1660 Langmuir 1994,10, 1660-1662

Atomic-Scale Resolution in Atomic Force Microscopy

F. Lin and D. J. Meier'

Michigan Molecular Institute, Midland, Michigan 48640

Received August 18, 1993. In Final Form: April 25,1994'

We raise a question concerning the ability of the atomic force microscope (AFM) to image neighboring atoms of condensed structures. Atomic-scale images produced by the AFM typically show features which do not correspond to the presumed atomic-scale structure; i.e., the symmetry and/or the spacing between satoms" in the image is incorrect. We then discuss the nature and interpretation of atomic-scale imaging by the AFM.

Introduction With the invention' of the scanning tunneling micro-

scope (STM), it became possible to observe conductive materials with atomic-scale resolution. This capability was then further enhanced with the following develop- ment2 of the atomic force or scanning force microscope (AFM), which allowed scanning of nonconductive materi- als, also at the atomic or molecular scale. During the past few years, AFM images of a number of materials a t the atomic or molecular scale have been shown, including graphite, mica, ionic crystals, various adsorbed species, polymers, Langmuir-Blodgett films, etc.%16

It is widely recognized that images produced by scanning tunneling techniques are not necessarily the images that the atomic structure would suggest. Quantum effects govern the behavior of the tunneling current, and thus the nature of the resulting image is governed by local quantum effects. But it is not as widely recognized that an image produced by an atomic force microscope is also not necessarily a direct image of the atomic structure. AFM atomic-scale scanning of condensed structures often produces images whose symmetry and/or spacing between neighboring "atoms" does not correspond to the atomic structure. A further problem arises when Fourier filtering is used improperly, since such filtering can produce atomic- scale features which are artifacts.

The details of the imaging process that results in images which do not correspond to the true atomic symmetry or spacing are important, not only for understanding of the

a Abstract published in Advance ACS Abstracts, June 1, 1994. (1) Binnig, G.; Rohrer, H.; Gerber, C.; Weibel, E. Phys. Rev. Lett.

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(5) Binnig, G.; Gerber, C.; Stoll, E.; Albercht, T. R.; Quate, C. F. Surf.

(6) Rugar, D.; Hansma, P. Phys. Today 1990,43 (lo), 23. (7) Abraham, F. F.; Batra, I. P. Surf. Sci. 1989,209, L125. (8) Meyer, E.; Gihtherodt, H.-J.; Haefke, H.; Gerth, G.; Krohn, M.

Europhys. Lett. 1991, 15 (3), 319. (9) Drake, B.; Prater, C. B.; Weisenhorn, A. L.; Gould, A. C.; Albrecht,

T. R.; Quate, C. F.; Cannell, D. S.; Hansma, H. G.; Hansma, P. K. Science 1989,243,1586. (10) Teao, Y.; Yang, S. X.; Evans, D. F.; Wennerstrdm, H. Langmuir

1991,12,3154. (11) Weisenhorn, A. L.; MacDougall, J. E.; Gould, S. A. C.; Cox, S. D.;

Wise, W. S.;Massie, J.;Maivald,P.;Elings,V.B.;Stucky,G. D.;Hansma, P. K. Science 1990,247, 1330. (12) Chen, C.; Vesecky, S. M.; Gerwith, A. A. J. Am. Chem. SOC. 1992,

114, 451. (13) Weisenhorn, A. L.; Henriken, P. N.; Chu, H. T.; Ramsier, R. D.;

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nature of the imaging process itself, but also from a practical standpoint. The calibration of the atomic size scale is invariably based on the "atomic" spacing of a standard sample, e.g., the atomic spacing of graphite or mica.

Experimental Section The images shown in this paper were obtained using a

TopoMetrix TMX 2000 AFM instrument, using the variable- force mode in the repulsive force (-10" N) region. Images were obtained in air on freshly cleaved surfaces. The images shown have been Fourier filtered, but a comparison of the raw images and the minimally-filtered images showed that no artifacta were introduced by the Fourier filtering.

Results and Discussion An example of an AFM atomic-scale image showing

symmetry which does not correspond to the known atomic symmetry is shown in Figure la. The image is of the basal plane of graphite, and the symmetry is hexagonal, Le., a central atom surrounded by six equispaced neighbors. However, the carbon atoms in the basal plane of graphite have trigonal symmetry, not hexagonalsymmetry; Le., each carbon atom in a graphite plane has three, not six, equispaced neighbors. Thus, this AFM image of graphite, which is similar to those obtained by others,%7 does not correctly display each carbon atom in the graphite surface. An AFM image apparently showing trigonal symmetry has been published: but the image has obviously been heavily filtered and processed, with unknown effects.

In other AFM atomic-scale images of condensed struc- tures which we have obtained or have noted in the literature, the spacing between the "peaks" in the images is too large for the peaks to correspond to the spacing between nearest neighbor atoms. For example (on the basis of the usual graphite size calibration, to be discussed below), the spacing between peaks in the image of a mica surface (Figure lb) is approximately 5.3 A, which is much larger than the spacing between any neighboring pair of atoms on the surface. Also, the images of sodium chloride (Figure IC) (and that of potassium bromide, not shown) give peak-to-peak spacing of 3.49 A for sodium chloride (and 4.70 A for potassium bromide). In both cases, the spacing is much larger than the alkali-metal/halogen ion distances.

Returning to the question of the hexagonal/trigonal puzzle for graphite, there are at least two plausible explanations. The first is based on the inability of the probe tip to actually probe into the space between closely- spaced atoms, with the result that such closely-packed atoms would be imaged as a single peak. Zhong, Overney,

0743-7463/94/2410-1660$04.50/0 0 1994 American Chemical Society

Letters Langmuir, Vof. 10, No. 6, 1994 1661

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Figure 1. Atomic-level AFM ima es: (a, top) graphite, showing hexagonal symmetry and 2.46-f spacing, (b, middle) mica, showing hexagonal symmetry and 5.3-81 spacing, (c, bottom) sodium chloride, showing simple cubic symmetry and 3.49-A spacing.

and Tomanek17 calculated the effect of probing a graphite surface with a single potassium atom. Their results show that in the contact mode with the usual tip forces (- 10-9 N) the corrugation will be insufficient to resolve neigh- boring carbon atoms which have a spacing of 1.41 A. They show that the corrugation depth from one atom to a

(17) Zhong, W.; Overney, G.; Tomanek, D. Europhys. Lett. 1991, 25, 49.

A 8

\ ---z scan dlrectlon

Figure 2. (A) Schematic of the graphite surface showing individualcarbonatoms (ascircles). The AMFimagepeakswhich result from the inability to resolve individual atomsare indicated by ellipses. (B) Model showing the arrangement of carbon atoms in two overlying layers of graphite, in which carbon atoms which are directly above one another are shown with black circles. The lower layer is indicated with dotted lines.

neighboring atom is too small for an AFM probe to resolve the two, and the resulting image will only show one peak for each pair of atoms (shown shaded in Figure 2A). However, the corrugation depth and spacing when tra- versing the hollow "H" are large enough such that the opposite pairs of atoms will be resolved. The resulting image will have hexagonal symmetry as shown in Figure 2A, with an "atom-to-atom" spacing of 2.46 A (not the true 1.41-A spacing between carbon atoms).

The actual size of the ultimate probe tip which interacts with a specimen is not known, although the "macroscopicn radiusof curvature is typically 100-400A. With the usual repulsive scanning forces (- N), it is unlikely that a single atom a t the probe tip is the point of contact with the surface. In the noncontact mode, ultralow attractive forces ( ~ 1 0 - l ~ N) might enable sensing with a single atom a t the tip.16

A different explanation7J8 for the discrepancy between the true symmetry of the graphite surface and that shown by atomic force microscopy is based on the response of the surface atoms to the force exerted by the probe tip. The compliance of a carbon atom to the force exerted by the probe will presumably be less when a carbon atom is directly over a carbon atom in the plane below than when it is not. Hence, there will be a difference in the displacement of a carbon atom depending on whether or not it is over a carbon atom in the lower plane, and the AFM image will show these differences. Half of the carbon atoms in an upper plane are over carbon atoms in a lower plane as shown in Figure 2B by black circles. The AFM image will show these sites as peaks which have hexagonal symmetry and also with a peak-to-peak spacing of 2.46 A.

Regardless of whether or not either of these two explanations (or others) is correct, it is certain that the AFM images of graphite do not show individual carbon atoms. Since the atomic-scale calibration of the atomic force instrument of the major manufacturers is based on graphite (assuming peak-to-peak spacing of 2.46 A), i t is important that a proof be given that this is the correct calibration. We have verified the correctness of this calibration from the image of a highly oriented polyeth- ylene sample. An atomic-scale image of the highly oriented chains (Figure 3) clearly shows the polyethylene chains as well as individual methylene groups of the chains (the image also shows a "hairpin" chain fold, with six methylene groups creating the fold). From analysis of the image and the known dimensions of the polyethylene crystal unit

(18) Batra, I. P.; Ciraci, S. J. Vac. Sci. Technol. 1988, A6, 313.

1662 Langmuir, Vol. 10, No. 6,1994 Letters

Figure 3. Atomic-level AFM image of highly oriented poly- ethylene showing individual chains and also a hairpin chain fold.

5.3 A4/ Figure 4. Crystal structure of mica showing the oxygen atoms which form the bases of S i 4 tetrahedra (shaded triangles) on the 001 cleavage plane. Oxygen atoms are indicated by circles. The peaks which appear in the AFM image are indicated by ellipses.

cell, size calibration based on the 2.46-A apparent lattice spacing of a graphite image showing hexagonal symmetry is verified.

Cleaving of muscovite mica occurs in the 001 plane, with oxygen atoms of the bases of the S i 4 tetrahedra a t the cleaved surface (Figure 4). Since the spacing between the oxygen atoms in these tetrahedra is less than 2 A, the theory of Zhong, Overney, and Tomanek17 would indicate that the corrugation depth and spacing are such that the tip will resolve neither each oxygen atom nor even each tetrahedron. However, the distances between pairs of tetrahedra are sufficiently large that such pairs will be imaged as a single peak (indicated with ellipses in Figure 4). The AFM image will then appear with hexagonal

symmetry and with a peak-to-peak spacing of 5.3 A. This is what we and others9J0 observe.

It is possible to image sodium chloride with what appears to be atomic-scale resolution (Figure 2c). However, the peak-to-peak spacing is much larger than the spacing between sodium and chloride ions. This results from the fact that the atomic size of the chloride ions is much greater than that of the sodium ions; the latter are "buried" and are not sensed by the AFM probe. The distance and depth between chloride ions is sufficient to allow imaging of each chloride ion, giving an observed ak-to-peak spacing of

not shown) corresponding to the distance between halogen ions. These values are very close to crystal lattice spacing between halogen ions, 3.98 and 4.66 A,19 respectively. The observed spacing for sodium chloride is approximately 10% smaller than that for the crystal, perhaps due to surface imperfection,20 or merely to the fact that the surface dimensions do not have to correspond exactly to those of the bulk crystal lattice. Meyer, et aL8 have shown an AFM image of silver bromide, and have arrived at a similar conclusion; i.e., only the bromide ions are imaged.

Conclusions In this paper, we have questioned whether at present

atomic force microscopy in the contact mode of operation can actually resolve all of the nearest neighboring atoms in a condensed structure because of insufficient corrugation depth and/or spacing. Of course, individual atoms can be resolved by AFM if the spacing between them is sufficiently large, as we have shown with the chloride atoms (ions) of sodium chloride, and others have shown with other systems involving adsorbed atoms.12-14 However, in each case, the spacing between the resolved features is larger than would correspond to the spacing between nearest neighbors of a condensed system. As a result, we believe that the ultimate resolution attainable by atomic force microscopy a t present is limited to atomic features which are spaced more than about 2.5 A from their neighbors. If the conclusion of Zhong, Overney, and Tomanek17 conceming the inability of a single (potassium) atom at an AFM tip to resolve neighboring atoms is correct, then enhanced tip sharpness is not the answer to the problem of imaging close-spaced features. The answer may lie in the further development of imaging in the noncontact mode with ultralow scanning forces.16

3.5 A for sodium chloride (and 4.7 r for potassium bromide,

(19) Pauling, L. The Nature of the Chemical Bond, 3rd ed.; Cornell University Prese: New York, 1960; p 526.

(20) Hoffman, R. Am. Sci. 1993,82,11.