J. Phys. Chem. 1993,97, 4764-4768
4764
Electronic Origin of the Low-Symmetry Scanning Tunneling Microscopy Image of the Layered Transition-Metal Halide cu-RuCls J. Ren and M.-H. Whangbo' Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204
H. Bengel and S. N. Magonov' Freiburg Materials Research Center (FMF), Albert- Ludwigs University, Freiburg D- 7800, Germany Received: December 4, 1992; In Final Form: February 18, I993
Scanning tunneling microscopy (STM) and atomic force microscopy (AFM) images of the layered transition-
metal halide a-RuC13 show striking differences. The AFM image shows a hexagonal symmetry of the surface C1 layer, while the STM images at various tunneling conditions exhibit a strong deviation from the hexagonal symmetry. The STM image was examined by calculating the partial electron density p(ro,ef), and the AFM image by calculating the total electron density p(ro), of a single RuQ layer. The patterns of the p(r0,ef) and p(r0) plots are similar to those of the STM and AFM images, respectively. The STM images of a-RuC13 for the surface-to-tip and tip-to-surface tunneling processes are similar, because a-RuCh is a magnetic semiconductor with d5 ions so that the partially filled levels lying at the top portion of the tz,-block bands are involved in both tunneling processes.
Introduction Scanning tunneling microscopy (STM)' provides atomic resolution real-space images of conducting surfaces.2 When the tip-to-surface distance ro is large, the tipsurface interaction is believed to be insignificant3 so that the tunneling current and hence the brightness of an STM image are proportional to the partial electron density p(ro,ef)," Le., the electronic density of the surfaceat the Fermi level erevaluated at the tipto-surfacedistance ro. The STM image can deviate considerably from the p(r0,er) plot (calculated on the basis of a bulk crystal structure) if the tipsurface interaction is stong or if there occurs a significant surface reconstruction. Atomic force microscopy (AFM)Sis an alternative technique to probe surface atomic arrangements. In a contact mode of AFM, a surface profile is examined by probing the repulsive interatomic force between the tip and surface atoms. A brighter spot of an AFM image corresponds to a stronger repulsiveforce the tip feels, a greater charge overlap the tip makes with the surface, and hence a higher electron density region of the surface. Thus, the total electron density of the surface p(r0) is expected to mimic the pattern of the interatomic repulsive forces registered by AFM. Because of the universal nature of repulsive interactions, AFM can be used to probe conducting as well as nonconducting surfaces. Layered materials such as transition-metal halides and chalcogenides are attractive for STM and AFM studies. The individual layers of these compounds are held by weak van der Waals interactions so that surface reconstruction is negligible, and clean surfaces are easily obtained by simple cleavage. Unless the tipsurface interaction is strong, therefore, the STM image of a layered transition-metal compound should be well described by the p(r0,ef) plot calculated for a single layer. Nevertheless, caution should be exercised in interpreting the STM image, because the p(r0,er) plot represents only those electrons lying in the vicinity of the highest occupied level (for the layer-to-tip tunneling) or the lowest unoccupied level (for the tip-to-layer tunneling).4,6-* In the present work, we analyze the experimental STM and AFM images of the layered transition-metal chloride a-RuC13. Earlier it was shown9that the STM images of a-RuC13 obtained for the layer-tetip and tip-to-layer tunneling processes are similar, and that these images are strongly distorted from the hexagonal
lattice of the surface C1 atoms. To explain these observations, we calculate the p(r0,ef)plots of a single RuC13 layer (taken from the X-ray crystal structurelo of a-RuCl3) on the basis of the extended Hiickel tight-binding (EHTB)" method. In addition, AFM images of a-RuCl3 are presented and analyzed on the basis of the p(r0) plot.
Experimental Section a-RuC13 was prepared as described earlieragThe X-ray crystal structure of a-RuC13I0shows that, in each RuCl3 layer of a-RuCl3, the Ru atoms are located at the octahedral sites between two hexagonal sheets of C1 atoms (Figure 1). Two-thirds of the octahedral sites between the two C1 atom sheets are occupied by Ru atoms to form a honeycomb pattern, and the remaining octahedral sites are empty. The RuCls octahedra are slightly squashed so that the shortest Cl-Cl distance within each C1 atom sheet is slightly longer than that between the two C1 atom sheets (Le., 3.46 vs 3.26 A).lo Experimental STM and AFM images were obtained with a commercial scanning probe microscope, Nanoscope 11,equipped with STM and AFM heads. Mechanically-sharpened Pt/Ir tips were used for STM, and Si3N4 and Si tips for AFM. Measurements were performed on freshly cleaved surfaces. Atomic-scale STM and AFM images were usually recorded in the current and force imaging modes, respectively. Rotation of a sample with respect to the scanning direction was used during AFM experiments. The observed images were filtered by using the fast Fourier transform (FFT) procedure to emphasize their periodic features. To exclude a possible influence of the tip shape on the symmetry of the images, a number of experiments were performed with different tips. In the following,we present the most characteristic images. The thermal drift and the nonlinear effects of the piezodrive lead to a variation in the geometrical parameters of the atomic-scale images. To reduce these effects, a series of images were collectedin two scanning directions,and the averaged values were used for image characterization. Our experience with various crystalline layered materials shows that the geometrical parameters derived from the STM and AFM images are correct within the limit of less than 10%. Other experimental details were described el~ewhere.~
0022-3654/93/2097-4764%04.00/0 0 1993 American Chemical Society
The Journal of Physical Chemistry, Vol. 97, No. 18, 1993 4765
STM Image of a-RuC13
TABLE I: Exponents h and the Valence Shell Ionization Potentials H.for Shter-Type Atomic Orbitals XP Yi ti N Hii (eV)
b ‘---a
c13s CI 3p
Ru 5s Ru 5p Ru 4d
2183 1.733 2.08 2.04 5.38 (0.5342)
-26.3 -14.2 -10.4
2.30 (0.6368)
-6.87 -14.9
Hi;s are the diagonal matrix elements (xilHcTxi),where Herris the effective Hamiltonian. In our calculations of the off-diagonal matrix elements H,, = (x,lHc”],), the weighted formula was used. For details, see: Ammeter, J. H.; Biirgi, H.-B.; Thibeault, J.; Hoffmann, R. J. Am. Chem. Soc. 1978,100,3686. The 4d orbitals of Ru are given as a linear combination of two different Slater-type orbitals, and each is followed by the weighting coefficient in parentheses.
Figure 1. Schematic projection view, along thecrystallographiccdirection, of a single RuCI3 layer found in a-RuC13. The Ru atoms are represented by small circles, and the CI atoms by the vertices of regular hexagons. The CI atoms form a hexagonal lattice.
-13
-12
-11
-10
-9
-E
Energy lev) Figure 2. Total and local density of states (DOS)calculated for a single RuC13 layer of a-RuC13. Only the energy region between the top portion of the t*,-block bands and the e,-block bands is shown. The solid line represents the total DOS. The long-dashed, the short-dashed, and the dotted lines refer to the projected DOS values for the Ru atom, the in-plane p orbitals of CI, and the out-of-plane porbital of CI,respectively.
Calculations Given theoxidation states of C1-and Ru3+,thed-electron count for a-RuCl3 is d5 so that the highest occupied bands (Le., the t2,-block bands) of a-RuC13are partially empty. Figure 2 shows the total and projected electronic densities of states calculated for a single RuC13 layer on the basis of the EHTB method.” The energy level efl is the Fermi level calculated by assuming that a-RuC13is a normal metal. A normal metal is described by an electronic state in which the energy levels below the Fermi level are all doubly occupied. a-RuC13 is a magnetic semiconductor and becomes antiferromagnetic below 13 KI2 so that, in the calculations of the p(ro,ef) plot for a-RuC13, we should first consider how to describe the electronic state of a magnetic semiconductor. A magnetic semiconducting state can be represented by an electronic state in which certain band levels are singly occupied.l3 Under this approximation, the energy levels of a-RuC13lying between efl and the top of the tz,-block bands (i.e., ea) may be regarded as singly occupied instead of being empty. (Consequently, some band levels lying below eflare singly filled instead of being doubly occupied.) Thus, it is expected that the layer-to-tip STM image of a-RuC13 is described by the p(r0,ef) plot obtained from the levels lying at the top portion of the tz,-block bands. For a normal semiconductor (Le., nonmagnetic semiconductor), the tip-to-layer STM image is described by the p(r0,ef) plot obtained from the levels lying at the bottom of the lowest unoccupied band.6.7 As mentioned above, the top portion of the tz,-block bands of a-RuC13 is not fully occupied
but partially empty. Thus, in the tip-to-layer tunneling process of a-RuC13, electrons may flow into the top portion of the tz,block bands instead of the bottom portion of the e,-block bands. If so, the tip-to-layer STM image of a-RuC13 should also be described by the p(ro,ef) plot calculated from the levels lying at the top portion of the tz,-block bands. This is indeed the case for a-RuC13 (see below). In the present study, we calculate the p(r0,ef) plots of a single RuCl3 layer on the basisof the X-ray crystal structureof a-RuC13. The atomic parameters employed for our EHTB calculationsare summarized in Table I. Strictly speaking, the simple STM theory’ relating the tunneling current to p(r0,ef) is valid for the ideal case of negligible surfacetip interaction, which is believed to be valid for ro > 4 A,3and is applicableonly to results of ultrahigh vacuum experiments. It is difficult to calculate p(ro,er) for large values of ro,l4especially with the LCAO (linear combination of atomic orbitals) method of electronic structure calculations, because the wave function of the surface practically vanishes for such a large ro value as 4 A. In the STM experiments at ambient conditions, the situation is more complicated. There is evidence that the mechanical force between the tip surface in STM ( l t 7 N) is stronger than the repulsive force in the contact AFM ( l t 9N).ISa In simultaneous STM and AFM experiments the current is not registered until the repulsive force is activated between the tip and surface.15b In addition, Barret et al.lsc obtained atomicresolution STM images of 1T-TaS2from their simultaneousSTM/ AFM measurements with the tip in contact with the sample surface. Their STM image is quite similar to that observed in the traditional STM measurements with no tipsurface contact. Although the tunneIing mechanism at ambient conditions is not well understood, the aforementioned findings suggest that scanning takes place when the tip is very close to, or even in contact with, the surface. Thus, in interpreting ambient-condition STM images, it is justified to use the p(ro,ef) plots calculated for small ro values. In fact, the STM images of a number of layered materials [e.g., ReSe2,7 NbSe3,8 Nb3Xs (X = C1, Br, 1),16 1TTax2 (X = S, Se),I7 and (BEDTTTF)2TlHg(SCN)41E]were successfully analyzed in terms of their p(ro,er) plots calculated for ro = 0.5 A by the EHTB method.]] This ro value was also employed in the present calculations of the p(r0) and p(r0,ef) plots. Results and Discussion All STM images of a-RuCl3 exhibit “distorted centered hexagons” that can be associatedwith the surfacesheet C1atoms. This pattern differs from the honeycomb arrangement of the Ru atoms. The symmetry of the STM images is lower than the hexagonal symmetry of the surface C1 atoms (Figure 3a). Each distorted centered hexagon of Figure 3a is characterized by three sides, 2.91, 3.28, and 3.67 A (Figure 3b), which were obtained after averaging several STM images at different tunneling conditions. The primary cause for the “distorted”images cannot be a tipsurface interaction, because they are found even at
4766 The Journal of Physical Chemistry, Vol, 97, No. 18, 1993
Ren et al.
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Figure 4. p(ro,er) plots of a-RuC13 calculated by employing the energy levels lying within 0.27 eV from the top of the tze-block bands. (a, top) Two-dimensional contour representation. The plot area consists of four unit cells, and a unit cell is indicated by a smaller rhombus. The contour electron/au3. For clarity, the values used are 10, 8, 6, 4, and 2 X bottom sheet CI atoms are not shown. The Ru and the top sheet CI atoms are shown by small and large circles, respectively. A distorted hexagon was drawn in on the basis of the HED spots. (b, bottom) Threedimensional surface representation. The plot area consists of one unit cell.
_ _ Figure 3. (A) 5 T M current image (filtered) ot a-RuC13 obtained with lo, = 5.7 nA and V,,,, = 300 mV. The vertical gray-scale bar indicates image variations [proportional to the logarithm of the tunneling current I, Le., In(f)] in the direction perpendicular to the examined surface. (B) Distorted hexagon of bright STM spots (filled circles and solid lines) deduced from the images of a - R ~ C l 3 . ~For comparison, the regular hexagon of CI atoms (empty circles and dashed lines) expected from the X-ray crystal of a-RuCI:, is also shown. The positions of the bright STM spots are displaced from the CI atom positions by approximately 0.36, 0.58, and 0.80 A. (C) AFM force image of a-RuCI,. Half of the image was shown after filtering. The vertical gray-scale bar indicates the force variation in nanonewtonsalong the direction perpendicularto the surface.
conditions when these interactions are negligible. (The STM measurements are characterizedby the bias voltage vbias, the set tunneling current It,,, and the gap resistance R g a p 3 vbias/Itun. The gap resistance R g a p is a qualitative measure of the tip-surface
distance ro in the sense that Rgapincreases with increasing ro.) With respect to the STM patterns at a high tunneling gap resistance, those at a small tunnelinggap are wide and truncated9 due most likely to tip-surface interactions. However, the STM images at high and small tunneling gaps exhibit a similar distorted symmetry. In an earlier study, the possibility of obtaining an atomicresolution AFM imageof the a-RuC13surface wasdemon~trated,~~ although the quality of the image was not high enough to address the problem of the "distortion" in the STM images. Shown in Figure 3c is a high-quality AFM image of a-RuC13, which exhibits a near hexagonal symmetry. Thus, an identicalsurfaceofa-RuC13 leads to quite different STM and AFM images. To explain this difference, we examine the p(ro,er) and p(r0) plots of a single RuC13 layer. Figure 4 shows the p(ro,er) plots of a single R u C I ~layer calculated by employing the energy levels lying within 0.27 eV from the top of the t2,-block bands (i.e., between en and en of Figure 2). Our calculations with the band levels lying within 0.25 eV below en give a p(ro,er) plot similar to that shown in Figure 4. Thus, the essential characteristics of the p(r0,er) plot do not depend strongly on the "energy window" used for calculations, as long as the energy window is chosen from the top portion of the t2,-block bands. Important findings to note from Figure 4 are as follows: (a) Only the top surface C1 atoms contribute to the p(ro,ef)plot. (b) The spot of the highest electron density (HED) associated with each C1 atom is at a location
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STM Image of a-RuC13 0 0.::::; ....... ...... .......... .......
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