Langmuir 1991, 7,442-443
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Morphology of Gold Islands on Highly Oriented Pyrolytic Graphite As Studied by Scanning Tunneling Microscopy Lisa Strong and D. Fennel1 Evans* Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455
Wayne L. Gladfelter Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455 Received September 13,1990 The morphologyof discontinuousgold filmson highly oriented pyrolytic graphite (HOPG) was investigated by using scanning tunneling microscopy (STM).Thermally evaporated films showed roughly triangular or diamond-shaped islands ca. 100 to 200 nm wide and 20 nm high composed of steeply rising stacked layers of (111) planes.
Introduction We have used scanning tunneling microscopy (STM) to image gold islands deposited on highly oriented pyrolytic graphite. Deposition of precious metals to form clusters, islands, or thin films is important in many industrial processes such as catalysis,' fabrication of microelectronic devices: and manufacture of computer boards.3 In such applications, the material properties of the nanoscale structures are determined by the initial nucleation and growth process. STM has been utilized to determine the structure of extremely small (6 to 20 atoms) gold clustersH and thick films,7-l0but has not been used to characterize island sizes which may be of industrial importance. In this paper, we show how STM measurements readily provide threedimensional structural information on such nanoscale materials.
Experimental Procedures Thermal Evaporation: Gold (99.99% purity) was vapor deposited onto a freshly cleaved HOPG surface at 104 Torr from a heated tungsten filament. Nominal film thicknesses and deposition rates were measured in situ by using a goldcoated quartz crystal microbalance. Three-dimensional morphology of gold clusters comprising 10 to 15 A thick films were evaluated as a function of two deposition rates, 0.03 and 3.0 A/s. These films were invisible to the naked eye. Work done by Arthur and Cho on this system indicate that at early stages of deposition, the sticking coefficient rises exponentially from 0 to 1." This change corresponds with the increasing coverage of the graphite surface from 0 to 100% gold. These very thin films fall within the steeply rising portion of the curve, which indicates that the measured film thicknesses are equivalent within the accuracy of the quartz crystal microbalance. (1) Eppell, S.; Chottiner, G. S.; Scherson, D. A.; Pruett, G. Langmuir 1990,6, 1316. (2) Ghandi, S. K. VSLZFabricationPrinciples; John Wiley and Sons: New York, 1983. (3) Moffatt, R.; Minten, K.; Yang,X.; Evans, D. F. J. Vac.Sci. Technol., in press. (4) Abraham, D. W.; Sattler,K.; Ganz, E.; Mamin, H. J.; Thomson,R. E.; Clarke, J. Appl. Phys. Lett. 1986,49,853. (5) Ganz, E.; Sattler, K.; Clarke, J. J. Vac. Sci. Technol. 1988,6 (21, 419. (6) Ganz, E.; Sattler,K.; Clarke,J. Am. Phys. SOC.1988,60(18),1856. (7) Binning, G.; Rohrer, H. Proc. ZX IVC ZCSS 1983,77. (8)Brodde, A,; Tosch, S.;Neddermeyer, H. J. Microsc. 1988,152,441. (9) Wiill, Ch.; Chiagn, H. S.;Wilson, R. J.; Lippel, P. H. Phys. Reu. B: Condens. Matter 1989,39,7988. (10)Lang, C. A.; Dovek, M. M.; Nogami, J.; Quate, C. F. Surf. Sci. 1989,224, L947. (11)Arthur, J.; Cho, A. Surf. Sci. 1973,36,641.
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Figure 1. Images of thermally depositedgold islands on HOPG substrate obtained by (a) scanning tunneling microscopy (STM) and (b) scanning electron microscopy (SEM). The STM image is unfiltered and was obtained by plotting vertical displacement of the tip with the feedback loop set to maintain constantcurrent (constantcurrent mode). The SEM image was obtained at 5 kV; the bar located at the bottom of the micrograph represents 1 pm.
After deposition, samples were transferred to a commercially available Nanoscope I1 STM from Digital Instruments for analysis with 80% Pt/20% Ir tip electrodes. STM results were compared to results obtained by scanning electron microscopy (SEM),and X-ray photoelectronspectroscopy (XPS) was
0 1991 American Chemical Society
Letters
Langmuir, Vol. 7, No. 3, 1991 443
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Figure 3. Unfiltered high-resolution constant current STM
image of Au islands deposited on HOPG substrate at 0.3 nm. Vertical distance between the arrows is 0.34 nm.
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Figure 2. (a) Unfiltered constant current STM image of Au
islands deposited on HOPG substrate at 0.003 nm/s. Vertical distance between the arrows is 8.4 nm. When this image is replotted and truncated in the manner shown in part b, the 0.35 nm steps which comprise the islands are clearly evident (this is the vertical distance between the arrows). used to determine film composition. All films contained small amounts (between 1.8 and 8 % ) of oxygen.
Results and Discussion A large scale STM image and an SEM micrograph of the same region of a thermally evaporated gold sample are shown in Figure 1. Both techniques show identical
large scale morphology suggesting that initial growth of islands occurs along the ledges present on the graphite surface. It is difficult, however, to use SEM to obtain three-dimensional information on smaller scale features such as island height. Figure 2 shows an STM image of several gold islands deposited on graphite. The cross-sectional profiles illustrate the three-dimensionalstructure implicitin the image. Atomic steps and ledge distances are readily discerned. The higher resolution image shown in Figure 3 permits quantitative structural evaluation. Analysis of 23 islands shows that for a 3A/s deposition rate, 79%of the measured steps are one atomic layer thick, 21 % of the steps are two atomic layers thick, and all ledges are less than 75 A wide. The effect of deposition rate on the structure of the islands was also determined. With a 0.03 A/s deposition rate, analysis of 14 islands shows that only 14% of the steps are one atomic layer thick, 86% of the steps are 2 to 4 atomic layers thick, and all ledges are greater than 75 wide with 46% greater than 175 A. When the top surface of the islands is imaged, a hexagonal atomic lattice with a nearest neighbor distance of 2.8 A is observed. This observation is consistent with the well-established fact that vapor deposited facecentered cubic crystals grow with the close-packed (111) face parallel to the surface. These observations illustrate how scanning tunneling microscopy provides three-dimensional information on nanoscale structure. The ability to consistently and nondestructively obtain atomic resolution on such materials constitutes a significantadvance in our ability to elucidate nucleation and initial growth mechanisms.
Acknowledgment. Support by the Center for Interfacial Engineering (CIE), a National Science Foundation Engineering Research Center, is gratefullyacknowledged.
