Bias-Dependent STM Image Contrast Study of Phenyloctadecyl Ethers

Control of a two-dimensional molecular structure by cooperative halogen and hydrogen bonds. Satoshi Yasuda , Atom Furuya , Kei Murakoshi. RSC Adv. 201...
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Langmuir 1998, 14, 7475-7483

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Bias-Dependent STM Image Contrast Study of Phenyloctadecyl Ethers Physisorbed onto Highly Oriented Pyrolytic Graphite H. S. Lee, S. Iyengar, and I. H. Musselman* Department of Chemistry, University of Texas at Dallas, P.O. Box 830688, Richardson, Texas 75083-0688 Received November 5, 1997. In Final Form: September 29, 1998 A homologous series of para-substituted phenyloctadecyl ethers (X-POEs, X ) H, Cl, Br, and I) was prepared using Williamson’s ether synthesis. The structure and purity of the ethers were confirmed using 1H NMR and GC/MS. Scanning tunneling microscopy (STM) images were acquired from monolayers of the ethers which formed at the surface of highly oriented pyrolytic graphite from a 2.5 wt % solution in phenylhexane. For all four ethers, the monolayers displayed a contrast which varied as a function of tip-sample bias. A comparison of STM images of the adsorbed ether molecules with electron density contours calculated using HyperChem suggested a bias-dependent participation to tunneling of individual bonding molecular orbitals (MOs). For example, at biases of -0.26 to -0.70 V, STM images of I-POE resembled the highest occupied molecular orbital (HOMO) exhibiting two bright spots for the pair of lobes of the phenyl ring and one bright spot for the halogen atom. At higher biases, contrast was observed for the phenyl ring alone (-0.8 to -1.2 V) and for the alkyl tail (-1.0 to -1.8 V) which was similar to that of the HOMO-1 and HOMO-4 contours, respectively. Owing to the measurement of an enhanced tunneling current simultaneous with the acquisition of atomically resolved bias-dependent STM images of the X-POE adsorbates, a resonance tunneling mechanism between the tip and substrate via MOs of molecular adsorbates adjacent to and including the HOMO is proposed.

Introduction Scanning tunneling microscopy (STM) has been widely applied to the field of surface science,1-3 as this technique can reveal the local information of atomic and molecular surface structures under low-temperature, vacuum, or ambient conditions. Despite the theoretical constraint that an STM sample be conductive, it was reported that individual molecules in a liquid crystal array formed on a conducting substrate could be resolved by STM.4 Accordingly, numerous STM studies of organic adsorbates, including physisorbed, chemisorbed, and LangmuirBlodgett films, at the gas/solid and liquid/solid interfaces have been accomplished.5-8 Physisorbed systems such as long-chain alkanes,9-15 functionalized alkanes,16-20 * To whom correspondence should be addressed. Phone: (972)883-2706. Fax: (972)883-2925. E-mail: [email protected]. (1) Magonov, S. N.; Whangbo, M.-H. Surface Analysis with STM and AFM; VCH Publishers: New York, 1996. (2) McGuire, G. E.; Weiss, P. S.; Kushmerick, J. G.; Johnson, J. A.; Simko, S. J.; Nemanich, R. J.; Parikh, N. R.; Chopra, D. R. Anal. Chem. 1997, 69, 231R. (3) Bottomley, L. A. Anal. Chem. 1998, 70, 425R. (4) Foster, J. S.; Frommer, J. E. Nature 1988, 333, 542. (5) Frommer, J. Angew. Chem., Int. Ed. Engl. 1992, 31, 1298. (6) Rabe, J. P. Ultramicroscopy 1992, 42-44, 41. (7) Delamache, E.; Michel, B.; Biebuyck, H. A.; Gerber, C. Adv. Mater. 1996, 8, 719. (8) Cyr, D. M.; Venkataraman, B.; Flynn, G. W. Chem. Mater. 1996, 8, 1600. (9) McGonigal, G. C.; Bernhardt, R. H.; Thompson, D. J. Appl. Phys. Lett. 1990, 57, 28. (10) McGonigal, G. C.; Bernhardt, R. H.; Yeo, Y. H.; Thompson, D. J. J. Vac. Sci. Technol. B 1991, 9, 1107. (11) Rabe, J. P.; Buchholtz, S. Science 1991, 253, 424. (12) McGonigal, G. C.; Bernhardt, R. H.; Yeo, Y. H.; Thomson, D. J. AIP Conf. Proc. 1991, 241, 190. (13) Watel, G.; Thibaudau, F.; Cousty, J. Surf. Sci. Lett. 1993, 281, L297. (14) Thibaudau, F.; Watel, G.; Cousty, J. Surf. Sci. Lett. 1993, 281, L303. (15) Venkataraman, B.; Breen, J. J.; Flynn, G. W. J. Phys. Chem. 1995, 99, 6608.

alcohols,10-12,15-19,21,22 dialkyl-substituted benzenes,23-25 liquid crystals,4,26-32 and dye pigments33,34 have been shown to form close-packed monolayers on highly oriented pyrolytic graphite (HOPG). This well-ordered structure results from van der Waals interactions among long-chain alkyl groups in neighboring molecules and between these groups and the hydrophobic HOPG substrate. The basic tunneling theory is understood; however, two curious aspects of STM imaging of molecular adsor(16) Venkataraman, B.; Flynn, G.; Wilbur, J. L.; Folkers, J. P.; Whitesides, G. M. J. Phys. Chem. 1995, 99, 8684. (17) Cyr, D.; Venkataraman, B.; Flynn, G. W.; Black, A.; Whitesides, G. M. J. Phys. Chem. 1996, 100, 13747. (18) 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. (19) Faglioni, F.; Claypool, C. L.; Lewis, N. S.; Goddard, W. A., III. J. Phys. Chem. B 1997, 101, 5996. (20) Giancarlo, L.; Cyr, D.; Muyskenes, K.; Flynn, G. W. Langmuir 1998, 14, 1465. (21) Buchholtz, S.; Rabe, J. P. Angew. Chem., Int. Ed. Engl. 1992, 31, 189. (22) Yackoboski, K.; Yeo, Y. H.; McGonigal, G. C.; Thompson, D. J. Ultramicroscopy 1992, 42-44, 963. (23) Buchholtz, S.; Rabe, J. P. J. Vac. Sci. Technol. B, 1991, 9, 1126. (24) Rabe, J. P.; Buchholtz, S. Phys. Rev. Lett. 1991, 66, 2096. (25) Eng, L. M.; Fuchs, H.; Buchholtz, S.; Rabe, J. P. Ultramicroscopy 1992, 42-44, 1059. (26) Spong, J. K.; Mizes, H. A.; LaComb, L. J., Jr.; Dovek, M. M.; Frommer, J. E.; Foster, J. S. Nature 1989, 338, 137. (27) Smith, D. P. E.; Ho¨rber, J. K. H.; Gerber, Ch.; Binnig, G. Science 1989, 245, 43. (28) Smith, D. P. E.; Ho¨rber, J. K. H.; Binnig, G.; Nejoh, H. Nature 1990, 344, 641. (29) Mizutani, W.; Shigeno, M.; Ono, M.; Kajimura, K. Appl. Phys. Lett. 1990, 56, 1974. (30) Nejoh, H. Appl. Phys. Lett. 1990, 57, 2907. (31) Walba, D. M.; Stevens, F.; Parks, D.; Clark, N. A.; Wand, M. D. Science 1995, 267, 1144. (32) Stevens, F.; Dyer, D. J.; Mu¨ller, U.; Walba, D. M. Langmuir 1996, 12, 5625. (33) Ludwig, C.; Gompf, B.; Petersen, J.; Eisenmenger, W.; Zimmermann, U.; Karl, N. Z. Phys. B 1992, 86, 397. (34) Ludwig, C.; Gompf, B.; Petersen, J.; Eisenmenger, W. Z. Phys. B 1994, 93, 365.

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bates still remain: (i) why do different atoms and molecules produce different contrast, and (ii) how can this contrast difference be interpreted to provide meaningful chemical information? STM image contrast mechanisms for molecular adsorbates have been proposed and can be categorized roughly into three groups: (1) tunneling from a conducting substrate, (2) tunneling from molecular orbitals of adsorbates, and (3) resonance tunneling. The proposed mechanisms in the first group are based on the local modulation of the substrate work function by the presence of polarizable adsorbates, as suggested by Spong et al.26 This model does not reconcile with the molecularly resolved images of alkane monolayers on HOPG which exhibit bright spots for the methyl/methylene groups despite their small polarizability.9,13 Thibaudau and co-workers suggested that the contrast in STM images of n-C36H74 on HOPG was dominated by elastic deformation of the substrate rather than by the polarizability of the alkane.14 Nevertheless, the potential role of adsorbate polarizability in the STM image contrast mechanism continues to be explored. Cyr et al. investigated the contrast of several functional groups including NH2, CH3, OH, SH, Cl, Br, and I in primary-substituted hydrocarbons physisorbed on HOPG.17 Among them, NH2, Br, I, and SH were identified by contrast differences which were related to molecular polarizability. Recently, Claypool et al. discounted the importance of molecular polarizability to image contrast.18 Images of functionalized alkanols revealed that the polarizable OH, Br, and Cl functionalities were dark relative to the less polarizable methylene chains whereas the iodide group was bright in comparison to these same units. While the tunneling current originated from a conducting substrate in the first group of mechanisms, Smith et al. proposed that STM images reflect the electron density of the molecular orbitals (MOs) of the adsorbates themselves.28 They presented STM images of alkylcyanobiphenyl molecules on HOPG in which the biphenyl group and the alkyl tail exhibited high and low contrast, respectively. The images were compared to electron density contours calculated for the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the compounds. The contours revealed contrast only for the biphenyl group and therefore did not support the additional contrast observed for the alkyl tails. A third mechanism proposed for STM image contrast is resonance tunneling. This model, suggested by Lindsay and co-workers,35 originated from STM images of uncoated DNA adsorbed on HOPG. The pressure between the tip apex and the sample was calculated to be in the range 0.1-10 GPa, sufficiently large to alter the positions of the MOs of the adsorbates to induce resonance tunneling. Mizutani et al. applied this mechanism to explain voltagedependent STM images of liquid crystals on HOPG.29 In their model, the physisorbed molecule was represented as a potential well between the tip and the substrate and contained discrete levels of adsorbate MOs which were shifted in energy from those of isolated molecules. They proposed that, at the quantized levels in the well, the tunneling probability increased dramatically by the resonance tunneling effect.36 Accordingly, it is important in the study of STM image contrast mechanisms to acquire images as a function of (35) Lindsay, S. M.; Sankey, O. F.; Li, L.; Herbst, C.; Rupprecht, A. J. Phys. Chem. 1990, 94, 4655. (36) Azbel, M. Phys. Rev. B 1983, 28, 4106.

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Figure 1. Structure of X-POE. X ) H, Cl, Br, I. Hydrogen atoms in the tail are not shown.

tip-sample bias. Studies of n-alkanes,24 functionalized alkanes,20 alcohols,18,22,24 and liquid crystals29,32 have demonstrated that image contrast can be controlled by changing the magnitude and polarity of the tip-sample bias. Most of these studies indicate that there is a threshold bias above which only the features of the monolayer can be discerned, thereby facilitating the observation of an adsorbate’s electronic and molecular structure independent of the underlying substrate. Interestingly, Yackoboski et al. observed a bias-dependent contrast at the n-decanol/HOPG interface which highlighted different regions of the molecule.22 Later, Claypool et al. observed bias-dependent contrast for the same system.18 In an STM study of functionalized alkanes physisorbed on HOPG, Giancarlo et al. observed biasdependent contrast for images of octadecanamide but not for 1-bromodocosane or octadecyl sulfide.20 Two factors (i.e. geometric and electronic) were considered to elucidate this difference. The geometric factor was related to the spatial overlap between the STM tip and the functional group, and the electronic structure addressed the coupling between the energy levels of the adsorbate and the surface Fermi level. They proposed that the former factor played a dominant role in the contrast of the bromide and the sulfide whereas the latter factor governed bias-dependent contrast in the amide possibly through a resonance tunneling mechanism. To better understand the mechanisms and resolution limits of STM image contrast in physisorbed systems, we designed a homologous series of phenyloctadecyl ethers (X-POEs, where X ) H, Cl, Br, I) (Figure 1). This novel molecular system possesses an electron-rich phenyl ring, a halogen atom substituted on the aromatic ring, and a C18 alkyl tail. The long alkyl tail ensures the stability of the physisorbed monolayer on a substrate like HOPG,10,37 and the phenyl ring serves as a marker for identifying the location of the halogen atom in the image. Varying the halogen atom (Cl, Br, I) affords a systematic study of any potential changes to contrast in the STM images. For para-substituted X-POEs, the ambiguity for halogen visualization observed previously in alkyl halides17 has been overcome by placing the halogen on the planar phenyl ring (sp2 hybridization) instead of on the hydrocarbon chain (sp3 hybridization). In this configuration, the halogen atom is always exposed and can potentially make a significant contribution to tunneling. In this paper, STM images, acquired as a function of tip-sample bias, are presented for ordered monolayers of X-POEs physisorbed onto HOPG. The orientation of the molecules in the monolayer and the identification of spots in the STM images are determined by comparing experimental metrics to values obtained from semiempirical calculations. The STM images are also compared with energy-minimized electron density contours to investigate the potential contribution of MOs of the molecular adsorbates to image contrast. A resonance tunneling mechanism for these physisorbed monolayers is proposed. (37) Findenegg, G. H.; Liphard, M. Carbon 1987, 25 (1), 119.

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Figure 2. (a) STM image of Br-POE monolayer formation. 400 × 400 nm2 scan size. Constant height mode. Bias ) -0.135 V. (b) STM image of I-POE monolayer. 50 × 50 nm2 scan size. Constant current mode. Bias ) -0.700 V. Tunneling current ) 830 pA. Arrow 1 shows the interface between differently oriented domains in the monolayer. Arrow 2 identifies a boundary between two domains which exhibit either reverse or identical symmetry but slightly different registration caused by a step edge in the HOPG surface.

Figure 3. (a) STM image of I-POE exhibiting the head-to-tail configuration. 10 × 10 nm2 scan size. Constant height mode. Bias ) -0.39 V. (b) STM image of Br-POE exhibiting the head-tail-tail-head configuration. 41 × 41 nm2 scan size. Constant height mode. Bias ) -1.77 V. The arrows indicate the locations of the head groups.

Experimental Section Synthesis. Para-substituted phenyloctadecyl ethers (X-POE, where X ) H, Cl, Br, I) were synthesized using a modified Williamson ether synthesis.38 The structure and purity of the compounds were confirmed by 1H NMR (JNM-FX200, JEOL Ltd.) and GC/MS (Finnigan MAT GCQ). STM Measurement. A 2.5 wt % solution of X-POE in phenylhexane was prepared. A drop of the solution was spread onto freshly cleaved HOPG (ZYA Grade, Advanced Ceramics, Cleveland, OH). Monolayers of the sample compounds were imaged under ambient conditions at the liquid/solid interface. Images were acquired using a Nanoscope III scanning tunneling microscope with a 0.7 µm A-scanner (Digital Instruments, Inc., Santa Barbara, CA). Mechanically sharpened Pt/Ir (80/20) tips (Digital Instruments, Inc., Santa Barbara, CA) were used in each experiment. Tip quality was tested by acquiring atomically resolved images of HOPG. A negative bias was chosen for imaging to effect tunneling from the filled states of the electron rich halogen atom and phenyl ring to the unfilled states of the tip thereby lending positive contrast to the image. The scanning

tunneling microscope was engaged in the constant current mode using a bias of -0.5 V and a set point of 800 pA. After the monolayer was observed, imaging conditions were changed to the constant height mode to avoid image distortion due to thermal drift. High-resolution images were obtained by making small changes to the bias until maximum contrast was observed.39 Analysis of STM Images. Nanoscope III software tools were used to process the STM images. Section analysis was used to calculate bond and molecular lengths. A low-pass filter was applied to remove undesired high-frequency noise from the raw images. Molecular Modeling. HyperChem version 5.0 for Win95 (Hypercube Inc., Waterloo, Canada) was used to make structural models and electron density contours of the sample compounds. Using “geometry optimization”, the most energetically stable configuration was obtained for each compound for which bond distances were measured. Semiempirical quantum mechanical calculations of the AM1 method were used to describe the electron density and energy of each MO, including the HOMO and the LUMO. The results of these calculations were plotted as 2-D or

(38) Leonard, N. J.; Felley, D. L.; Nicolaides, E. D. J. Am. Chem. Soc. 1952, 74, 1700.

(39) Roach, J. S.; Musselman, I. H.; Honeyman, J. J. Vac. Sci. Technol. A 1996, 14 (3), 1205.

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Figure 4. Bias-dependent STM images of I-POE: 25 × 25 nm2 scan size; constant height mode; (a) bias ) -0.55 V; (b) bias ) -0.80 V; (c) bias ) -0.50 V; (d) bias ) -1.82 V. 3-D isosurfaces displaying charge density contour lines for a selected orbital. The contrast predicted for the ethers by HyperChem was compared to that observed in the STM images.

Results Monolayer Formation and Molecular Arrangement on HOPG. Monolayers of X-POEs formed on cleaved HOPG from 2.5 wt % solutions in phenylhexane. Generally, higher mass and lower temperature, which reduce Brownian motion, favored monolayer formation. Solution deposition of Br-POE and I-POE at 20-22 °C often resulted in the immediate observation of a stable monolayer, whereas H-POE and Cl-POE monolayers were more difficult to obtain. Above 27 °C, monolayers of the X-POEs were not observed. Ease of monolayer formation can be attributed to the presence of van der Waals forces of attraction between the alkyl tails of neighboring molecules and between the alkyl tails and the HOPG substrate.10 Once formed, all X-POE monolayers remained stable for STM imaging. The I-POE monolayer could be sustained for 24 h. The STM image of Br-POE in Figure 2a shows the beginning stage of monolayer formation. Arrow 1 in Figure 2b shows the interface

between two differently oriented domains, whereas arrow 2 identifies a boundary between two domains which exhibit either reverse or identical symmetry but slightly different registration caused by a step edge in the HOPG surface. The organization of the molecules was generally of headto-tail type (Figure 3a), but sometimes a head-tail-tailhead configuration was observed (Figure 3b). In this case, the periodicity was double the molecular length. The arrows in Figure 3b indicate the locations of the head groups. In many STM images, it was observed that adjacent molecules were offset from one another, presumably to reduce the steric hindrance among neighboring head groups. The trend in offset distance, I > Br > Cl, is related to the atomic size of the halogen. Frequently, I-POE molecules formed dimers with opposing tail directions (Figure 8a). This configuration resulted in a lower energy surface, owing to the cancellation of the molecular dipole. Bias-Dependent STM Images. Ordered monolayers of X-POE molecules were imaged as a function of tipsample bias (sample negative: -0.15 to -2.0 V). The monolayers were sometimes observed at -0.15 V; however,

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Figure 5. High-resolution bias-dependent STM images (constant height mode, zoomed to 10 × 10 nm2) and MO electron density contours (HyperChem, AM1 method) of H-POE. (a) Bias ) -0.41 V. The z range is 0.4 nA. (b) Bias ) -1.3 V. The z range is 1.5 nA. (c) HOMO. (d) HOMO-1. (e) HOMO-2. (f) HOMO-3. (g) HOMO-4. (h) LUMO.

most STM images were acquired at biases more negative than -0.50 V. Strikingly, image contrast within the monolayer was dramatically altered by changing the bias. Four STM images of I-POE are presented in Figure 4 which reveal the relationship between STM image contrast and tip-sample bias. The images were acquired during a 30-min experiment in which the sample bias was gradually increased negatively. The image of I-POE acquired at -0.55 V (Figure 4a) showed a bright band whose width was 8-9 Å. At -0.80 V (Figure 4b), the width of this band was reduced to about 4 Å. This change in contrast was reversible when the bias was returned to -0.50 V (Figure 4c). Subsequently, the width of the bright band increased to 22 Å at -1.8 V (Figure 4d). This trend in contrast was observed routinely; however, the magnitude of the bias at which each bandwidth appeared varied from experiment to experiment. For example, STM images of I-POE resembling Figure 4a were observed between -0.26 and -0.70 V whereas images similar to Figure 4b and d were acquired from -0.8 to -1.2 V and from -1.0 to -1.8 V, respectively. In Figures 5-8, high-quality STM images acquired from monolayers of H-POE, Cl-POE, Br-POE, and I-POE, respectively, clearly display the variation in contrast observed for these ethers as a function of tip-sample bias. The contrast can be grouped into three types. One type (Type I) includes a short array of bright spots for each

ether molecule, three for H-POE (Figure 5a), exhibiting an average length of 6.2 ( 0.5 Å, and four for the halogenated ethers (Figures 6a, 7a, and 8a), exhibiting average lengths of 6.8 ( 0.3, 7.0 ( 0.5, and 7.2 ( 0.5 Å for Cl-POE, Br-POE, and I-POE, respectively. These lengths correspond to those of the respective head groups (hereinafter referring to the halogen atom, the phenyl ring, the oxygen atom, and the R-carbon atom) (Table 1). The periodicities of the spot arrays in these images, 29.0 ( 2.0 Å for H-POE, 29.0 ( 0.5 Å for Cl-POE, 29.5 ( 1.5 Å for Br-POE, and 30.5 ( 0.5 Å for I-POE, correspond to the molecular lengths of the ether molecules (Table 1). The second type of contrast (Type II), observed only for I-POE (Figure 8b), consists of one large spot per ether molecule approximately 4 Å in diameter and with a 30 Å periodicity. The large spot was partially resolved into two smaller spots. The images belonging to the third contrast type (Type III), shown in Figures 5b, 6b, 7b, and 8c, reveal a substantially longer array of eight or nine bright spots. Whereas the length of the spot array in images of the first contrast type was unique for each ether, it was constant for the third contrast type, ∼22 Å, indicating the presence of the C18 alkyl tail (Table 1). Molecular Modeling. In addition to providing information about molecular geometry (Table 1), HyperChem was used to predict the energy levels and electron density contours of the molecular orbitals of free-standing X-POE

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Figure 6. High-resolution bias-dependent STM images (constant height mode, zoomed to 10 × 10 nm2) and MO electron density contours (HyperChem, AM1 method) of Cl-POE. (a) Bias ) -0.78 V. The z range is 5.0 nA. Arrow identifies a Cl atom. (b) Bias ) -1.2 V. The z range is 0.7 nA. (c) HOMO. (d) HOMO-1. (e) HOMO-2. (f) HOMO-3. (g) HOMO-4. (h) LUMO.

molecules. These calculations do not take into consideration the effect of molecular physisorption to a substrate or the presence of a tip. However, as suggested by Faglioni et al.,19 the small van der Waals forces of attraction between the molecules and the graphite surface are not expected to change significantly the electronic structure of the physisorbed molecules relative to that of freestanding molecules. Electron density contours of the HOMO, the next four bonding MOs (i.e. HOMO-1, HOMO-2, HOMO-3, and HOMO-4), and the LUMO of the ethers, calculated using HyperChem, are presented with the STM images in Figures 5-8. It can be observed that the two lobes of the phenyl ring and the halogen atom are the strongest contributors to the HOMO contour of each ether whereas the oxygen and the R-carbon atoms contribute weakly (Figures 5c, 6c, 7c, and 8d). Observed differences in the distribution of electron density between H-POE (Figure 5c) and the halogenated POEs (Figures 6c, 7c, and 8d) enable one to verify the presence of a halogen atom. The variation in the size of the halogen spot (Figures 6c, 7c, and 8d) implies that they can be differentiated from one another. In the HOMO-1 contours of the X-POEs (Figures 5d, 6d, 7d, and 8e), the only contribution to electron density is the phenyl ring, again represented as a pair of lobes, but oriented at 90° with respect to those in the HOMO

contours (Figures 5c, 6c, 7c, and 8d). Electron density on the alkyl tail is observed only for bonding MOs below the HOMO. For example, contours for HOMO-2 of H-POE (Figure 5e), Cl-POE (Figure 6e), and Br-POE (Figure 7e) and HOMO-4 of I-POE (Figure 8h) reveal electron density between adjacent carbon atoms along the entire length of the alkyl tail. Figure 9 presents the HOMOs and LUMOs of the X-POEs in an energy level diagram with the Fermi levels of the HOPG substrate and the Pt/Ir tip. Since STM images reflect the partial electron density distribution near the Fermi level of a sample, the table in Figure 9 provides the energies (eV) for the first five bonding MOs (HOMO to HOMO-4) and for the LUMO of each compound. Comparison of STM Images and Electron Density Contours. Highly resolved images of the ethers (Figures 5a, 6a, 7a, and 8a) that resemble the HOMO electron density contours (Figures 5c, 6c, 7c, and 8d) were selected to identify the spots in the head groups. The STM image for H-POE (Figure 5a) shows two bright circular spots that are contributed by the lobes of the phenyl ring, as predicted by HyperChem (Figure 5c). STM images of the halogenated POEs (Figures 6a, 7a, and 8a) reveal three or four spots for the head group. On the basis of the HyperChem results (Figures 6c, 7c, and 8d), two larger

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Figure 7. High-resolution bias-dependent STM images (constant height mode, zoomed to 10 × 10 nm2) and MO electron density contours (HyperChem, AM1 method) of Br-POE. (a) Bias ) -0.92 V. The z range is 5.0 nA. Arrow identifies a Br atom. (b) Bias ) -1.717 V. The z range is 0.7 nA. (c) HOMO. (d) HOMO-1. (e) HOMO-2. (f) HOMO-3. (g) HOMO-4. (h) LUMO.

spots are expected for the phenyl ring, one large spot is predicted for the halogen atom, and one small spot each is anticipated for the oxygen and R-carbon atoms. Apparently, the electron densities revealed for the oxygen and R-carbon atoms in the STM images are sometimes weak, owing to the brightness of the adjacent spot of the phenyl ring, and/or unresolved, owing to the short bond distance of 1.4 Å. Taking into consideration geometric factors (Table 1), as well as the trace of the alkyl tail which appeared in the images, the halogen atoms are identified in Figures 6a, 7a, and 8a by an arrow. Discussion Interpretation of Bias-Dependent STM Images. The contrast patterns observed in the STM images acquired from the X-POEs as a function of bias were compared to those exhibited by the electron density contours calculated using HyperChem (Figures 5-8). Even though the AM1 method is simple and the model contours were calculated for free-standing molecules in the absence of a STM tip, the contours describe the experimental data remarkably well. The HOMO contours of the X-POEs presented in Figures 5c, 6c, 7c, and 8d resemble the Type I contrast pattern shown in Figures 5a, 6a, 7a, and 8a, respectively. The Type II contrast observed for I-POE in Figure 8b corresponds to the HOMO-1 electron density

contour for I-POE presented in Figure 8e. The HOMO-2 contours of H-POE (Figure 5e), Cl-POE (Figure 6e), and Br-POE (Figure 7e) and the HOMO-4 contour of I-POE (Figure 8h) match the Type III contrast exhibited in Figures 5b, 6b, 7b, and 8c, respectively. From these results, it becomes clear that the contrast changes observed in STM images of X-POEs as the bias is increased negatively follow the same trend in “contrast” observed for the electron density contours when proceeding from the HOMO to other bonding MOs. The ability to acquire bias-dependent STM images of X-POEs corresponding to electron density contours suggests that the electronic structure of this class of molecular adsorbates plays an important role in the image contrast mechanism (Figures 5 - 8). Generally, the electronic structure for the X-POEs (Figure 9) includes 1 eV differences between the HOMO, HOMO-1, and HOMO-2 energy levels followed by a quasicontinuous energy band. This unique electronic structure apparently enables the scanning tunneling microscope to probe the individual HOMO, HOMO-1, and HOMO-2/HOMO-4 molecular orbitals. Bias-dependent STM images have been reported for other studies of molecular adsorbates on HOPG, including liquid crystals28,29 and functionalized alkanes.17,18,20,22 These images revealed a contrast representative of the

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Figure 8. High-resolution bias-dependent STM images (constant height mode) and MO electron density contours (HyperChem, AM1 method) of I-POE. (a) 10 × 10 nm2 scan size. Bias ) -0.5 V. The z range is 3.0 nA. Arrow identifies an I atom. (b) Zoomed to 10 × 10 nm2. Bias ) -0.52 V. The z range is 10.0 nA. (c) Zoomed to 10 × 10 nm2. Bias ) -1.7 V. The z range is 1.0 nA. (d) HOMO. (e) HOMO-1. (f) HOMO-2. (g) HOMO-3. (h) HOMO-4. (i) LUMO. Table 1. Geometric Factors for X-POEsa H-POE molecular length (Å) length of head group (Å) length of alkyl tail (Å) X-P1 distance (Å) P1-P2 distance (Å) P2-O distance (Å) P2-CR distance (Å) a

Cl-POE

Br-POE

I-POE

HyperChem

exp

HyperChem

exp

HyperChem

exp

HyperChem

exp

28.0 6.1 22.1 N/A 2.8 1.4 2.4

29.0 ( 2.0 6.2 ( 0.5 21.5 ( 0.5 N/A 3.0 ( 0.5

28.6 6.7 22.1 1.7 2.8 1.4 2.4

29.0 ( 0.5 6.8 ( 0.3 22.0 ( 0.5 1.8 ( 0.2 2.7 ( 0.3 1.4 ( 0.2 2.4 ( 0.5

28.8 6.9 22.1 1.9 2.8 1.4 2.4

29.5 ( 1.5 7.0 ( 0.5 21.8 ( 1.0 1.9 ( 0.1 2.5 ( 0.2 1.6 ( 0.3 2.4 ( 0.5

29.0 7.0 22.1 2.0 2.8 1.4 2.4

30.5 ( 0.5 7.2 ( 0.5 22.3 ( 1.0 1.9 ( 0.2 2.5 ( 0.3 1.5 ( 0.2 2.5 ( 0.5

3.0 ( 0.5

X ) H, Cl, Br, I; P1 ) first lobe of phenyl ring; P2 ) second lobe of phenyl ring; O ) oxygen; CR ) alpha carbon.

substrate at low bias,18,22,28,29 of the substrate and adsorbate at an intermediate bias,18,22 and of the adsorbate alone at a higher bias.18,22,28,29 In 1992, Yackoboski et al. published STM images of n-decanol which revealed biasdependent contrast. Specifically, the hydroxyl group and alkyl tail were observed at low and high bias, respectively.22 Later, Claypool et al. confirmed bias-dependent contrast for n-decanol.18 Recently, Giancarlo et al.20 investigated bias-dependent contrast in three functionalized alkanes. The topographic heights of the functional groups (amide, bromide, sulfide) were compared to that of the hydrocarbon backbone. For octadecanamide, the ratio peaked at |V| ) 1.4 V whereas it remained constant for docosane bromide and octadecyl sulfide for all voltages sampled. The difference in bias-dependent contrast observed for these functional groups was attributed to a

variation in the level of participation of geometric and electronic structure factors. These results are consistent with earlier results from the same research group in which bias-dependent STM images for monolayers of CH3(CH2)21Br and CH3(CH2)21SH were not observed.17 That individual MOs of functionalized alkanes were not differentiated in bias-dependent STM images17,20 may result from an electronic structure which differs from that of the X-POEs. To investigate this assumption, we calculated energy level diagrams and electron density contours for free-standing molecules of CH3(CH2)21Br, CH3(CH2)21SH, CH3(CH2)17S(CH2)17CH3, and CH3(CH2)17CONH217,20 and compared them to those of the X-POEs. Unlike the case for the X-POEs, CH3(CH2)21Br has a quasicontinuous band of bonding MOs including the HOMO. The differences in energy between the adjacent

Physisorbed Phenyloctadecyl Ethers

Figure 9. Energy level diagram containing Fermi levels of the HOPG substrate and the tip as well as the HOMOs (lower lines) and LUMOs (upper lines) of the X-POEs (energies in electronvolts): (a) Fermi energy of Pt; (b) H-POE; (c) Cl-POE; (d) BrPOE; (e) I-POE; (f) Fermi energy of HOPG.

MOs may be too small for STM to probe them individually. The HOMO-1 of CH3(CH2)21SH is situated 2 eV more negative than the HOMO. It appears that this energy gap was not overcome by changing the applied bias under the specified imaging conditions.17,20 In the case of CH3(CH2)17CONH2, a 1 eV energy difference exists between the HOMO and a quasicontinuous band of bonding MOs. Although Giancarlo et al. did not report an STM image corresponding to a HOMO contour, their results did reveal a trend of increasing contrast of the alkyl tail with a higher bias, as predicted by energy level calculations. Image Contrast Mechanism. During the past 10 years, three unique contrast mechanisms have been proposed for STM images of molecular adsorbates: (1) tunneling from a conducting substrate, (2) tunneling from molecular orbitals of adsorbates, and (3) resonance tunneling. Recently, Claypool et al.18 and Giancarlo et al.20 proposed an image contrast mechanism for functionalized alkanes which combines topographic/geometric and electronic structure factors. The bias-dependent STM images acquired in this study of X-POEs suggest that the tunneling process occurs through a resonance mechanism involving the energy states of the adsorbate as well as those of the tip and the substrate. The energy level diagram in Figure 9 indicates that the frontier bonding MOs of the X-POEs are lower in energy than the Fermi levels of the HOPG substrate (experimental,17 -4.70 eV; calculated,40 -4.35 eV) and the tip (experimental,41 -5.64 eV). If the energy level of the tip, as controlled by the bias, is in the vicinity of the adsorbate energy levels, tunneling through those levels is achieved and STM images then reflect the electron density of adsorbate MOs. Even though the HOMOs of the X-POEs lie below the Fermi level of HOPG by 4 or 5 eV, an argument for the coupling between the two energy states can be made, like in the case of Xe on Ni, as described by Eigler et al.,42 and as was related to functionalized

Langmuir, Vol. 14, No. 26, 1998 7483

alkanes by Cyr et al.17 In the energy diagram for Xe adsorbed on a Ni surface, the Xe 5p (HOMO) and 6s (LUMO) levels lie far in energy from the Fermi level of the Ni. Due to adsorption, the LUMO energy is lowered below the vacuum level and is closer to the Fermi level of the Ni than is the HOMO. A calculation of the contribution of the adsorbed Xe to the local density of states of this system revealed that the resonance of the adsorbed Xe 6s develops a long energy tail which extends to the Fermi level of the substrate. The existence of orbital diffuseness in systems of physisorbed molecular adsorbates was described by Claypool et al.,20 who revealed that it plays an important role in defining the overall electronic coupling matrix elements. In addition, Sautet et al. calculated that the orbitals of benzene, which are located far from the Fermi level of Pt(111) (tens of electronvolts), make a significant contribution to an STM image.43 In addition to a bias-dependent contrast, an enhanced tunneling current is expected for a resonance tunneling mechanism. According to this mechanism, the tunneling efficiency increases as the bias is changed to align the Fermi levels of the tip and substrate with the energy levels of the adsorbate’s MOs.44,45 In this study, STM images of X-POEs were acquired in the constant height mode. Small changes in bias, which resulted in STM images exhibiting atomic resolution, were accompanied by a 5- to 10-fold increase in the tunneling current. This enhancement in tunneling current with applied bias has been observed for other physisorbed monolayers10,26,29 and has been attributed to a reduction in the substrate work function32 and to a resonance tunneling mechanism.29 Conclusions X-POEs (X ) H, Cl, Br, I) were prepared using a modified Williamson ether synthesis. The structure and purity of the ethers were confirmed using 1H NMR and GC/MS. Atomically resolved STM images were acquired from monolayers of these molecules physisorbed on HOPG which clearly displayed a bias-dependent variation in contrast. Comparison of the STM images with HyperChem electron density contours suggested a bias-dependent participation in tunneling of the HOMO and adjacent molecular orbitals. This result and the observed enhancement of the tunneling current accompanying the acquisition of high-resolution STM images may support a resonance tunneling mechanism. Acknowledgment. The support of this research by a grant from the Robert A. Welch Foundation is gratefully acknowledged. The authors thank Professor J. P. Ferraris and Drs. M. Hmyene and M. S. K. Dhurjati for helpful suggestions regarding the synthesis of the ethers. The authors are grateful to Professor M. Banaszak Holl for fruitful discussion of results. Supporting Information Available: A description of the modified Williamson ether synthesis used to prepare the X-POEs (X) H, Cl, Br, I) as well as 1H NMR and GC/MS data confirming the structures and purity levels of the ethers (4 pages). Ordering information is given on any current masthead page. LA9712092 (40) Painter, G. S.; Ellis, D. E. Phys. Rev. 1970, 1, 4747. (41) CRC Handbook of Chemistry and Physics, 79th ed.; CRC Press: Boca Raton, FL, 1998-1999; pp 12-124. (42) Eigler, D. M.; Weiss, P. S.; Schweizer, E. K.; Lang, N. D. Phys. Rev. Lett. 1991, 66, 1199. (43) Sautet, P.; Bocquet, M.-L. Phys. Rev. B 1996, 53, 4910. (44) Kuznetsov, A. M.; Sommer-Larsen, P.; Ulstrup, J. Surf. Sci. 1992, 275, 52. (45) Tao, N. J. Phys. Rev. Lett. 1996, 76, 4066.