J. Phys. Chem. B 2001, 105, 10465-10467
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Effect of Chemically Modified Tips on STM Imaging of 1-Octadecanethiol Molecule Qing-Min Xu, Li-Jun Wan,* Shu-Xia Yin, Chen Wang, and Chun-Li Bai* Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China ReceiVed: May 21, 2001; In Final Form: August 6, 2001
The effect of chemically modified tips on STM imaging was investigated. An electrochemical technique was used to prepare a monolayer of sulfur and iodine on the apex of STM tips. The chemically modified STM tips were first employed to image a 1-octadecanethiol (1-C18H37SH) adlayer on a highly oriented pyrolytic graphite (HOPG) surface. The images acquired with S-modified tips could ascertain the structural detail of thiol groups of the adsorbed molecules, while that acquired with I-modified tips appeared in a low resolution. The contrast variations of STM images are attributed to the changes of electronic states of the tips.
Introduction Scanning tunneling microscopy (STM) is a powerful tool for investigating surface microstructure with atomic resolution in ambient, ultrahigh vacuum (UHV), and even electrolyte solution.1-4 However, it is difficult for STM to identify chemical species because the electronic structures of different atoms or functional groups often do not differ sufficiently from one another. The use of chemically modified STM tips seems to be a simple and promising approach to the differentiation of chemical species at an atomic or molecular scale, although photoemission spectroscopy5 and scanning tunneling spectroscopy (STS)6 have also been suggested to be useful for this purpose. Adsorption of an atom or a molecule on an STM tip will change the structure and electronic state of the tip. The contrast of an STM image acquired with a modified tip will vary in accordance with the different chemical species. Therefore, the information contained in the so-acquired STM image can be used to identify different atoms and functional groups. For example, a C60 adsorbed Pt/Rh tip has been reported to yield an improved image of HOPG.7,8 The bright ball-like image of the NO molecule changes to a ring-shaped feature with the use of a CO-modified tip.9 Chemisorbed O atoms at the tip could discriminate between O and Cu or Ni atoms.10 Tips modified with a self-assembled monolayer or a conducting polymer allow to detect functional groups because of chemical interactions such as hydrogen bonds between the functional groups on the tip and the sample.11,12 In the present study, we investigated the effect of chemically modified tips on STM imaging of 1-C18H37SH, an alkanethiol intensively studied for both fundamental research and industrial applications.13-15 A 1-C18H37SH molecular adlayer on the highly oriented pyrolytic graphite (HOPG) surface was first imaged by using STM with I (iodine)- and S (sulfur)-modified Pt/Ir tips. To prevent the formation of multilayers, the conventional tip preparation method was improved. An electrochemical technique was employed to modify the tips by applying electrode potential. This method allows to control surface coverage, structure, and even the molecular orientation of the adsorbates on tip. The experiment was successfully carried out in ambient * Corresponding authors: E-mail:
[email protected]; clbai@ infoc3.icas.ac.cn; fax: (+86)10-62558934.
environment. Details of both the alkyl skeleton and the thiol group were clearly imaged. Experimental Section I- or S-modified tips were prepared by the electrochemical method which we used previously for the preparation of the surface-modified anion specimens for STM studies.16 This method allows to control surface coverage, structure, and even the molecular orientation of the adsorbates on the tip. Briefly, mechanically prepared Pt/Ir (90/10) tips (0.25 mm in diameter) were immersed into an aqueous solution containing 2 mM sodium iodide or sulfide (Kanto Chem., Japan) under potential control. The potential was controlled at the value where a submonolayer or monolayer of I or S was formed.16 The tips were then washed thoroughly by Millipore-Q water. They were characterized by cyclic voltammetry for the confirmation of a monolayer formation. 1-C18H37SH was dissolved in toluene (HPLC grade, Aldrich Inc.). A droplet of the solution was deposited onto a freshly cleaved HOPG surface. After the liquid was dried out, STM experiments were performed on a Nanoscope III SPM (Digital Instruments, Santa Barbara, CA). All STM images shown here were collected in the constant-current mode under ambient conditions at temperature between 22 and 25 °C. Images were obtained with different tips and samples to check the reproducibility and to ensure that the images were free from artifacts caused by the tip, sample, and ambient contamination. Results and Discussion Figure 1 shows a typical STM image of 1-C18H37SH on HOPG acquired with a bare Pt/Ir tip. A herringbone structure can be seen in this image consisting of two reversible V-type lamellae with a period of a ) 8.7 ( 0.02 nm. The lamellae have an essentially identical molecular configuration. Each lamella consists of two molecular rows crossing each other at an angle of 120° ( 2°. The bright band at the center of the two molecular rows indicated by arrows is attributed to the thiol functional groups according to the previous results.13-15 The bright dots in the alkyl chain of the molecules correspond to the locations of the hydrogen atoms of the methylene units of adsorbed molecules.13-15 The length of one molecule is measured to be ca. 2.4 nm, consistent with the molecular
10.1021/jp011916a CCC: $20.00 © 2001 American Chemical Society Published on Web 10/09/2001
10466 J. Phys. Chem. B, Vol. 105, No. 43, 2001
Figure 1. STM image of 1-C18H37SH on HOPG obtained with a bare Pt/Ir tip. Bias was 478 mV, tunneling current 959 pA, and scanning rate 11 Hz. A highly ordered molecular adlayer can be seen. The bright bands indicated by arrows correspond to the location of thiol groups.
Figure 2. STM image of 1-C18H37SH on HOPG obtained with an I-modified Pt/Ir tip. Bias was 464 mV, tunneling current 1 nA, and scanning rate 11 Hz.
structure, indicating that the molecules are oriented in parallel to the HOPG surface. The STM image shows that the alkyl skeletons take the orientation parallel to the plane of the HOPG surface. The herringbone structure is similar to that of alkanol adlayers15 but different from the structure of the 1-C22H45SH adlayer.13,14 The length of the alkyl chain and the sample preparation may be responsible for this difference. A further study of the effect of alkyl chain length on the adlayer structure is in progress. It should be noted that structural details near thiol groups are not apparent in this image. To obtain further understanding of the adlayer structure of 1-C18H37SH, chemically modified tips were used. Figure 2 is a typical STM image acquired with an I-modified tip. This image
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Figure 3. STM image of 1-C18H37SH on HOPG obtained with a S-modified Pt/Ir tip. Bias was 351 mV, tunneling current 882 pA, and scanning rate 11 Hz. Structural details of thiol groups are clearly seen.
was recorded in the same area of the sample as that used for obtaining Figure 1. It can be seen that the contrast of the image is almost reversed compared with that of the image in Figure 1. The bright thiol groups become dark, while the part of the alkyl skeleton near the thiol groups becomes bright. The molecular adlayer appears in a low resolution. From this image it is difficult to determine the orientation of the alkyl chains on HOPG surface. The structural details of 1-C18H37SH molecules cannot be clearly identified. The image resolution is obviously lower than that obtained with the bare Pt/Ir tip. Although effort has been made to the STM observation such as varying bias voltage and tunneling current and preparing high quality tip, no clear image could be achieved. The resolution difference would be attributed to the electronic state of the I-modified tip. On the other hand, S-modified tips were able to obtain higher resolution STM images and ascertain details of the thiol groups. Figure 3 is an STM image acquired with an S-modified tip. The structural details of both thiol groups and carbon skeletons of alkyl chains can be discerned in this image. Especially, the structure of the bright band in Figure 1 corresponding to the thiol groups of 1-C18H37SH molecules is well recognized. It is seen clearly that the bright bonds in each lamella consist of two sets of bright spots indicated by two arrows. The molecules in a lamella lie preferentially “head to head” with thiol groups facing each other. The width of the bright bands in the neighboring lamella is found to be different: 0.56 ( 0.02 nm indicated by arrow A and 0.35 ( 0.02 nm indicated by arrow B, respectively. This configuration may imply the coordination with hydrogen bonds between the neighboring thiol groups in different molecules. Stable STM images of 1-C18H37SH could be seen at positive or negative bias voltages. However, after the modification of I or S, the range of bias voltages became narrower than that with a bare Pt/Ir tip. The results may imply changes of the electronic states of the tips. It is known that the contrast of images recorded in the constant-current mode usually reflects the local density of states of the surface near its Fermi level.17 The imaging mechanism here is similar to that of STM imaging S/Cu(11,1,1).18 The contrast variation of the STM images was dependent on different electronic states of tips. When STM
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Figure 4. A schematic presentation of 1-C18H37SH adlayer structure on the basis of STM observation and theoretical optimization.
imaging a surface, the tip follows a contour of constant density of states at the Fermi level. The path deviates markedly from the contour because of the presence of the sulfur 3p resonance below the Fermi level. On the other hand, owing to electronegativity, the S-modified tip induces a stronger interaction between tip and surface. In this case, the Fermi-level state density of the tip could be diminished and closer to the surface. So the STM image resolution of 1-C18H37SH molecules is improved. Although iodine presents a 5p orbital, the electronegativity of iodine is lower than that of sulfur, which may reduce the resolution of STM images. The theoretical details for the imaging mechanism are in progress. Based on STM images acquired with various tips, the possible arrangements of 1-C18H37SH molecules on HOPG are schematically shown in Figure 4. All of the C-S-H bonds of the molecules are parallel to the HOPG surface and orient in the same direction as in Figure 4a or in reverse as in Figure 4b. In these arrangements, the hydrogen bonded S-H‚‚‚S angle is kept close to 180° and results in the formation of strong hydrogen bonds. The S‚‚‚S distance between two thiol molecules is 0.56 nm in Figure 4a and 0.35 nm in Figure 4b, respectively. The distance is consistent with the width of the bright bands A and B in Figure 3. The two reversible V-type lamellae have a period of a ) 8.7 nm. The model thus built can explain the reversible V-type lamella structure in STM images. Conclusion The contrast variation of STM images is dependent on different surface structures and electronic states of the tips. The results described above show that the different tips produce STM images with different contrasts. By choosing a proper tip, the 1-C18H37SH adlayer can be well resolved. The bare Pt/Ir tip discerns the alkyl skeleton clearly. The S-modified tip reveals the structural details of thiol groups. On the other hand, the I-modified tip causes a change in contrast and results in low
resolution of STM images. The preliminary results presented here suggest that the S-modified tip is useful to distinguish thiol functional groups. Acknowledgment. This work was supported by National Natural Science Foundation of China (No. 20025308), the National Key Project on Basic Research (Grant G2000077501), and Chinese Academy of Sciences. The authors also thank Dr. Y. Okinaka for his help in editing the manuscript. References and Notes (1) Bai, C. L. Scanning Tunneling Microscopy and its Applications; Springer: Shanghai, 1995. (2) Gewirth, A. A.; Niece, B. K. Chem. ReV. 1997, 97, 1129. (3) Moffat, T. P. Scanning Tunneling Microscopy Studies of Metal Electrodes in Electroanalytical Chemistry; Bard, A. J., Rubinstein, I., Eds.; Marcel Dekker: New York, 1999; Vol. 21, p 211. (4) Itaya, K. Prog. Surf. Sci. 1998, 58, 121. (5) Berndt, R.; Gaisch, R.; Gimzewski, J. K. Science 1993, 262, 1425. (6) Kelly, K. F.; Tromp, R. M.; Demuth, J. E. Phys. ReV. Lett. 1986, 56, 1972. (7) Resh, J.; Sarkar, D.; Kulik, J.; Brueck, J.; Ignatiev, A.; Halas, N. J. Surf. Sci. 1994, 316, L1601. (8) Kelly, K. F.; Srakar, D.; Prato, S.; Resh, J. S.; Hale, G. D.; Halas, N. J. J. Vac. Sci. Technol. B 1996, 14, 593. (9) Xu, H.; Ng, K. Y. S. Surf. Sci. 1996, 355, L350. (10) Ruan, L.; Besenbacher, F.; Stensgaard, I.; Laegsgaard, E. Phys. ReV. Lett. 1993, 70, 4079. (11) Ito, T.; Buhlmann, P.; Umezawa, Y. Anal. Chem. 1998, 70, 255. (12) Ito, T.; Buhlmann, P.; Umezawa, Y. Anal. Chem. 1999, 71, 1699. (13) Venkataramann, B.; Flynn, G. W.; Wilbur, J. L.; Folkers, J. P.; Whitesides, G. M. J. Phys. Chem. 1995, 99, 8684. (14) Cyr, D. M.; Venkataramann, B.; Flynn, G. W.; Black, A.; Whitesides, G. M. J. Phys. Chem. 1996, 100, 13747. (15) Claypool, C. L.; Faglioni, F.; Goddard, W. A., III; Gray, H. B.; Lewis, N. S.; Marcus, R. A. J. Phys. Chem. 1997, 101, 5978. (16) Wan, L. J.; Shundo, S.; Inukai, J.; Itaya, K. Langmuir 2000, 16, 2164. (17) Tersoff, J.; Hammann, D. R. Phys. ReV. B 1985, 31, 805. (18) Rousset, S.; Gauthier, S.; Siboulet, O.; Sacks, W.; Belin, M.; Klein, J. Phys. ReV. Lett. 1989, 63, 1265.
