Microscopy: A Surface Structural Tool Shirley Chiang Robert J. Wilson
IBM Almaden Research Center San Jose, Calif. 95120
The scanning tunneling microscope (STM) is a new tool for measuring surface topography on a subatomic scale. The instrument has been used to make three-dimensional, real-space images of solid surfaces with vertical resolution as high as 0.01 A and horizontal resolution as high as -2 A. Scanning tunneling microscopy was developed by G. Binnig, H. Rohrer, Ch. Gerber, and E. Weibel a t the IBM Zurich Research Laboratory in 1982. Their work attracted attention because it provided for the first time a method of measuring the structure of solid surfaces in three-dimensional real space with atomic resolution. In their early work, the technique was applied to such diverse systems as the Si(ll1) I X I reconstruction, reconstructed Au(ll0) and Au(100) surfaces, and chemisorbed oxygen atoms on Ni(ll0) and Ni(100). Binnig and Rohrer were awarded half of the 1986 Nobel Prize in physics for their achievements (I). Since the time of Binnig and Rohrer's pioneering work, the field of scanning tunneling microscopy has grown rapidly; many groups have built new instruments and applied them to various problems. Early practitioners of the art included teams headed by R. M. Feenstra at the IBM T. J. Watson Research Center (Yorktown Heights), C. F. Quate at Stanford University, J. A. Golovchenko a t Bell Laboratories, and N. Garcia a t Universidad Authnoma (Madrid). The popularity of this technique has grown substantially in a short period of time. In 1985 approximately 35 people attended a workshop on scanning tunneling microscopy held in Oberlech, Austria, under the auspices of the IBM Europe Institute. Several of the parti0003-2700/87/A359- 1267/$01.50/0 0 1987 American Chemical Society
cipants displayed beautiful atomic resolution images of surfaces; others were heavily involved in the instrumentbuilding stage. Only two years later, the 2nd International Conference on Scanning Tunneling Microscopy/Spectroscopy (STM '87) attracted more than 300 people, and high-resolution
surface microscopy seemed routine. The instrument has now been applied successfully to such diverse problems as metal surface reconstruction in the presence of chemisorbed molecules (R. J. Behm and co-workers,University of Munich), spectroscopy of electronic states of metals on semiconduc-
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rlgure I. scnematic aiagram snowing me operation of an SIM. me tip is mounted onto a piezoelectric tripod with three athogom1scanners marked x, y. and I.Circles
in me mapilied region reprewnl IndiViduI amms on me tip and hsample, which are separated by d i r lance S. Tunneling current I lrom the tip to me campis is indicated by anow.
ANALYTICAL CHEMISTRY. VOL. 59, NO. 21, NOVEMBER 1. 1987
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surface across an atom$ step, showing the characteristic 12 adatoms per unit ceii and a corrugation of 2 A. Image-processlngIeQlniqws have been used to enhance lhe observationd &toms bwderingthe step in this -290 X 170 A image of filled states of lhe sample.
tors (R. J. Hamers and co-workers, IBM Yorktown Heights), surface states on metals (W. J. Kaiser and R. C. Jaklevic, Ford Motor Co.), scanning tunneling potentiometry (P. Muralt and D. W. Pohl, IBM Zurich), reconstructions of metal alloys (Y.Kuk and co-workers, Bell Laboratories), imaging of charge density waves in layered compounds (R. V. Coleman and coworkers, University of Virginia), imaging of electrodes in aqueous solution (P. K. Hansma and eo-workers, University of California, Santa Barbara),
on me tanaces
imaging of metal clusters on graphite (J. Clarke and co-workers, University of California, Berkeley), and STM lithography (H.-J. Giintherodt and coworkers, University of Basel). Several review articles present more detailed information on scanning tunneling microscopy research (2,3). A schematic diagram demonstrating the operation of an STM is shown in Figure 1.When one applies a bias voltage to a sharp tip within a few atom diameters of the sample under study, a quantum mechanical tunneling cur-
rent can flow from the tip to the sample through the vacuum gap. Because the magnitude of this current depends on the overlap of the electronic wave functions between the tip and the sample, it depends exponentially on the distance S between the two electrodes. For normal samples with work functions of a few eV, the current will change by an order of magnitude as S changes by 1A, so that the current will always flow between the closest protrusions on the tip and the sample. In an STM, the tip is mounted onto three orthogonal piezoelectric scanners that allow its movement in three-dimensional space. As the tip is scanned laterally in a raster pattern across the surface, a feedback circuit maintains a constant tunneling current by moving the tip perpendicular to the sample. In the absence of chemical effects causing spatially varying work functions, the tip will trace a path equidistant from the atoms on the surface. Because we know the voltages applied to the piezoelectric elements, we know the path of the tip and can make a three-dimensional topographic map of the surface under study. In fact, detailed theoretical analysis of the tunneling process in an STM is extremely complicated because, in principle, one must perform a threedimensional tunneling calculation involving wave functions in both the sample and the tip. Our own work bas recently focused on the study of submonolayer coverages of metals on the Si(ll1) surface. We have built an ultrahigh vacuum (UHV)STM with in situ interchange of samples and tips for surface analysis by a system equipped with SAMEEM, XPS,LEED, and sample cleaning and
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t rlgure a. Largearea STM scan 01 the (,Pi3
constniction of NiISi(111).
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x ,119) R23.4' re-
This filled stam Imaged a - 2 5 0 X 130 A BTBB shows the characteristic sixfold rings 01 Silicon adatoms on this low coverage surface. with clear domains of varying orientation 86 well as d i d r e d regions an *mic step. The brimnew is proportional to the h e i m of lhe atom.
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Figure 4. Three-dimensional STM image of a region of Si(l11) 7 X 7 character (top) next to a domain of X $) R30' AgISi(ll1) Structure (bottom).
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he image hss been conbast-enhanced mtiSticBIdifferencingto @ow me upbay of the 2 A corrugation of the 7 x 7 with me 0.2 A COW. gation 01 the \13 sbuc1ure. The -63 A X 50 A I w e of empty states 01 the sample was obtained 1w tip bias 01 -2.0 V and tunneling cunent of 2.0 nA.
ANALYTICAL CMMISTRY. VOL. 59, NO. 21, NOVEMBER 1, 1987
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preparation facilities ( 4 ) . Figure 2 shows a three-dimensional view of the reconstruction of the clean Si(ll1) 7 X 7 surface acros an atomic step, with the characteristic 12 adatoms per unit cell and a corrugation of -2 A. This particular surface has now been observed by many groups and is often used as a test sample for new UHV STMs. When a small amount of nickel is added to the Si(l11) surface, it is possible to form the X R23.4O reconstruction. This surface has only one nickel atom per unit cell of 19 silicon atoms (5).The STM image appears to have six large protrusions a t the corners of the unit cell, plus one isolated hump in each cell. By registering the observed STM image with an Si(ll1) unit mesh, we have successfully made a model that explains this structure and all of the features in our images (6). Figure 3 shows a large-area STM scan of this structure near an atomic step. We have also studied several reconstructions of silver on the Si(ll1) surface (7).By depositing silver on a substrate held a t 500 "C, it is possible to make domains of the (8 X 8) R30' (hereafter called $3) Ag/Si(lll) structure that coalesce along step edges next to regions of the clean Si(ll1) 7 X 7 surface. Figure 4 shows a three-dimensional view of such domains after image-processing contrast enhancement, which facilitates the display of the 2 A high Si adatoms in the 7 X 7 region next to the 0.2 A corrugation of the 8structure. By registering the adatoms of the Si 7 X 7 region with their known binding sites from the Takayanagi DAS (Dimer-adatom-stacking fault) model (8).we have been able to use a computerized least-squares tit to determine that the honeycomb of protrusions observed by the STM in the 8region can be associated with silver atoms in threefold hollow sites (9). An alternative structure has been proposed by E. J. VanLoenen et al. ( I O ) . The STM is an excellent tool for understanding surface structure on an atomic scale on well-characterized surfaces. It can also he used to obtain parameters relating to surface roughness on the scale of a few hundred angstroms. A large number of scientists are now working in the field, and developments are occurring rapidly in the application of the STM to many new types of materials, including organic and biological samples. In addition, a related development is the invention of the atomic force microscoDe bv Binnie, Quate, and Gerber (11). instruneit that permits both atomic resolution topographic measurements of insula. tors and detailed investigations of
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ANALYTICAL CHEMISTRY. VOL. 59, NO. 21. NOVEMBER 1. 1987
interatomic forces. The extraction of chemical information, however-even on samples studied extensively by other, more established techniques-is still quite difficult. Therefore, the use of scanning tunneling microscopy as a general analytical technique will require further research and development. Reference8 innie. G.: Rohrer. H. Re". Mod. Phvs. ,59,-615.'
~ --innip, , G.; Rohrer, H. IBM J . Res. Deu. 1985.30,355. (3) Hansma, P. K.; Tersoff, J. J. Appl.
Phys. 1987,61, R1. Chiang, S.;Wilson, R. J.; Gerber, Ch.; Hallmark, V. J. Vac. Sci. Teehnol., in press. ( 5 ) Hansson,G.V.;Bachrach,R.Z.;Bauer, RiS.; Chiaradia, P. Phys. Reu. Lett. 1981, (4)
46, 1033.
(6) Wilson, R. J.; Chiang, S. Phys. Reu.
Lett. 1981,58,2575.
(7) Wilson, R. J.; Chiang,
S. Phys. Reu. Lett. 1987,58,269. (8)Takayanaa K.; Tanishiro, Y.; Takahashi, s.;T ahashi, M.Surf. Sei. 1985, 164,367.
(9) Wilson.
R. J.: Chiane, S., Phys. Reu.
Shirley Chiang, a research staff member in the Micro, Surface, and Anulytical Science Department, joined IBM in 1983afterreceiuingaPh.D.inphysics from the University of California, Berkeley. Her research interests include the we of the STM to study the relationship of surface structure to the chemical properties of reconstructed surfaces and adsorbates on metals. Robert J. Wilson, also a research staff member in the Micro, Surface, and Analytical Science Department, receiued a Ph.D. in physics from the Uniuersity of California, Berkeley. Prior to joining IBM in 1983, he was inuolued in postdoctoral research a t Bell Laboratories. His research interests include the application of the STM and other surface analysis techniques to study nucleation and growth phenomena and adsorbate structure.
