0.78 nA. In this image, the dim area between two adjacent lamellae corresponds to hydrogen-bonded OH groups. The image includes molecules adsorbed on HOPG with the herringbone packing model and the parallel packing model (e1). (e) Parallel packing model of di-ol, in which all molecules are parallel to each other. CH3(CH2)7OH and HOCH2(CH2)6CH2OH represents the typical n-alcohol and di-ol HOCH2(CH2)14CH2OH, respectively, for a clearer presentation.

2) is defined as the molecule with a 140° (or 220°) clockwise angle from the long molecular axis to its OH group upon packing on HOPG. Figure 8b1,2 schematically presents this definition. This definition can also be used for the di-ol molecules (Figure 8b3,4). Also due to the hydrogen-bonding network structure, the face used to adsorb on HOPG can be identified with the clockwise chain-to-trough angle. If the angle is 120°, the molecules in this lamella use face 1 to pack. If it is 60°, the molecules use face 2 to self-assemble. Moreover, the previous method identifies the face of the di-ol packing on HOPG with the herringbone structure as seen in each lamella of Figure 7a. With this method, the face of n-alcohol or di-ol used to pack in terms of its two-dimensional chirality can be easily identified, although as compared to face 1, face 2 does not present a distinguishable difference in the STM image.

Previous investigations of the self-assembled monolayer of triacontanol24,25 show that alcohol molecules can form another hydrogen-bonding network as shown in Figure 8c. In this structure, molecules in adjacent lamellae are parallel to each other. Notably, this hydrogen-bonding network is weaker than that of the chevron pattern (Figure 8a) due to a larger O-H‚‚‚O distance in half of the hydrogen bonds of this network as marked with a red box in Figure 8c1. This may explain why the second hydrogen-bonding network has only been observed in very few n-alcohols with long chains such as triacontanol.24,25 In addition to the herringbone structure of Figure 7, long chain di-ol molecules can form the second hydrogen-bonding network (24) Venkataraman, B.; Breen, J. J.; Flynn, G. W. J. Phys. Chem. 1995, 99, 6608. (25) Gunning, A. P.; Kirby, A. R.; Mallard, X.; Morris, V. J. J. Chem. Soc., Faraday Trans. 1994, 90, 2551.

3522 Langmuir, Vol. 23, No. 7, 2007

as described previously. In Figure 8d, lamellae A, B, D, E, F, H, and I use the second hydrogen-bonding network shown in Figure 8c2. Figure 8e presents the models of the second hydrogenbonding network for the di-ol. On the basis of the clockwise chain-to-trough angle of 60 ( 3°, these lamellae use the mode of Figure 8e1. The coexistence of two different hydrogen-bonding networks in the image of Figure 8d shows the diversity of selfassembled structures of these bifunctional molecules.

Conclusion The self-assembly behavior of three series of bifunctional molecules including HOOC(CH2)nCOOH (n ) 20, 18, 16, 14, 12, 10), HOOC(CH2)nCH2OH (n ) 13, 14), and HO(CH2)16OH were studied with high-resolution STM. Bifunctional molecules display diverse self-assembled structures due to the interplay between hetero-functional groups and interactions between homofunctional groups in the self-assembled domain as well as van der Waals and molecule-substrate interactions. Di-acids HOOC(CH2)nCOOH have a high selectivity for the trans-rotamer (I) due to the maximization of molecule-molecule van der Waals interactions. Whether a di-acid coadsorbs with octanoic acid or not strongly depends on the hydrogen-bond density of the formed monolayer. Octanoic acid can coadsorb with a di-acid HOOC(CH2)nCOOH (n ) 20), which has a lower hydrogen-bond density,

Tao and Bernasek

but not with shorter di-acids. HOOC(CH2)nCH2OH (n ) 13, 14) displays multiple adsorption structures. A new hydrogen-bonding network of COOH‚‚‚OH was seen in the self-assembled structure of HOOC(CH2)13CH2OH. HOOC(CH2)14CH2OH can selfassemble with octanoic acid at the molecular level, producing homogeneously arranged openings with a size of 12.5 ( 0.3 Å × 5.0 ( 0.2 Å × 1.8 ( 0.1 Å. This demonstrates that the coadsorption of mixtures with different molecular chains may provide a new strategy for the fabrication of thin film materials with nanometer-scale structural features. In the self-assembled monolayer of HO(CH2)16OH, the molecular conformation is distorted because of the mismatch between the hydrogen atom chain and substrate lattice spacing, thereby increasing the molecule-substrate interaction. The preservation of the planar geometry of O-H‚‚‚O-H hydrogen bonds also affects this distortion. The distortion of the molecular chain breaks the original symmetry and induces chirality for this self-assembled structure, providing another example of adsorption-induced chirality in these self-assembled monolayer systems.15 Acknowledgment. This work was partially supported by the National Science Foundation (CHE-0313801). LA0613631