2986
Langmuir 1993,9, 2986-2994
Imaging and Modification of Au( 11 1) Monatomic Steps with Atomic Force Microscopy Charles A. GOSS,~ Jay C. Brumfield, Eugene A. Irene, and Royce W. Murray* Kenan Laboratories of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27599-3290 Received March 26,1993. In Final Form: August 20,199P
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AFM images of Au(ll1) films on mica substrates show crystalliteplateaus averaging 230 nm across and separated by ca. 20-nm-deepvalleys. Images obtained at relativelylow ( 20 nN) tip-sample forces reveal well-defined monatomic-high (0.24 nm) terraces on top of individualcrystallites. At high tip-sample force (-200 nN), scanningthe AFM tip results in selectiveremoval of Au atoms at and near monatomic terrace edges. Generation of monolayer-deep pits as small as 12 X 12 nm was observed; however, no such deformationswere observed in regions void of defects. In order to compare our observations with those of other researchers, a model is presented for estimation of the AFM tip dimension from high-resolution step profiles of the monatomic terrace edges. The analysis gave a tip radius of curvature of 43 nm, from which further calculation of tip-sample pressures employing Hertzian and Johnson-Kendall-bberta theories was possible. The results suggest that the contact mechanics of the Au surface are a function of defect density within and near the tip-sample contact area. Scanning probe microscopies are powerful toola for the study of surface structure, reactivity, and mechanical properties. Sincethe initial scanning tunnelingmicroscopy (STM)l images of resolved atoms2 and large atomically flat areas,3y4Au(lll)has been widely investigated by STM and atomic force microscopy (AFM)5in studies of nanoscopicsurface topography and diffusion,'-13 corrosion,lP18 underpotential deposition>s21 atomic adlayer structure,%24
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Present address: Burroughs-Wellcome Co., Research Triangle Park, NC. * Abstract Dublished in Advance ACS Abstracts. October 1,1993. (1) Binnig, 6.;Rohrer, H. Sci. Am. 1986,253, 50. . (2) Hallmark, V. M.; Chiang, S.; Rabolt, J. F.; Swalen, J. D.; Wilson, R. J. Phys. Rev. Lett. 1987,59, 2879-2882. (3) Schneir, J.; Sonnenfeld, R.; Marti, 0.; Hansma, P. K.; Demuth, J. E.; Hamers, R. J. J. Appl. Phys. 1988,63, 717-721. (4) (a) Emch. R.: Noeami. J.: Dovek. M. M.: Lane. C. A.: Quate. C. F. J. Appl.'Phys. 1989,65;79-83.' (b) L&g, C. A.; Dobk, M.'M.; Nogami, J.; Quate, C. F. Surf. Sei. 1989,224, L947-L955. (5) Binnig, G.;Quate, C. F., Gerber, Ch. Phys. Reu. Lett. 1986, 56, 930-933. (6) Chidsey, C. E. D.; Loiacono, D. N.; Sleater, T.; Nakahara, S. Surf. Sci. 1988.200. 45-66. (7) Pukam', A.; Blackford, B. L.; Jericho, M. H.; Watanabe, M. 0. Surf.Sci. 1989,217,276-288. (8)Holland-Moritz,E.; Gordon,J.; Borges,G.;Sonnenfeld, R.Langmuir 1991, 7,301-306. (9) Sommerfeld, D. A.; Cambron, T.; Beebe, T. P., Jr. J. Phys. Chem. 1990,94,89268932. (10) Snyder, S. R. J. Electrochem. SOC. 1992,139,5C-8C. (11) Barth, J. V.; Brune, H.; Erti, G.;Behm, R. J. Phys. Rev. E 1990, 42,9307-9318. (12) (a) Oden, P. I.; Nagahara, L. A.; Graham, J. J.; Pan, J.; Tao, N. J.; Li, Y.; Thundat, T. G.; DeRose, J. A.; Lindsay, S. M. Ultramicroscopy 1992,42-44,580-586. (b) Oden, P. I.; Tao, N. J.; Lindsay,S.M. J . Vac. Sci. Technol., B 1993,11, 137-140. (13)Hagan, H. P.; Campbell, P. A.; Smith, K. W.; Turner, R. J.; Walmsley, D. G. Ultramicroscpy 1992,42-44, 587-593. (14) (a) Trevor, D. J.; Chidsey, C. E. D. J. Vac. Sci. Technol., B 1991, 9,964-968. (b) Trevor, D. J.; Chidsey, C. E. D.; Loiacono, D. N. Phys. Reu. Lett. 1989, 62, 929-932. (15) Manne, S.; Massie, J.; Elings, V. B.; Hansma, P. K.; Gewirth, A. A. J. Vac. Sci. Technol., B 1991, 9, 950-954. (16) McCarley, R. L.; Bard, A. J. J. Phys. Chem. 1992,96,7410-7416. J. J.Electroanal. (17) Wiechers,J.;Twomey,T.;Kolb,D.M.;Behm,R. Chem. 1988,248, 451-460. (18) Honbo, H.; Sugawara, S.; Itaya, K. Anal. Chem. 1990,62,24242429. (19) (a) Green, M. P.; Hanson, K. J. Surf. Sci. Lett. 1991,259, L743L749. (b) Green, M. P.; Hanson, K. J.; Scherson, D. A.; Xing, X.;Richter, M.; Ross, P. N.; Carr, R.; Lindau, I. J. Phys. Chem. 1989,93,2181-2184. (20) Hachiya, T.; Honbo, H.; Itaya, K. J. Electroanal. Chem. 1991, 315, 275-291.
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and self-assembledmonolayers.25-28Atom-resolvedimages of bare214110~12b11~17~~22 and monolayer-coated23-2s Au(111) surfaces, as well as images of larger-scale features like crystallite grain structure, monatomic steps, and dislocations, have been reported with b o t h STM24,6-11,14,18,19,25b,2Bb and AFM.12,15,29,30 Following our interest in nanoscopic imaging and modification of electrode surfaces,31we have obtained and present here large-feature AFM images of Au(ll1) vapor deposited onto heated mica. Using commercially available tips, we have been successful in high-resolution AFM imaging of clearly defined monatomic steps atop individual Au(ll1) crystallites. We further demonstrate AFM tip-driven alterations of the shapes of individual Au terraces and creation of monolayer-deep pits in the Au surface. Deliberate and adventitious consequences of AFM imaging in modifying surface structure have been exem(21) (a) Chen, C.; Gewirth, A. A. J. Am. Chem. SOC. 1992,114,54395440. (b) Chen, C.; Vesecky, S. M.; Gewirth, A. A. J. Am. Chem. SOC. 1992,114,451-458. (c) Manne, S.; Hansma, P. K.; Maesie, J.; Elings, V. B.;Gewirth, A. A. Science 1991,251,183-186. (d) Chen, C.; Gewirth, A. A. Ultramicroscopy 1992,42-44,437-444. (22) McCarley, R. L.; Bard, A. J. J.Phys. Chem. 1991,96,9618-9620. (23) Tao, N. J.; Lindsay, S. M.; J. Phys. Chem. 1992,96,5213-5217. (24) (a) Gao, X.; Weaver, M. J. J. Am. Chem. SOC. 1992,114,85448551. (b) Gao, X.; Zhang, Y.; Weaver, M. J. J. Phys. Chem. 1992, W, 4156-4159. (25) (a) Alves, C. A.; Smith, E. L.; Porter, M. D. J. Am. Chem. SOC. 1992,114,1222-1227. (b) Widrig, C. A.; Alves, C. A.;Porter,M. D. J.Am. Chem. SOC. 1991,113,2805-2810. (26) (a) Kim, Y.-T.; McCarley, R. L.; Bard, A. J. J.Phys. Chem. 1992, 8,7416-7421. (b) Kim, Y.-T.; Bard,A. J. Langmuir 1992,8,1096-1102. (27) Sun, L.; Crooks, R. M. J. Electrochem. SOC. 1991,138, L23-L25. (28) Yeo,Y.H.;McGonigd,G.C.;Yackoboeki,K.;Guo,C.X.;Thomson, D. J. J. Phys. Chem. 1992,96, 6110-6111. (29) Thundat, T.; Zheng, X.-Y.; Sharp, S. L.; &on, D. P.;Warmack, R. J.; Joy, D. C.; Ferrell, T. L. Scanning Microsc. 1992,6,903-910 and references therein. (30) Keller, R. W.; Keller, D. J.; Bear, D.; Vasenka, J.; Bustamante, C. Ultramicroscopy 1 9 9 2 , 4 2 4 , 1173-1180. (31) (a) Goss, C. A.; Brumfield, J. C.; Irene, E. A.; Murray, R. W. Langmuir 1992,8, 1459-1463. (b) Brumfield, J. C.; h a , C. A.; Irene, E. A.; Murray, R. W. Langmuir 1992, 8, 2810-2817. (c) Goes, C. A.; Brumfield, J. C.; Irene, E. A.; Murray, R. W. Anal. Chem. 1993,65,13781389. (32) We refer to these as mica/Au(lll) to distinguish them from bulk Au(ll1) single-crystal surfaces, or the Au(ll1) regions present on gold spheres produced by melting Au wire in aflame and subsequent annealing. PreviousSTMstudies'sllindicate thateachofthesesurfaceahasdietinct large-scale topography that depends on the details of preparation.
0 1993 American Chemical Society
Imaging a n d Modification of Au(l11) Monatomic Steps
Langmuir, Vol. 9, No. 11,1993 2987
cleaned high-vacuum evaporator (Model KV-301, Key High plified by imaging and creation of defects in LangmuirVacuum, Nesconset, NY) on an Al holder about 30 cm above the Blodgett f i l m ~ , observations 3~~~ of ledges and etch pits Au source. A 0.64-cm-thick matching Al block, fitted with a during quartz diss0lution,3~ layer-by layer etching of metal J-type thermocouple, was placed on top of the mica. After chalcogenides,36and pattern formation in metal oxides,3’ evacuation to ca. 4 X 1W Torr, a 500-W quartz lamp (KurtJ. calcite,M*39 polymer^$^*^^ and proteins.30*Lesker Co., Clairton, PA) heated the mica/Al assembly from the The AFM tip-driven removal of ultrathin polymer films backside to 573 2 K for 1-1.5 h; a slight pressure rise, to ca. l ~ ~ ~ dependent on HOPG graphite e l e c t r o d e ~is~ strongly 7 X 1W Torr, resulted. Au (Canadian Mint, 99.99%) was on appropriateadjustment of tip-sample forces. Our new evaporated from a resistively heated Mo or W boat (Model S3results on nanoscopically controlled AFM modifications 010, R.D. Mathia Co., Long Beach, CAI at about 0.2 nm/s and 2 X 1PTorr, maintaining the Al/mica temperature at 573 K, to of Au(ll1) add to the growing body of literature on t i p a final film thickness of ca. 150 nm, whereupon heating was sample interactions. terminated. After the substrate cooled radiatively to
