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Synthesis of Hexanedithiolate/Decanethiolate Mixed Monolayer Protected Gold Clusters and Scanning Tunneling Microscope Tip Induced Patterning on the Clusters/Au(111) Surface Wu Yang,*,†,‡ Miao Chen,‡,§ Wolfgang Knoll,‡ and Hualing Deng† Chemistry and Chemical Engineering College, Northwest Normal University, Lanzhou 730070, China, The State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China, and Max-Planck-Institute for Polymer Research, D-55128, Mainz, Germany Received September 4, 2001. In Final Form: February 21, 2002 Hexanedithiolate/decanethiolate mixed monolayer protected gold clusters (DC6/C10-MPCs) with an average core diameter of 1.78 ( 1.1 nm and with an average composition formula Au174(S(CH2)9CH3)31(S(CH2)6SH)19 have been synthesized through a phase transfer method, and cluster monolayer films were formed by self-assembly of the clusters on Au(111) surface. Both the clusters and the monolayer films were characterized by absorption spectra, Fourier transform infrared reflection spectroscopy, 1H NMR spectroscopy, elemental analysis, differential scanning calorimetry and thermogravimetry, transmission electron microscopy, and scanning probe microscopy. Some geometrically well-defined patterns have been made on the cluster film surface by scanning tunneling microscopy (STM) lithography, and detailed etching conditions were investigated. It was found that STM directed patterning on the cluster film surface was mechanically controlled.

Introduction Nanometer-sized metallic and semiconducting particles have been a very active area of substantial research attention over the last 2 decades.1,2 Because such materials represent an “intermediate” dimension between bulk materials and small molecules, they offer a potential for generating unusual chemical, electronic, and physical properties.3 Gold colloids represent one of the most widely studied4 of these nanoparticle systems, and they have a rich history even dating back to the original work of Faraday.5 However, compared with other numerous synthetic procedures of colloidal gold particles,4,6 the description by Brust and co-workers7 of nanoscale alkanethiolate monolayer protected gold clusters (alkanethiolate-MPCs) is more interesting, for it provides one convenient way to prepare materials with properties akin to those of large, robust molecules, namely, stability in air and in solvent-free forms, ease of characterization by standard analytical techniques8,9 and of production in macroscopic quantities and of purification to a single size,10 and convenience of further surface functionalization11-14 * To whom correspondence should be addressed. Email: yangw@ nwnu.edu.cn. Tel: 0086-931-7973004. Fax: 0086-931-7971989. † Northwest Normal University. ‡ Lanzhou Institute of Chemical Physics. § Max-Planck-Institute for Polymer Research. (1) Schmid, G. Clusters and Colloids. From Theory to Applications; VCH: New York, 1994. (2) Haberland, D. Clusters of Atoms and Molecules; SpringerVerlag: New York, 1994. (3) Mayye, K. S.; Sastry, M. Langmuir 1998, 14, 74-78. (4) Hayat, M. A. Colloidal Gold: Principles, Methods, and Applications; Academic Press: San Diego, CA, 1989; Vol. 1. (5) Faraday, M. Philos. Trans. R. Soc. London 1857, 147, 145. (6) Han, M. Y.; Quek, C. H. Langmuir 2000, 16, 362-367. (7) Brust, M.; Walker, M., Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801-802. (8) Templeton, A. C.; Chen, S. W.; Gross, S. M.; Murray, R. W. Langmuir 1999, 15, 66-76. (9) Badia, A.; Cuccua, L.; Demers, L.; Morin, F.; Lennox, R. B. A. J. Am. Chem. Soc. 1997, 119, 2682-2692.

and of immobilization on given surface.15-17 In addition, MPCs can be regarded as three-dimensional (3D) selfassembled monolayer structures, in comparison to alkanethiol self-assembled monolayers (SAMs) on flat gold surfaces (two-dimensional, 2D). Synthesis of functionalized MPC molecules is a prerequisite to their use as multifunctional reagents, catalysts, chemical sensors, and immediates of fabrication of 2D and 3D structures. At present, there are mainly three routes for the preparation of mixed thiolate monolayer protected gold clusters: (i) a one-step route in which mixed thiolate MPCs are synthesized directly from a mixed solution of an unsubstituted-alkanethiol and a ω-functionalized alkanethiol,18,19 (ii) a two-step route in which unsubstituted-alkanethiolate monolayer protected MPCs are first synthesized, followed by a ligand place-exchange reaction between ω-substituted alkanethiol and the MPC cluster solution,20,21 and (iii) an indirect route in which further chemical modifications of ω-functional groups in (10) Whetten, R. L.; Khoury, J. T.; Alvarez, M. M.; Murthy, S.; Vezmar, I.; Wang, Z. L.Stephens, P. W.; Cleveland, C. L.; Luedtke, W. D.; Landman, U. Adv. Mater. 1996, 8, 428. (11) Hostetler, M. J.; Green S. J.; Stokes, J. J.; Murray, R. W. J. Am. Chem. Soc. 1996, 118, 4212-4213. (12) Templeton, A. C.; Hostetler, M. J.; Kraft, C. T.; Murray R. W. J. Am. Chem. Soc. 1998, 120, 1906-1911. (13) Templeton A. C.; Hostetler, M. J.; Warmoth, E. K.; Chen, S.; Hartshorn, C. M.; Krishnamurthy, V. M.; Forbes, M. D. E.; Murray, R. W. J. Am. Chem. Soc. 1998, 120, 4845-4849. (14) Chen, S. W. Langmuir 1999, 15, 7551-755. (15) Andres, R. P.; Bielefeld, J. D.; Henderson, J. I.; Janes, D. B.; Kolagunta, V. R.; Kubiak, C. P.; Mahoney, W. J.; Osifchin, R. G. Science 1996, 273, 1690-1693. (16) Gutierrez-Wing, C.; Santiago, P.; Ascencio, P.; Ascencio, J. A.; Camacho, A.; Jose-Yacaman, M. Appl. Phys. A 2000, 71, 237-243. (17) Bethell, B.; Brust, M.; Schiffrin, D. J.; Kiely, C. J. Electroanal. Chem. 1996, 409, 137-143. (18) Chen, S. W.; Murray, R. W. J. Phys. Chem. 1999, 103, 999610000. (19) Leff, D. V.; Brandt, L.; Heath, J. R. Langmuir 1996, 12, 47234730. (20) Hostetler, M. J.; Templeton, A. C.; Murray, R. W. Langmuir 1999, 15, 3782-3789.

10.1021/la011389m CCC: $22.00 © 2002 American Chemical Society Published on Web 04/19/2002

Cluster Patterning on Gold

MPCs are carried out through convenient chemical procedures, such as amidation reaction between carboxylic acid group and amine, esterification reaction between carboxylic acid group and alcohol, substitution reaction between amine and halogen, and so on.13 The first route is the most economic and time-saving one. Ordered films of alkanethiolate MPCs have been formed on surfaces and studied with transmission electron microscopy (TEM)22 and scanning tunneling microscopy (STM).23,24 The ligand monolayer is found to isolate the metal core allowing single electron effects to dominate the cluster’s electron transport properties even at room temperature.25 These phenomena are very interesting for future electronic devices and they may form the basis of room-temperature single electron transitions.26 However, to use coated clusters in future devices, it will be necessary to pattern a surface such that only selective areas are covered by the clusters. STM-tip-induced patterns on the surfaces of flat thiolate/ Au SAMs have been widely studied,27-30 and surface etching mechanisms have also been evaluated.27,28 But few STM characterizations of MPC/Au monolayer films have been made, and especially STM-tip-directed patterns on a MPC/Au monolayer surface have not been demonstrated so far. To test the properties of MPC monolayer films and the STM patterning conditions, in this present paper, hexanedithiolate/decanethiolate mixed monolayer protected gold clusters (DC6/C10-MPCs) with an average core diameter of 1.78 nm were synthesized by a one-step route, and self-assembled monolayers of these clusters on gold(111)/mica surface were imaged by STM and tapping mode atomic force microscopy. Moreover, the STM patterning conditions were systematically investigated. Experimental Section Chemicals. 1,6-Hexanedithiol (Aldrich, 96%), decanethiol (Aldrich, 96%), sodium borohydride (Sigma-Aldrich, 98%), tetraoctylammonium bromide, and toluene (Aldrich, 99.9%) were used as received. Deionized water was prepared by a Milli-Q purification system, resistance is 18.2 MΩ‚cm). Substrate Preparation. Au-coated substrates for scanning tunneling microscopy and atomic force microscopy were prepared by thermal evaporation of 100 nm of gold on freshly cleaved mica annealed in N2 atmosphere (BAE 250 coating system, BALZERS). Au was deposited at a pressure of ∼1 × 10-6 mbar at a rate of 0.1 nm/s with constant sample rotation to ensure uniform deposition. Self-assembly experiments of nanoparticles were carried out immediately after the Au-coated substrates were annealed at 650 °C under a constant flow of N2 for several minutes to decrease the surface roughness of the evaporated layer. Cyclic voltammetric and STM experiments supported the characteristics of 200-300 nm of gold (111) terraces. For FTIR reflection spectroscopy the substrates were prepared by thermal evaporation of 2 nm of chromium followed by 100 nm of gold on freshly (21) Ingram, R. S.; Hostetler, M. J.; Murray, R. W. J. Am. Chem. Soc. 1997, 119, 9175-9178. (22) Harfenist, S. A.; Wang, Z. L.; Alvarez, M. M.; Vezmar, J.; Whetten, R. L. J. Phys. Chem. 1996, 100, 13904-13910. (23) Durston P. J.; Palmer, R. E.; Wilcoxon, J. P. Appl. Phys. Lett. 1998, 72, 176-178. (24) Durston, P. J.; Schmidt, J.; Palmer, R. E.; Wilcoxon, J. P. Appl. Phys. Lett. 1997, 71, 2940-2942. (25) Dorogi, M.; Gomez, J.; Osifchin, R.; Andres, R. P.; Reifenberger, R. Phys. Rev. B 1995, 52, 9071-9077. (26) Fulton, T. A.; Dolan, G. J. Phys. Rev. Lett. 1987, 59, 109-112. (27) Nyffenegger, R. M.; Penner, R. M. Chem. Rev. 1997, 97, 11951230. (28) Schoer, J. K.; Zamborini, F. P.; Crooks, R. M. J. Phys. Chem. 1996, 100, 11086-11091. (29) Schoer, J. K.; Crooks, R. M. Langmuir 1997, 13, 2323-2332. (30) Delamarche, E.; Hoole, A. C. F.; Michel, B.; Wilkes, S.; Despont, M.; Welland, M. E.; Biebuyck, H.; J. Phys. Chem. B 1997, 101, 92639269.

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Figure 1. Absorption spectrum of hexanedithiolate/decanethiolate mixed monolayer protected gold clusters in toluene solution with toluene as blank. annealed BK7 glasses in N2 atmosphere. Prior to the evaporation of gold, the glasses were cleaned by soaking them in a piranha solution (30% H2O2 + 70% H2SO4) for 10 min and then ultrasonically washing successively with deionized water and ethanol and drying in a stream of N2. Monolayers of mixed thiolate MPCs on Au/mica surface were prepared by placing the substrates in a toluene solution of MPCs with a concentration of 1-2 mg/ mL for 4 h, removing them from solution, rinsing with toluene and absolute ethanol, and then drying under a stream of N2. DSC and TGA Measurements. TGA analysis was performed on a Mettler 50 thermogravimetric analyzer (heating rate 10 °C/min, N2 atmosphere). Differential scanning calorimetry (DSC) data were obtained on a Mettler DSC 30 thermoanalysis system (heating rate 10 °C/min, N2 atmosphere). Spectroscopic Measurements. FTIR reflection spectra were recorded using a Nicolet Magna 850 FTIR spectrometer (series II) equipped with a reflection accessory and a liquid-N2-cooled system. All spectra are a sum of 50 or fewer individual scans. Absorption measurements were taken from λ ) 400 to λ ) 800 nm on a Lambda 9 UV-vis-NIR spectrophotometer (PerkinElmer, USA) in 1.0 cm path length quartz cuvettes with toluene as blank. 1H NMR spectra (in CDCl3) were obtained with a Bruker WS 250 MHz spectrometer. Transmission Electron Microscopic Measurements. The samples for transmission electron microscopy (TEM) were prepared by dropping a dilute solution containing the nanoparticles on a carbon-coated copper grid and allowing the solvent to evaporate. TEM micrographs were obtained with a Pilips CM12 electron microscope operated at 100 kV. Scanning Probe Microscopy (SPM). Scanning tunneling microscopic imaging and patterning of SC6/C10-MPCs films were investigated with a Nanoscope II scanning probe microscopy (Digital Instruments, Santa Barbara, CA). Mechanically cut Pt-Ir (90/10) tips (Goodfellow, England) were used for all STM experiments. The STM tip-sample bias voltage and tunneling current setpoint were respectively kept as 300 mV and 0.2 nA for STM imaging. Tapping mode atomic force microscopic imaging was made by a Nanoscope III scanning probe microscope (Digital Instruments, Santa Barbara, CA), using a Si3N4 cantilever. Synthesis of Hexanedithiolate/Decanethiolate Mixed Monolayer Protected Gold Clusters (DC6/C10-MPCs). Hexanedithiolate/decanethiolate mixed monolayer protected gold clusters were prepared by a slightly modified Brust reaction.7 To a vigorously stirred solution of 1.5 g of tetraoctylammonium bromide in 80 mL of toluene was added 0.43 g of HAuCl4‚3H2O in 25 mL of deionized water. The yellow HAuCl4‚3H2O aqueous solution quickly cleared, and the toluene phase became orange brown as the AuCl4- species was transferred into it. The organic layer was isolated, and desired amounts of thiols (overall mole ratio of thiol to Au was 3:1 and decanethiol/hexanedithiol ) 9:1) were added. After the mixture was vigorously stirred for 15 min at room temperature, 0.38 g of NaBH4 in 25 mL of deionized water was added. After the mixture was continually stirred for another 4 h, the organic phase was collected, and the majority

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Figure 2. (a) Transmission electron microgram of hexanedithiolate/decanethiolater mixed monolayer protected gold clusters. (b) Highly resolved transmission electron micrograms of the clusters. of the solvent was removed on a rotary evaporator. Then 50 mL of ethanol was added, and the black rough product was separated by centrifugation and washed successively with ethanol and acetone and then dried in vacua for 1 h. The product was proved to be spectroscopically pure (1H NMR). It can be dissolved in dichloromethane, chloroform, toluene, cyclohexane and other nonpolar solvents. Anal. Found: C, 11.94; H, 1.99; S, 5.21; Au, 80.86. Calcd for Au174C424H898S69: C, 11.98; H, 2.13; S, 5.21; Au, 80.68. Mixed monolayer MPC products, following their decomposition with iodine, were analyzed by NMR. Iodine-induced decomposition quantitatively liberates the thiolate ligands as disulfide, within which the relative quantities of alkanethiolate and alkanedithiolate can be analyzed by proton NMR. A typical analytical procedure could be found in the literature.31 The proton NMR of the disulfides from iodine-decomposed mixed monolayer MPCs dissolved in CDCl3 exhibited peaks at δ (ppm) 0.8(1H), 1.2(5.20H), 1.6(1.86H), and 2.7(1.22H). The relative intensities of the methyl group (δ ) 0.8 ppm, residual -SH showed peaks in this region too) and R-methylene group peaks (δ ) 2.70 ppm) revealed the MPC to have a CH3(CH2)9SH/HS(CH2)6SH ligand mole ratio of 1.69:1, which translated to ca. 19 HS(CH2)6SH and 31 CH3(CH2)9SH ligand molecules per MPC. It is very consistent with elemental analysis and thermogravimetric results. DC6/ C10-MPCs in CDCl3 had very similar 1H NMR spectrum to their I2-decomposed succession except for broader bands.

Figure 3. FT IR reflection spectra of hexanedithiolate/Au, decanethiolate/Au, and hexanedithiolate/ decanethiolate mixed monolayer protected gold clusters/Au films.

UV-vis Absorption Spectrum. Figure 1 shows the absorption spectrum of decanethiolate/hexanedithiolate mixed monolayer protected gold clusters (DC6/C10-MPCs) in toluene. This absorption spectrum was obtained after a light brown MPC solution was filtered with a 0.2 µm nylon filter. One plasmon absorption band is located at λ ) 521 nm. It is a typical characteristic for particulate gold solutions ascribed to a collective oscillation of the conduction electrons in response to optical excitation.32 However, Figure 1 only shows a broad, weak, shoulder characteristic due to the small particle size. It is similar to the results of Duff et al.33 for gold colloid solution without

large organic stabilizing molecules. In that reference,33 it was found that the plasmon band was only present for a scarlet solution of 4.3-nm gold particles but nearly absent (shoulder) for an orange-brown solution of 1.4 nm gold particles. Kreibig et al.34 also found a dramatic increase in width of plasmon band of metal clusters with decreasing grain size. The weakening of the plasmon absorption band can be attributed to quantum-size effects accompanying the reduction of the gold particle dimensions. Thermoanalysis (TGA). TGA is a convenient method for determining the organic weight fraction of Au thiolateMPCs, because they have been known to decompose thermally to give elemental gold residue and volatile disulfides.35 The mixed monolayer capped nanocrystals DC6/C10-MPCs showed a maximal rate of weight loss of

(31) Aguila, A.; Murray, R. W. Langmuir 2000, 16, 5949-5954. (32) Bohren, C. F.; Huffman, D. R. Absorption and Scattering of Light by Small Particles; Wiley: New York, 1983. (33) Duff, D. G.; Baiker, A.; Edwards, P. P. Langmuir 1993, 9, 23012309.

(34) Kreibig, U,; Genzel, L. Surf. Sci. 1985, 156, 678-700. (35) Hostetler, M. J.; Wingate, J. E.; Zhong, C. J.; Harris, J. E.; Vachet, R. W.; Clark, M. R.; Londono, J. D.; Green, S. J.; Stokes, J. J.; Wignall, G. D.; Glish, G. L.; Porter, M. D.; Evans, N. D.; Murray, R. W. Langmuir 1998, 14, 17-30.

Results and Discussion

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Figure 4. (a) Topographical STM image of CH3(CH2)9SH/HS(CH2)6SH mixed monolayer MPC film on Au(111) surface: setpoint is 0.2 nA; bias voltage is 300 mA. (b) Topographical AFM image of the MPC film on Au(111) surface: setpoint is 1.047 V; tapping mode. Table 1. Mode Assignments for DC6/C10-MPCs/Au Film and Corresponding Thiolate/Au SAMsa

18.98% at 223 °C, roughly consistent with elemental analysis. Taken with the average core size data from TEM (discussed in the last paragraph), TGA, elemental analysis, and 1H NMR results yielded an average number of thiolate ligands per MPC. DC6/C10-MPCs clusters had an average composition formula of Au174(S(CH2)9CH3)31(S(CH2)6SH)19. In the DSC spectrum of the MPCs there was a strong endothermic transition extending from 210 to 230 °C with a peak near 223 °C (3.8 W‚g-1) which is higher than the boiling point of neat hexanedithiol (bp 118-119 °C) but lower than that of neat decanethiol (bp 240 °C). It was evident that this endothermic peak was derived from thermal decomposition of DC6/C10-MPCs. Transmission Electron Microscopy (TEM). Figure 2a shows a typical TEM micrograph of the MPCs. The sample is characterized by a broad size distribution. The average size was measured by TEM as 1.78 ( 1.1 nm, which was very close to the STM result deducting the ligand monolayer thickness. This value can be used to extract a single quantity, the core mass, or approximate number of Au atoms per cluster using the density (59 atoms/nm3) of bulk face-center cubic (fcc) Au: NAu ) (59 nm-3)(π/6)(Deff)3 ) 174, where Deff is the effective core diameter of MPCs. Figure 2b is a highly resolved transmission electron microgram of the clusters; monodisperse nanoparticles can be distinguished clearly. Fourier Transform Infrared (FT IR) Spectroscopy. Infrared spectroscopy is an important tool for the study of monolayers on metal nanoparticles. The frequency and bandwidth in the C-H and C-C stretching regions, in particular, are indicative of order in the 3D SAM.12,36 Figure 3 shows the FT IR reflection spectrum of the DC6/ C10-MPCs/Au film. The S-H stretching band is too weak to be found. In the 2800-3000 cm-1 region, the methyl and methylene stretching vibration region, the intensities of MPCs are much weaker than those of the corresponding monothiolate and dithiolate SAM, which is attributed to the poor ordering of the methylene chains. Symmetric and antisymmetric methylene C-H stretching frequencies of the MPCs/Au film were respectively 2849 and 2921 cm-1, slightly higher than those of SH(CH2)6SH/Au SAMs (2848 and 2918 cm-1, respectively) and of CH3(CH2)9SH/ Au SAMs (2850 and 2920 cm-1, respectively, here the symmetric methylene stretching frequency of CH3(CH2)9-

SH is a little higher). The increase in vibration energy (higher wavenumber) and broadening of the stretching peaks indicate a slightly lower monolayer ordering of the alkyl chains and a higher population of gauche defects in the MPCs/Au films compared to the corresponding alkanethiolate/Au SAMs. In this region, the much higher intensity of methylene stretching vibrations relative to the methyl ones supports the idea that multilayers do not play a significant role in monolayer formation in MPCs.37 By comparison of the energies of the symmetric and the antisymmetric stretching modes of the methylene group, we conclude that the ordering of the alkyl chains in the mixed monolayer MPCs was higher than that in liquid films of CH3(CH2)9SH or HS(CH2)6SH on silicon but lower than that of the corresponding CH3(CH2)9SH or HS(CH2)6SH gold MPCs. For liquid film samples, ν(CH2) and νa(CH2) are at 2854 and 2924 cm-1 for decanethiol, and at 2855 and 2930 cm-1 for hexanedithiol, respectively. But for MPCs samples, they shifted to 2850 and 2920 cm-1 for

(36) Shon, Y. S.; Gross, S. M.; Dawson, B.; Porter, M.; Murray, R. W. Langmuir 2000, 16, 6555-6561.

(37) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559-3568.

position (cm-1) DC6/C10-MPCs/Au CH3(CH2)9SH/Au HS(CH2)6SH/Au assignment 2958(w) 2921(vs) 2882(w) 2849(s) 1478(s) 1409(s) 1348(w) 1251(w) 900(w) 859(m) 784(w) 738(w) 706(m)

2954(s) 2920(s) 2872(s) 2850(s) 1456(s) 1465(s) 1414(s) 1342(m) 1252(m) 891(w) 770(w) 750(w) 719(m)

2954(s) 2918(s) 2870(s) 2848(s) 1456(s) 1465(sh) 1409(s) 1338(m) 1252(s) 892(w) 857(w) 786(w) 758(w) 723(m)

νa(CH3) νa(CH2) ν(CH3) ν(CH2) R(CH3) δ(CH2) δs WE ω(CH2) β Px Px Px ν(C-S)

a In this table the following abbreviations were used: ν(CH ) 3 and νa(CH3) are symmetric and antisymmetric methyl C-H stretching, respectively; ν(CH2) and νa(CH2) are symmetric and antisymmetric methylene C-H stretching, respectively; ν(C-S) is C-S stretching; R(CH3) is methyl asymmetric bending; δ(CH2) is methylene scissoring; β is methyl rocking; ω(CH2) is non-in-plane rocking mode of methylene; W is methylene wagging; P is methylene rocking-twisting; E is end-gauche defect; x is unknown portion of various progression bands.

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Figure 5. 600 × 600 nm STM images of hexanedithiolate/decanethiolate mixed monolayer protected gold cluster films before and after patterning a nominally 200 × 200 nm feature. Imaging conditions: setpoint ) 0.2 nA, bias voltage ) +0.3 V, scan rate ) 4.35 Hz. Patterning conditions: setpoint ) 0.2 nA, scan rate ) 78 Hz, bias voltage +2.00 V (a), +2.50 V (b), +2.55 V (c), +2.60 V (d), +2.80 V (e), +3.00 V (f); four scans.

decanethiolate/Au MPCs, and to 2848 and 2918 cm-1 for hexanethiolate/Au MPCs, respectively. See Table 1. Scanning Probe Microscopy. The morphology of the MPC layers on a Au(111) surface was imaged using scanning tunneling microscopy (STM) and atomic force microscopy (AFM). Distinct surface morphology could be observed. STM grain analysis gave a diameter of d ) 2.9 nm for maximum population clusters, which is larger than the TEM result by 1.1 nm. This difference is compatible to the calculated thickness of a decanethiol layer of 1.2 nm. So, STM tip broadening effects are not so remarkable. It is because the employed STM tip is sharp enough. Figure 4a shows a region of a cluster monolayer on the Au(111) surface imaged by STM. The measured film thickness of 2.6 ( 0.4 nm confirms the formation of a monolayer of the MPCs which was also supported by the IR results. STM section analysis results showed that the clusters had an average vertical dimension of 2.6 ( 0.3 nm and an average lateral dimension of 15.3 ( 0.6 nm. Figure 4b is a typical topographical image of the MPC film. An AFM grain analysis program supported that the clusters had a mean grain diameter of 3.0 nm. And the section analysis of the AFM images gave an average vertical dimension of 2.5 ( 0.3 nm and an average lateral dimension of 17.0 ( 3.3 nm for the MPCs. STM-Tip-Induced Patterns. Some geometrically welldefined patterns have been successfully written into the cluster films by removing the cluster molecules in selective regions, leaving behind a stable cluster-free area. The detailed patterning conditions have also been tested. An example of patterning of the cluster film is shown in Figure

5. As shown in Figure 5, however, some disordered region around the primary pattern was clearly observed due to accumulation of the removed clusters and long-range interaction between the tip and sample. Patterns with lateral direction of 10 nm and above can easily be prepared by scanning the cluster film surface four times at high scan rate (78 Hz) and at high bias voltages (higher than +2.55 V (positive bias indicates electron tunneling from the tip to the substrate)) with the z-piezo feedback of the STM being enabled. Smaller patterns (down to 5 nm) were also possible, but welldefined shapes were difficult to obtain. It was found that when the setpoint was kept to the imaging current of 0.2 nA, no patterns could be obtained until the bias voltage exceeded +2.55 V. This value is slightly higher than that of n-alkanethiolate/Au SAMs. It was found that the patterning follows a different mechanism from that for flat n-alkanethiolate SAM etching by the STM tip for which the primary patterning mechanism is electrochemical.29 We found no significant influence of the environmental humidity on the etching process, and patterning could be easily finished under convenient laboratory conditions. We also found a very interesting phenomenon in our experiments that the etching was more sensitive to the current setpoint than to bias voltage. At an imaging bias of 0.3 V no patterning occurred until the applied setpoint was higher than 0.45 nA. If the setpoint exceeded 0.5 nA, although the bias voltage was much less than the imaging bias (for example, 10 mV) etching could be easily carried out. At the same

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time due to a too short tip-surface separation, it was always accompanied with sample surface destruction. Our patterning experiments were made under convenient laboratory conditions without controlling the environmental humidity on purpose. To ascertain whether the humidity performed an important function in patterning procedure, we took contrast tests with the flat hexanedithiolate/Au SAMs and the flat decanethiolate/ Au SAMs. It was found that under the same humidity conditions as the cluster film etching, patterning of these two SAMs by STM tip did not occur at all. Obviously, failure to pattern on the corresponding flat alkanethiolate/ Au surfaces was attributable to the lack of necessary electrochemical environment (due to too low humidity). On the basis of the mention above, the predominant patterning mechanism should be not an electrochemical mechanism but a mechanical one. Effects of the scan rate and number of patterning cycles were also examined. It was showed that the scan rate had no detectable influence on the etched situation of the cluster films. However, when the scan rate was too low, thermal shift would make the pattern so irregular and the patterned size so difficult to control. So in our etching experiments a scan rate of 78 Hz was employed throughout. During imaging when sample surface was scanned two times at high scan rate the cluster morphology would be more distinct and better resolved, but more scans would get image blurred. Under the bias conditions for patterning, the number of patterning cycles influenced the depth of patterns and patterning thoroughness. At least four cyclic scans were

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necessary for removing all cluster molecules in the patterning area. However more cycles would make the disordered region larger around the primary pattern. At the same time, it was shown that the smaller the clusters were, the more difficult they were removed, because the average size of the residual clusters among patterning area decreased with the increasing of patterning cycle number when it was less than 4 times. Therefore, it is possible that smaller clusters are more strongly bound to the surface than larger ones and so are less easily disturbed by STM tip interactions. Similar results were also found in previous STM imaging of small metal clusters on graphite.38 When the selective clusters were removed by a STM tip on a freshly exposed gold surface, the herringbone restructure of gold (111) could be distinguished. It was one more proof that the clusters were immobilized on a gold (111) surface. Acknowledgment. W. Yang is thankful for a postdoctoral stipend from the Max-Planck-Society. M. Chen gratefully acknowledges the support from the Alexander von Humboldt Foundation. And the authors are also grateful to CXJJ-001 of Northwest Normal University and National Natural Science Foundation of China (29875018) for their financial support. LA011389M (38) Carroll, S. J.; Weibel, P.; von Issendorff, B.; Kuipers, L.; Palmer, R. E. J. Phys.: Condens. Matter 1996, 8, L617-L624.