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Surface valence charge distributions and scanning tunneling microscopy of WTe2
280
Surface
Science 23X (1990) 280-288 North-Holland
Surface valence charge distributions and scanning tunneling microscopy of WTez S.L. Tang, R.V. Kasowski,
A. Suna and B.A. Parkinson
E.I. DuPont de Nemours & Co., Cenlrul Research & Development Department, Experlmenral Statmn, Wilmington, DE 19898. USA Received
21 March
1990; accepted
for publication
28 June 1990
We have studied the surface electronic structures of the van der Waals surfaces of tungsten ditelluride (WTe,) with first principles calculations of the spatial distribution of the surface valence charge densities and compared the results to images obtained with the scanning tunneling microscope (STM). The energy- and z(distance from the surface)-dependent calculations show that the valence charge density distribution above the Te surface could be derived from the surface Te layer, as we previously calculated, but the charge density distribution close to but below the Fermi energy has a distortion that coincidentally makes it appear to have a symmetry close to the paired, zig-zag and buckled rows of the W layer. These results dramatically illustrate that in highly covalent compounds, the surface valence charge density distribution does not necessarily follow the surface atomic positions even on ideal. unreconstructed surfaces. An alternative interpretation of the STM images of this surface is proposed in light of this new surface electronic structure. Our calculated and experimental results are also discussed with reference to recent STM results on other transition metal dichalcogenides.
1. Introduction The layered transitional metal dichalcogenides have been extensively studied with scanning tunneling microscopy (STM) especially because many of them exhibit charge density waves transitions [l]. Moreover, the flatness, the inertness even in water [2], and also the ease of obtaining nearly defect-free, atomically clean surfaces make these materials extremely suitable for STM studies and may offer opportunities for novel STM applications. Because these materials consist of two different elements in two different layers - the chalcogen at the surface and the transition metal underneath - there has been some controversy concerning the role played by the subsurface metal layer in their STM images. The problem was made more complicated by the fact that the surface chalcogen and the subsurface metal layers both have identical hexagonal symmetries in the compounds studied [3-51. Since STM in the topographical mode cannot distinguish between different elements by their chemical nature, it was not 0039-602X/90/$03.50
@ 1990 - Elsevier
Science Publishers
possible to definitively determine the effect of the subsurface layer in these studies. We have discussed this controversy in detail earlier [6]. WTe2 has a different structure from the compounds studied to date. The distinctive structure of this surface described briefly below allows us to investigate the influence of the subsurface metal layer on the STM images of the surface. In our previous study [6] of the WTe, surface, we observed images that resembled the structure of the subsurface W layer. These images, however, were not explained by the first principles pseudofunction calculation performed on the surface valence charge density around the Fermi energy at the surface. The goal of the present study is twofold. First we would like to reconcile the difference between experimental and calculated results; and second, further investigate the effect of the subsurface metal layer on the surface electronic structure of these compounds. To these ends we have improved our calculations of the spatial distribution of the surface valence charge density of WTe, as
B.V. (North-Holland)
S. I.. Tung et ul. / Sur~&e calence charge distnhutions
functions of distance from the surface Te atoms and energy. These calculated results will be compared to our STM images on WTeZ. The significance of these results will also be discussed with reference to other recent STM images on related surfaces. The structure of WTez is briefly described here. A detailed account of the structure can be obtained elsewhere [7]. Like many transition metal dichalcogenides, WTe, crystals are made of XMm X (X = chalcogen, M = transition metal) layers separated by van der Waals0 gaps. 0 WTe, has a surface unit cell size of 3.5 A x 6.3 A. The surface Te layer is made up of undulating rows of Te atoms. The height difference between the top and bottom Te atoms is 0.61 A. The subsurface W layer forms paired, zig-zag and slightly buckled rows. The metal-metal bonding resulted from the pairing reduces the perpendicular distance between the two W rows to 2.2 A and also pushes one member of the pair upward by 0.21 A. The W layer is therefore distinctly different in symmetry from the surface Te layer. The mode1 structure in fig. 1 summarizes the structural features of the surface Te and subsurface W layers. The low
und STM of WTe,
281
energy electron diffraction (LEED) pattern of the surface shows spots with the symmetry of the unit cell of the unreconstructed surface [8].
2. Experimental The experiment was carried out in ambient atmosphere with two different scanning tunneling microscopes. The first microscope is of our own design and has been described briefly elsewhere [9]. The second microscope is a commercial instrument [lo]. The tunneling images were obtained with the STM operating in both the constant current mode and the constant height mode. Tips used were either electrochemically etched tungsten or platinum wires. Results obtained from different operational modes and different tips are essentially the same. The WTe, samples were grown by iodine vapor transport. Flat, shiny thin crystals were chosen for these experiments. WTe, is metallic. All results on clean surfaces reported here were taken within three hours of cleaving a new surface and during this time, large clean areas (over a thousand angstriims squared) were readily located. X-ray photoemission spectroscopy was used to ensure that the sample surfaces were essentially free of contaminants except for a small amount of carbon.
3. Results 3. I. WTe,
Fig. I. A W layers outhned. lower Te
schematic drawing of the surface Te and subsurface in WTe,. The surface unit cell of 3.5 Ax6.3 A is Big open circle: upper Te atom, big hatched circle: atom. small closed circle: upper W atom, small open circle: lower W atom.
images
Fig. 2 shows a 33 A x 33 A greyscale image obtained on the WTe, surface with the constant height mode. The average current was 1.1 nA and the sample bias was -0.04 V. In the lower half of the image, the paired, zig-zag and buckled rows characteristic of the W layer is observed. In the middle of the scan, there was a tip change as indicated by the abrupt horizontal break in the picture. The atomic rows of the surface after the tip change appear to have the symmetry of the top-most Te layer. This image dramatically illustrates the different kinds of images we obtained on this surface: W-like, Te-like and a mixture of
282
XL. Tang et al. / Surface valence charge dmtributions and STM of WTe,
of four Te-W-Te sandwiched layers. The van der Waals region between two of the Te-W-Te sandwiched layers has been expanded by 3 A so as to emulate the condition of a surface. The other two van der Waals gaps are left unchanged. The surface charge density plots obtained are those of the surface inside the expanded van der Waals gap.
Fig. 2. A 33 AX 33 A greyscale image of the WTe, surface. The average current was 1.1 nA and the sample bias was -0.04 V. The lower half of the image resembled the paired, zig-zag symmetry of the W layer whereas the upperhalf of the image had the symmetry of the top-most Te layer.
both. The difficulty in interpreting images such as that in fig. 2 prompted us to perform the calculations below.
3.2.1. Energy-dependence Fig. 4a presents two plots of the charge density of the van der Waals surface of WTe, at (i) (E,-0.5 eV)
Te w Te
3.2. Valence charge density calculations of the WTe, surfaces
Te w
We have performed first principles pseudofunction calculations of the spatial distribution of the valence charge densities on the WTe, surface as functions of the height from the surface and energy. The method used is similar to the one used previously on WTe, [6] in which the top-most Te atoms were found to be the centers of the surface valence charge density. The present calculations are aimed at refining the previous result to obtain information on the energy- and z(distance from the surface)-dependences of the Te charge densities. The basis function set has been doubled and the number of k points sampled for the charge density plots has been tripled when compared to the previous calculation. The geometry of the structure used for the present calculations is shown in fig. 3. The structure contains two bulk unit cells
Te
Te w Te
Te W Te
Te Fig. 3. A schematic
drawing of the structure the calculation.
of WTe,
used in
d~sfribu~io~s and STM of W’Te?
283
bii
bi 7.91
3.96 7 3 h
0.00
-3.96
I-
-72% 5 -4..3:
0.00 x
ta.UJ
4.35
-4.35
0.00 x
4.35
ia.u.)
Fig. 4. (a) The spatial distribution of the valence charge density at a distance of 1.32 A above the top-most Te surface at (i) (E, - 0.5 eV) <: E < E, and (ii) EF < E -C ( EF + 0.5 ev). The atomic positions are marked as follows: Big open circle: upper Te, big hatched circle: lower Te, small closed circle: upper W, small open circle: lower W. The highest constant charge contour in each plot is labeled in relative units. The intervals of each successive contour are 0.3 unit in (i) and 0.5 unit in (ii). (b) The spatial distribution of the valence charge density witbin the energy windows (i) (E, - 0.5 eV) i E < E, and (ii) E, c E < (E, + 0.5 e\q_ The distance from the surface top-most Te atom is 1.9 A. The atom postions are tabled as in (a). The intervats of each successive contour are 0.03 unit in fi) and 0.045 units in (ii).
284
S.L. Tang et al. / Surface r&wcr
lower row of Te atoms. The shift of the charge density from the top-most Te atom positions coupled with the asymmetric shapes of the charge clouds give an overall paired. zig-zag and buckled appearance to the constant charge contours on this surface. The distance between the centers of the two paired Te charge clouds is 2.4 A. The charge density distribution due to the unfilled states (from E, to E, + 0.5 eV) centers around the top-most Te atoms with only a slight distortion. 3.2.2. Distance dependence Figs. 4bi and 4bii show another pair of charge density plots with the same energy windows as those in fig. 4a. The heights from the surface Te layer for these plots are z = 1.9 A. The charge density distortion around the top-most Te atoms remains essentially the same as at z = 1.32 A, i.e. it is independent of the distance from the surface. There are however some subtle changes at this height in addition to the overall decrease in charge density. The asymmetric shapes of the charge clouds are not so distinct and the charge density around the lower Te moves a little off the Te atom position such that the perpendicular distance between the two adjacent rows is about 2.9 A. The charge intensities of the two paired charge clouds are very weak but almost equal. The overall symmetry of the charge density distribution at this height is somewhat different from that at z = 1.32 A. We have also calculated the charge density distribution within the same energy window of E, - 0.5 eV to E, in the middle of the unexpanded van der Waals gap (1.44 A from the top-most Te atoms). Again the charge density centers directly on the top-most Te atoms.
4. Discussion 4. I. Energy dependence The energy-dependent spatial distribution of the valence charge density reveals an interesting and surprising phenomenon: the valence charge density distribution below 0.5 eV of E, shifts off
charge drsrrrhurms
und STM of U’Te,
the top-most Te atoms even though the calculation is based on the perfect, unreconstructed surface structure. The shift is a “surface effect” since it is not found in the charge density distribution within the regular van der Waals gap. There the charge clouds center on the top-most Te atoms up to the middle of the van der Waals gap. The detailed mechanism for the shift in the positions of the charge centers when the bulk structure is terminated is not clear without a complete picture of the bonding charge density distribution. It is reasonable, however. to suggest that the underlying W layer provides the driving force for the distortion of the charge density distribution. Otherwise it would be difficult to understand why the charge clouds originally surrounding the Te atoms should shift in a particular direction (in this case toward the W atom that is buckled downward) when there is a surface symmetry plane running through these Te atoms. The pairing of the adjacent W rows to form W-W bonds and the resulting buckling of these rows in the subsurface W layer destroys this symmetry plane. We did not detect this distortion of the charge density distribution on the WTe, surface with our previous calculation on this surface because the previous charge density was averaged over an energy window straddling E, , i.e. (E,. - 0.45 eV) < E < (E, + 0.45 eV). The charge density distribution obtained was the sum of those of the filled states and the unfilled states. which has the approximate symmetry of the undistorted Te atoms. To see this, one needs only sum the charge densities of fig. 4ai and fig. 4aii. It is clear that the resulting plot will be dominated by the charge density distribution in fig. 4aii because the magnitude of the charge in fig. 4aii is twice that of fig. 4ai. We used this energy window previously because different sample bias polarities did not seem to produce a difference in our STM images. By integrating the charge density over a larger energy window. better signal to noise ratio for the charge density distributions calculated at large distances from the surface was obtained. Fig. 5 shows two images taken simultaneously with sample bias voltages of -t 0.03 and -0.03 V. The two images are essentially identical. The near identity of the two images presents another problem in the con-
S. L. Tang et al. / Surface valence charge distributions
text of the present calculations because the calculated results do predict a sample bias polarity dependence. This problem will be discussed below. Note also that the paired, zig-zag and buckled rows are not observed in these images. 4.2. Height dependence The height-dependent calculation is performed at distances within an angstrom outside of the surface. Calculating charge density at larger distances from the surface is not feasible because the charge contours will become too weak to be meaningful. Our results should be valid for tunneling in ambient atmosphere because tunneling on layered materials has been shown to occur so close to the surface that large forces exerted by the tip on the surface [ll-131 are routinely observed. Also, the height at which the charge density distortion as described in fig. 4ai was observed was probably the lower limit. The same amount of charge distortion could possibly be observed at a greater distance from the surface than calculated if the large charge distribution distortion induced a slight shift in the atomic positions to follow the positions of the charge clouds (analogous to the atomic displacement in charge density wave formation [14]). The calculated results can be used to explain the experimental images if one considers the well-
Fig. 5. Two greyscale
image of the WTe, surface
and STM of WTe,
285
accepted tunneling theory that STM contours are proportional to the surface total valence charge density for materials with extended Fermi surfaces [15]. The STM images showing the paired, zig-zag and buckled rows have almost the same symmetry as the surface charge density distribution when tunneling from the surface valence filled states. The charge density on the surface is most likely derived from the surface chalcogen layer because of its proximity to the surface, even though states around the Fermi energy are mixed such that some contribution from the W layer is also possible [7,16]. These results effectively show that features in STM images cannot be associated directly with atomic positions even on this ideal, unreconstructed surface. Although the STM theory shows that STM images reflect the electronic structures rather than the atom positions [17], and experiments on surfaces with strong surface states such as Si(1 ll)-2 X 1 [18] have nicely demonstrated this effect, STM images on transitional metal dichalcogenides [3-51 and metal surfaces without reconstruction [19] are frequently compared directly to surface topographical structures. The distortion in the spatial distribution of the surface charge density gives the coincidental resemblence to the paired, zig-zag and buckled rows of the W layer. Indeed the resemblence is quite remarkable. At distances close to the surface, the perpendicular distance between the two paired
taken simultaneously
with sample biases of (a) - 0.03 V and (b) + 0.03 V.
286
XL. Tang et al. / Surface ualence charge distrihutrons and STM of WTe,
charge clouds is 2.4 A, extremely close to the perpendicular distance of 2.2 A in the paired W rows. Because of this, we have previously interpreted the STM images to be those of the subsurface W layer. Detection of the subsurface layer has been thought to be possible in other transition metal dichalcogenides [3,5]. The present interpretation of the STM results, that the surface valence charge density is imaged, offers advantages over our previous interpretation that the atomic positions of the subsurface W layer was imaged. A similar charge density calculation [20] of the surface valence charge density on the graphite (HOPG) surface produces a charge density distribution plot at E, that shows that every other atom in the surface hexagon of graphite has enhanced charge density due to interactions with the atoms in the subsurface layer, exactly as observed in STM images [21]. Graphite has an extremely small Fermi surface (the Fermi surface collapses to one k point at E, [17]). Yet the tunneling theory seems to be adequate to explain the experimental images. WTe,, being a metallic material with a more extended Fermi surface, should be more appropriately explained with the existing tunneling theory. The interpretation that the subsurface W layer is imaged cannot be reconciled with this theory. We have also looked into several possibilities that might cause the observation of the W layer, but none of them seem satisfactory. A previously considered possibility [6] of surface reconstruction has been proven energetically unfavorable by a total energy calculation of the assumed reconstructed surface. Moreover, we considered the previous speculation that there might be some unique features in the tunneling matrix element in WTe, that make it possible to observe the second layer at least occasionally with the STM. One such feature that suggests itself would be that the tunneling matrix element was wave-vector selective, which has been shown to be likely to occur in low dimensional materials [17]. The effect of wave-vector selective tunneling can be demonstrated in the calculation of charge density distribution for HOPG [20]. The difference in heights between the two adjacent C atom positions in graphite is already apparent in the total valence charge density
distribution, although the difference seems small. The different heights manifest themselves in the surface charge density plots at E,. Since the graphite band structure has an anomaly at E, where the band collapse to one k point, the charge density plots at E, that explain the STM image is in effect k-specific. WTe, does not have such an anomaly in its band structure, but since the band does cross E, at a narrow range of k points, the calculations in this study are therefore also to an extent k specific. We suspect that calculating the charge density at specific k points would enhance the charge distortion rather than introduce new features, as in the case of HOPG. The second feature that we considered was the possible involvement of the tip’s electronic structure in the tunneling process. The STM theory [15] assumes the tip states to be spherical, i.e. mostly s-like. However, recently it has been shown that the assumption of tips having s-wave is not a stringent requirement for the tunneling theory to work on metallic surfaces [22]. Lastly, we considered the possible existence of a resonance enhancement of the tunneling probability between the d states of the tungsten tips and the d states of the subsurface tungsten layer. The results obtained with Pt tips, which are identical to those obtained with W tips, however, again make this possibility unlikely. There are obviously other possibilities that we could also have considered, for example. tip-pressure induced local reconstruction, local variations in the tunneling barrier, etc. However, in view of the findings in the calculations presented here, we did not think it necessary to pursue these rather speculative assumptions. The comparison between the calculated surface valence charge density distribution and the experimental images based on the well-accepted tunneling theory can explain the observation of paired, zig-zag and buckled rows in the STM images taken with a negative sample bias, but do not explain the undistorted Te-like images and the lack of difference in the images taken with different sample bias polarities. We have previously attributed the multiple symmetries observed on this surface to be images from multiple-atoms tunneling, and the fine structures of the paired, zig-zag and buckled rows were obtained only with
S. L. Tang et al. / Surface valence charge distributions
the sharpest tips. Because of the corrugated nature of the surface, unequal tip sharpness can be a problem that is difficult to identifiy and avoid. Recent calculations [23] show that when a cluster of W atoms is taken as the tunneling tip, images obtained even on a flat surface like graphite can be complicated. The tip problem on this surface is further complicated by the height dependence of the surface charge density distribution. For example. as the tip moves away from the surface because of a change in the tip geometry, the symmetry of the surface charge distribution switches from the paired, buckled and zig-zag rows to resemble somewhat that of the surface Te atoms. We attempted to minimize tip problems by choosing only data that were obtained with the most stable tunneling currents and the best tunneling gap stability, and with identical forward and backward scans. The paired, buckled and zig-zag rows were scanned in perpendicular directions to ensure that the features were not due to multiple tip atoms or the surface single-atom wide ridges scanning a blunt tip. Without an independent means such as field ion microscopy to determine the sharpness of the tips, it is extremely difficult to decide whether certain images are a result of multiple-tip effects. Aside from multiple-tip effects, it is interesting to note that the potential (van der Waals interaction) that causes the charge distribution distortion on the WTe, surface is extremely weak. The multiple images could perhaps also be caused by a combination of strong perturbations from the strong field of the tip and the high polarizability of Te. In other transition metal dichalcogenides such as MoTe,, MO&, etc., it is possible to obtain STM greyscale images showing two sets of hexagons, a bright one and a faint one superimposed onto each other [5,6]. It is believed that one set of hexagons represents the surface chalcogenide layer while the other represents the metal layer [5]. In WTe,, we have shown that the STM images of the surface are better explained with surface charge density distribution rather than atomic positions. However, from the results on WTe, alone, we cannot in a straightforward manner draw the same conclusion on the semiconducting materials such as MO&, MoTe,, etc. i.e., the images of the
and STM of WTe,
287
surfaces of these materials do not coinside with the atomic positions of the surface atoms. Because of the electronic and structural differences between the metallic WTe, with distorted octahedral metal coordination, and the semiconducting group of transition metal dichalcogenides with trigonal prismatic metal coordination, a similar calculation should be applied to these materials in order to better explain the observation of the two sets of hexagons.
5. Conclusion In conclusion, our energy dependent first-principles calculations of the surface valence charge density distribution of WTe, show that the termination of the bulk structure can lead to a subtle change in the charge density distribution on the surface, even though there is no gross physical rearrangement of the surface atoms. This change is likely due to the interaction between the surface charge density and that of the subsurface metal layer. The resulting shift of the charge clouds from the top-most Te atoms closer to the lower Te atoms on the corrugated surface creates a symmetry very similar to that of the underlying W layer. What we previously interpreted as the images of the subsurface W layer can be the images of the charge density distributions that are not in registry with the surface atomic positions. We supported the present conclusion by comparing our calculations to those performed on graphite (HOPG) and discussing the difficulties in finding theoretical or experimental justification for the observation of the subsurface W atoms in WTe,. Our calculations explain some but not all of the STM images on the surface. We attribute the observations of multiple images to the the dependences of the surface charge distribution to both energy and height from the surface, which are made more complicated by the difficulties associated with obtaining sharp enough tips to scan a corrugated surface. These results caution the assigning of STM features to atomic positions in a complex system with highly covalent and metalmetal bonding such as WTe,. Spatial positions of the charge density due to states near the Fermi
288
S. L. Tang et al. / Surface valence charge distributions and STM of WTe,
level (from and to which tunneling occurs) can be considerably shifted from the atomic positions. Very subtle changes in tip heights and tip geometries may then produce rather dramatic changes in the STM images due to variable contributions from these states.
Acknowledgements We would like to thank Dr. F. Ohuchi and Dr. L.E. Firment for performing the XPS measurements, Professor J. Dow , Dr. W.Y. Hsu, and Dr. T.R. Albrecht for helpful discussions. The technical assistance of J. Koston is gratefully acknowledged.
References 111See for example:
R.V. Coleman, B. Drake, P.K. Hansma and G. Slough, Phys. Rev. Lett. 55 (1985) 394; C.G. Slough, W.W. McNairy, R.V. Coleman, B. Drake and P.K. Hansma, Phys. Rev. B 34 (1986) 994; B. Giambattista, A. Johnson, R.V. Coleman, B. Drake and P.K. Hansma, Phys. Rev. B 37 (1988) 2741; X.L. Wu and C.M. Lieber, J. Am. Chem. Sot. 110 (1988) 5200; X.L. Wu, P. Zhou and C.M. Lieber, Nature 335 (1988) 55; E. Meyer, R. Wiesendanger, D. Anselmetti, H.R. Hidber, H.-J. Guntherodt, F. Levy and H. Berger, J. Vat. Sci. Technol. A 8 (1990) 495. [21 J.L. Stickney, S.D. Rosasco, B.C. Schardt, T. Solomun, A.T. Hubbard and B.A. Parkinson, Surf. Sci. 136 (1984) 15. and M.S. Leung, Appl. Phys. Lett. 51 [31G.W. Stupian (1987) 1560. and L.S. Bell, Appl. [41 D. Sarid, T. Henson, N.R. Armstrong Phys. Lett. 52 (1988) 2252.
[5] M. Weimer, J. Kramar, C. Bai and J.D. Baldeschwieler, Phys. Rev. B 37 (1988) 4292. [6] S.L. Tang, R.V. Kasowski and B.A. Parkinson. Phys. Rev. B 39 (1989) 9987. [7] B. Brown, Acta Cryst. 20 (1966) 268. [8] M.L. Bortz, F.S. Ohuchi and B.A. Parkinson, Surf. Sci. 223 (1989) 285. [9] S.L. Tang, R.V. Kasowski, M.A. Subramanian and W.Y. Hsu, Physica B+C 156 (1988) 177. [lo] Nanoscope II, Digital Instrument Inc., Santa Barbara. CA. (111 H.J. Mamin, E. Ganz, D.W. Abraham, R.E. Thomson and J. Clarke, Phys. Rev. B 34 (1986) 9015. [12] S.L. Tang, J. Bokor and R.H. Storz, Appl. Phys. Lett. 52 (1988) 188. [13] CM. Mate, R. Erlandsson, G.M. McClelland and S. Chiang, Surf. Sci. 208 (1989) 473. [14] J.A. Wilson. F.J. Di Salvo and S. Mahajan, Adv. Phys. 24 (1975) 117. [15] J. Tersoff and D.R. Hamann, Phys. Rev. Lett. 50 (1983) 1998; Phys. Rev. B 31 (1985) 805. [16] G.H. Jeung and M.H. Whangbo, private communications. [17] J. Tersoff, Phys. Rev. Lett. 57 (1986) 440. [18] R.M. Feenstra, W.A. Thompson and A.P. Fein, Phys. Rev. Lett. 56 (1986) 608; also see J.A. Stroscio, R.M. Feenstra, D.M. Newns and A.P. Fein, J. Vat. Sci. Technol. A 6 (1988) 499. [19] See, for example, J. Wintterlin, J. Wiechers, H. Brune, T. Gritsch, H. Hofer and R.J. Behm, Phys. Rev. Lett. 62 (1989) 59. [20] 1.P. Batra, N. Garcia, H. Rohrer. H. Salemink, E. Stall and S. Ciraci, Surf. Sci. 181 (1987) 126. [21] G. Binnig, H. Fuchs, Ch. Gerber, H. Rohrer. E. Stoll and E. Tosatti, Europhys. Lett. 1 (1986) 31. [22] J. Tersoff. Phys. Rev. B 41 (1990) 1235. [23] K. Kobayashi and M. Tsukada. J. Phys. Sot. Jpn. 58 (1989) 2238; M. Tsukada, K. Kobayashi and S. Ohnishi, J. Vat. Sci. Technol. A 8 (1990) 160; S. Ohnishi and M. Tsukada, J. Vat. Sci. Technol. A X (1990) 174.
Surface
Science 23X (1990) 280-288 North-Holland
Surface valence charge distributions and scanning tunneling microscopy of WTez S.L. Tang, R.V. Kasowski,
A. Suna and B.A. Parkinson
E.I. DuPont de Nemours & Co., Cenlrul Research & Development Department, Experlmenral Statmn, Wilmington, DE 19898. USA Received
21 March
1990; accepted
for publication
28 June 1990
We have studied the surface electronic structures of the van der Waals surfaces of tungsten ditelluride (WTe,) with first principles calculations of the spatial distribution of the surface valence charge densities and compared the results to images obtained with the scanning tunneling microscope (STM). The energy- and z(distance from the surface)-dependent calculations show that the valence charge density distribution above the Te surface could be derived from the surface Te layer, as we previously calculated, but the charge density distribution close to but below the Fermi energy has a distortion that coincidentally makes it appear to have a symmetry close to the paired, zig-zag and buckled rows of the W layer. These results dramatically illustrate that in highly covalent compounds, the surface valence charge density distribution does not necessarily follow the surface atomic positions even on ideal. unreconstructed surfaces. An alternative interpretation of the STM images of this surface is proposed in light of this new surface electronic structure. Our calculated and experimental results are also discussed with reference to recent STM results on other transition metal dichalcogenides.
1. Introduction The layered transitional metal dichalcogenides have been extensively studied with scanning tunneling microscopy (STM) especially because many of them exhibit charge density waves transitions [l]. Moreover, the flatness, the inertness even in water [2], and also the ease of obtaining nearly defect-free, atomically clean surfaces make these materials extremely suitable for STM studies and may offer opportunities for novel STM applications. Because these materials consist of two different elements in two different layers - the chalcogen at the surface and the transition metal underneath - there has been some controversy concerning the role played by the subsurface metal layer in their STM images. The problem was made more complicated by the fact that the surface chalcogen and the subsurface metal layers both have identical hexagonal symmetries in the compounds studied [3-51. Since STM in the topographical mode cannot distinguish between different elements by their chemical nature, it was not 0039-602X/90/$03.50
@ 1990 - Elsevier
Science Publishers
possible to definitively determine the effect of the subsurface layer in these studies. We have discussed this controversy in detail earlier [6]. WTe2 has a different structure from the compounds studied to date. The distinctive structure of this surface described briefly below allows us to investigate the influence of the subsurface metal layer on the STM images of the surface. In our previous study [6] of the WTe, surface, we observed images that resembled the structure of the subsurface W layer. These images, however, were not explained by the first principles pseudofunction calculation performed on the surface valence charge density around the Fermi energy at the surface. The goal of the present study is twofold. First we would like to reconcile the difference between experimental and calculated results; and second, further investigate the effect of the subsurface metal layer on the surface electronic structure of these compounds. To these ends we have improved our calculations of the spatial distribution of the surface valence charge density of WTe, as
B.V. (North-Holland)
S. I.. Tung et ul. / Sur~&e calence charge distnhutions
functions of distance from the surface Te atoms and energy. These calculated results will be compared to our STM images on WTeZ. The significance of these results will also be discussed with reference to other recent STM images on related surfaces. The structure of WTez is briefly described here. A detailed account of the structure can be obtained elsewhere [7]. Like many transition metal dichalcogenides, WTe, crystals are made of XMm X (X = chalcogen, M = transition metal) layers separated by van der Waals0 gaps. 0 WTe, has a surface unit cell size of 3.5 A x 6.3 A. The surface Te layer is made up of undulating rows of Te atoms. The height difference between the top and bottom Te atoms is 0.61 A. The subsurface W layer forms paired, zig-zag and slightly buckled rows. The metal-metal bonding resulted from the pairing reduces the perpendicular distance between the two W rows to 2.2 A and also pushes one member of the pair upward by 0.21 A. The W layer is therefore distinctly different in symmetry from the surface Te layer. The mode1 structure in fig. 1 summarizes the structural features of the surface Te and subsurface W layers. The low
und STM of WTe,
281
energy electron diffraction (LEED) pattern of the surface shows spots with the symmetry of the unit cell of the unreconstructed surface [8].
2. Experimental The experiment was carried out in ambient atmosphere with two different scanning tunneling microscopes. The first microscope is of our own design and has been described briefly elsewhere [9]. The second microscope is a commercial instrument [lo]. The tunneling images were obtained with the STM operating in both the constant current mode and the constant height mode. Tips used were either electrochemically etched tungsten or platinum wires. Results obtained from different operational modes and different tips are essentially the same. The WTe, samples were grown by iodine vapor transport. Flat, shiny thin crystals were chosen for these experiments. WTe, is metallic. All results on clean surfaces reported here were taken within three hours of cleaving a new surface and during this time, large clean areas (over a thousand angstriims squared) were readily located. X-ray photoemission spectroscopy was used to ensure that the sample surfaces were essentially free of contaminants except for a small amount of carbon.
3. Results 3. I. WTe,
Fig. I. A W layers outhned. lower Te
schematic drawing of the surface Te and subsurface in WTe,. The surface unit cell of 3.5 Ax6.3 A is Big open circle: upper Te atom, big hatched circle: atom. small closed circle: upper W atom, small open circle: lower W atom.
images
Fig. 2 shows a 33 A x 33 A greyscale image obtained on the WTe, surface with the constant height mode. The average current was 1.1 nA and the sample bias was -0.04 V. In the lower half of the image, the paired, zig-zag and buckled rows characteristic of the W layer is observed. In the middle of the scan, there was a tip change as indicated by the abrupt horizontal break in the picture. The atomic rows of the surface after the tip change appear to have the symmetry of the top-most Te layer. This image dramatically illustrates the different kinds of images we obtained on this surface: W-like, Te-like and a mixture of
282
XL. Tang et al. / Surface valence charge dmtributions and STM of WTe,
of four Te-W-Te sandwiched layers. The van der Waals region between two of the Te-W-Te sandwiched layers has been expanded by 3 A so as to emulate the condition of a surface. The other two van der Waals gaps are left unchanged. The surface charge density plots obtained are those of the surface inside the expanded van der Waals gap.
Fig. 2. A 33 AX 33 A greyscale image of the WTe, surface. The average current was 1.1 nA and the sample bias was -0.04 V. The lower half of the image resembled the paired, zig-zag symmetry of the W layer whereas the upperhalf of the image had the symmetry of the top-most Te layer.
both. The difficulty in interpreting images such as that in fig. 2 prompted us to perform the calculations below.
3.2.1. Energy-dependence Fig. 4a presents two plots of the charge density of the van der Waals surface of WTe, at (i) (E,-0.5 eV)
Te w Te
3.2. Valence charge density calculations of the WTe, surfaces
Te w
We have performed first principles pseudofunction calculations of the spatial distribution of the valence charge densities on the WTe, surface as functions of the height from the surface and energy. The method used is similar to the one used previously on WTe, [6] in which the top-most Te atoms were found to be the centers of the surface valence charge density. The present calculations are aimed at refining the previous result to obtain information on the energy- and z(distance from the surface)-dependences of the Te charge densities. The basis function set has been doubled and the number of k points sampled for the charge density plots has been tripled when compared to the previous calculation. The geometry of the structure used for the present calculations is shown in fig. 3. The structure contains two bulk unit cells
Te
Te w Te
Te W Te
Te Fig. 3. A schematic
drawing of the structure the calculation.
of WTe,
used in
d~sfribu~io~s and STM of W’Te?
283
bii
bi 7.91
3.96 7 3 h
0.00
-3.96
I-
-72% 5 -4..3:
0.00 x
ta.UJ
4.35
-4.35
0.00 x
4.35
ia.u.)
Fig. 4. (a) The spatial distribution of the valence charge density at a distance of 1.32 A above the top-most Te surface at (i) (E, - 0.5 eV) <: E < E, and (ii) EF < E -C ( EF + 0.5 ev). The atomic positions are marked as follows: Big open circle: upper Te, big hatched circle: lower Te, small closed circle: upper W, small open circle: lower W. The highest constant charge contour in each plot is labeled in relative units. The intervals of each successive contour are 0.3 unit in (i) and 0.5 unit in (ii). (b) The spatial distribution of the valence charge density witbin the energy windows (i) (E, - 0.5 eV) i E < E, and (ii) E, c E < (E, + 0.5 e\q_ The distance from the surface top-most Te atom is 1.9 A. The atom postions are tabled as in (a). The intervats of each successive contour are 0.03 unit in fi) and 0.045 units in (ii).
284
S.L. Tang et al. / Surface r&wcr
lower row of Te atoms. The shift of the charge density from the top-most Te atom positions coupled with the asymmetric shapes of the charge clouds give an overall paired. zig-zag and buckled appearance to the constant charge contours on this surface. The distance between the centers of the two paired Te charge clouds is 2.4 A. The charge density distribution due to the unfilled states (from E, to E, + 0.5 eV) centers around the top-most Te atoms with only a slight distortion. 3.2.2. Distance dependence Figs. 4bi and 4bii show another pair of charge density plots with the same energy windows as those in fig. 4a. The heights from the surface Te layer for these plots are z = 1.9 A. The charge density distortion around the top-most Te atoms remains essentially the same as at z = 1.32 A, i.e. it is independent of the distance from the surface. There are however some subtle changes at this height in addition to the overall decrease in charge density. The asymmetric shapes of the charge clouds are not so distinct and the charge density around the lower Te moves a little off the Te atom position such that the perpendicular distance between the two adjacent rows is about 2.9 A. The charge intensities of the two paired charge clouds are very weak but almost equal. The overall symmetry of the charge density distribution at this height is somewhat different from that at z = 1.32 A. We have also calculated the charge density distribution within the same energy window of E, - 0.5 eV to E, in the middle of the unexpanded van der Waals gap (1.44 A from the top-most Te atoms). Again the charge density centers directly on the top-most Te atoms.
4. Discussion 4. I. Energy dependence The energy-dependent spatial distribution of the valence charge density reveals an interesting and surprising phenomenon: the valence charge density distribution below 0.5 eV of E, shifts off
charge drsrrrhurms
und STM of U’Te,
the top-most Te atoms even though the calculation is based on the perfect, unreconstructed surface structure. The shift is a “surface effect” since it is not found in the charge density distribution within the regular van der Waals gap. There the charge clouds center on the top-most Te atoms up to the middle of the van der Waals gap. The detailed mechanism for the shift in the positions of the charge centers when the bulk structure is terminated is not clear without a complete picture of the bonding charge density distribution. It is reasonable, however. to suggest that the underlying W layer provides the driving force for the distortion of the charge density distribution. Otherwise it would be difficult to understand why the charge clouds originally surrounding the Te atoms should shift in a particular direction (in this case toward the W atom that is buckled downward) when there is a surface symmetry plane running through these Te atoms. The pairing of the adjacent W rows to form W-W bonds and the resulting buckling of these rows in the subsurface W layer destroys this symmetry plane. We did not detect this distortion of the charge density distribution on the WTe, surface with our previous calculation on this surface because the previous charge density was averaged over an energy window straddling E, , i.e. (E,. - 0.45 eV) < E < (E, + 0.45 eV). The charge density distribution obtained was the sum of those of the filled states and the unfilled states. which has the approximate symmetry of the undistorted Te atoms. To see this, one needs only sum the charge densities of fig. 4ai and fig. 4aii. It is clear that the resulting plot will be dominated by the charge density distribution in fig. 4aii because the magnitude of the charge in fig. 4aii is twice that of fig. 4ai. We used this energy window previously because different sample bias polarities did not seem to produce a difference in our STM images. By integrating the charge density over a larger energy window. better signal to noise ratio for the charge density distributions calculated at large distances from the surface was obtained. Fig. 5 shows two images taken simultaneously with sample bias voltages of -t 0.03 and -0.03 V. The two images are essentially identical. The near identity of the two images presents another problem in the con-
S. L. Tang et al. / Surface valence charge distributions
text of the present calculations because the calculated results do predict a sample bias polarity dependence. This problem will be discussed below. Note also that the paired, zig-zag and buckled rows are not observed in these images. 4.2. Height dependence The height-dependent calculation is performed at distances within an angstrom outside of the surface. Calculating charge density at larger distances from the surface is not feasible because the charge contours will become too weak to be meaningful. Our results should be valid for tunneling in ambient atmosphere because tunneling on layered materials has been shown to occur so close to the surface that large forces exerted by the tip on the surface [ll-131 are routinely observed. Also, the height at which the charge density distortion as described in fig. 4ai was observed was probably the lower limit. The same amount of charge distortion could possibly be observed at a greater distance from the surface than calculated if the large charge distribution distortion induced a slight shift in the atomic positions to follow the positions of the charge clouds (analogous to the atomic displacement in charge density wave formation [14]). The calculated results can be used to explain the experimental images if one considers the well-
Fig. 5. Two greyscale
image of the WTe, surface
and STM of WTe,
285
accepted tunneling theory that STM contours are proportional to the surface total valence charge density for materials with extended Fermi surfaces [15]. The STM images showing the paired, zig-zag and buckled rows have almost the same symmetry as the surface charge density distribution when tunneling from the surface valence filled states. The charge density on the surface is most likely derived from the surface chalcogen layer because of its proximity to the surface, even though states around the Fermi energy are mixed such that some contribution from the W layer is also possible [7,16]. These results effectively show that features in STM images cannot be associated directly with atomic positions even on this ideal, unreconstructed surface. Although the STM theory shows that STM images reflect the electronic structures rather than the atom positions [17], and experiments on surfaces with strong surface states such as Si(1 ll)-2 X 1 [18] have nicely demonstrated this effect, STM images on transitional metal dichalcogenides [3-51 and metal surfaces without reconstruction [19] are frequently compared directly to surface topographical structures. The distortion in the spatial distribution of the surface charge density gives the coincidental resemblence to the paired, zig-zag and buckled rows of the W layer. Indeed the resemblence is quite remarkable. At distances close to the surface, the perpendicular distance between the two paired
taken simultaneously
with sample biases of (a) - 0.03 V and (b) + 0.03 V.
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XL. Tang et al. / Surface ualence charge distrihutrons and STM of WTe,
charge clouds is 2.4 A, extremely close to the perpendicular distance of 2.2 A in the paired W rows. Because of this, we have previously interpreted the STM images to be those of the subsurface W layer. Detection of the subsurface layer has been thought to be possible in other transition metal dichalcogenides [3,5]. The present interpretation of the STM results, that the surface valence charge density is imaged, offers advantages over our previous interpretation that the atomic positions of the subsurface W layer was imaged. A similar charge density calculation [20] of the surface valence charge density on the graphite (HOPG) surface produces a charge density distribution plot at E, that shows that every other atom in the surface hexagon of graphite has enhanced charge density due to interactions with the atoms in the subsurface layer, exactly as observed in STM images [21]. Graphite has an extremely small Fermi surface (the Fermi surface collapses to one k point at E, [17]). Yet the tunneling theory seems to be adequate to explain the experimental images. WTe,, being a metallic material with a more extended Fermi surface, should be more appropriately explained with the existing tunneling theory. The interpretation that the subsurface W layer is imaged cannot be reconciled with this theory. We have also looked into several possibilities that might cause the observation of the W layer, but none of them seem satisfactory. A previously considered possibility [6] of surface reconstruction has been proven energetically unfavorable by a total energy calculation of the assumed reconstructed surface. Moreover, we considered the previous speculation that there might be some unique features in the tunneling matrix element in WTe, that make it possible to observe the second layer at least occasionally with the STM. One such feature that suggests itself would be that the tunneling matrix element was wave-vector selective, which has been shown to be likely to occur in low dimensional materials [17]. The effect of wave-vector selective tunneling can be demonstrated in the calculation of charge density distribution for HOPG [20]. The difference in heights between the two adjacent C atom positions in graphite is already apparent in the total valence charge density
distribution, although the difference seems small. The different heights manifest themselves in the surface charge density plots at E,. Since the graphite band structure has an anomaly at E, where the band collapse to one k point, the charge density plots at E, that explain the STM image is in effect k-specific. WTe, does not have such an anomaly in its band structure, but since the band does cross E, at a narrow range of k points, the calculations in this study are therefore also to an extent k specific. We suspect that calculating the charge density at specific k points would enhance the charge distortion rather than introduce new features, as in the case of HOPG. The second feature that we considered was the possible involvement of the tip’s electronic structure in the tunneling process. The STM theory [15] assumes the tip states to be spherical, i.e. mostly s-like. However, recently it has been shown that the assumption of tips having s-wave is not a stringent requirement for the tunneling theory to work on metallic surfaces [22]. Lastly, we considered the possible existence of a resonance enhancement of the tunneling probability between the d states of the tungsten tips and the d states of the subsurface tungsten layer. The results obtained with Pt tips, which are identical to those obtained with W tips, however, again make this possibility unlikely. There are obviously other possibilities that we could also have considered, for example. tip-pressure induced local reconstruction, local variations in the tunneling barrier, etc. However, in view of the findings in the calculations presented here, we did not think it necessary to pursue these rather speculative assumptions. The comparison between the calculated surface valence charge density distribution and the experimental images based on the well-accepted tunneling theory can explain the observation of paired, zig-zag and buckled rows in the STM images taken with a negative sample bias, but do not explain the undistorted Te-like images and the lack of difference in the images taken with different sample bias polarities. We have previously attributed the multiple symmetries observed on this surface to be images from multiple-atoms tunneling, and the fine structures of the paired, zig-zag and buckled rows were obtained only with
S. L. Tang et al. / Surface valence charge distributions
the sharpest tips. Because of the corrugated nature of the surface, unequal tip sharpness can be a problem that is difficult to identifiy and avoid. Recent calculations [23] show that when a cluster of W atoms is taken as the tunneling tip, images obtained even on a flat surface like graphite can be complicated. The tip problem on this surface is further complicated by the height dependence of the surface charge density distribution. For example. as the tip moves away from the surface because of a change in the tip geometry, the symmetry of the surface charge distribution switches from the paired, buckled and zig-zag rows to resemble somewhat that of the surface Te atoms. We attempted to minimize tip problems by choosing only data that were obtained with the most stable tunneling currents and the best tunneling gap stability, and with identical forward and backward scans. The paired, buckled and zig-zag rows were scanned in perpendicular directions to ensure that the features were not due to multiple tip atoms or the surface single-atom wide ridges scanning a blunt tip. Without an independent means such as field ion microscopy to determine the sharpness of the tips, it is extremely difficult to decide whether certain images are a result of multiple-tip effects. Aside from multiple-tip effects, it is interesting to note that the potential (van der Waals interaction) that causes the charge distribution distortion on the WTe, surface is extremely weak. The multiple images could perhaps also be caused by a combination of strong perturbations from the strong field of the tip and the high polarizability of Te. In other transition metal dichalcogenides such as MoTe,, MO&, etc., it is possible to obtain STM greyscale images showing two sets of hexagons, a bright one and a faint one superimposed onto each other [5,6]. It is believed that one set of hexagons represents the surface chalcogenide layer while the other represents the metal layer [5]. In WTe,, we have shown that the STM images of the surface are better explained with surface charge density distribution rather than atomic positions. However, from the results on WTe, alone, we cannot in a straightforward manner draw the same conclusion on the semiconducting materials such as MO&, MoTe,, etc. i.e., the images of the
and STM of WTe,
287
surfaces of these materials do not coinside with the atomic positions of the surface atoms. Because of the electronic and structural differences between the metallic WTe, with distorted octahedral metal coordination, and the semiconducting group of transition metal dichalcogenides with trigonal prismatic metal coordination, a similar calculation should be applied to these materials in order to better explain the observation of the two sets of hexagons.
5. Conclusion In conclusion, our energy dependent first-principles calculations of the surface valence charge density distribution of WTe, show that the termination of the bulk structure can lead to a subtle change in the charge density distribution on the surface, even though there is no gross physical rearrangement of the surface atoms. This change is likely due to the interaction between the surface charge density and that of the subsurface metal layer. The resulting shift of the charge clouds from the top-most Te atoms closer to the lower Te atoms on the corrugated surface creates a symmetry very similar to that of the underlying W layer. What we previously interpreted as the images of the subsurface W layer can be the images of the charge density distributions that are not in registry with the surface atomic positions. We supported the present conclusion by comparing our calculations to those performed on graphite (HOPG) and discussing the difficulties in finding theoretical or experimental justification for the observation of the subsurface W atoms in WTe,. Our calculations explain some but not all of the STM images on the surface. We attribute the observations of multiple images to the the dependences of the surface charge distribution to both energy and height from the surface, which are made more complicated by the difficulties associated with obtaining sharp enough tips to scan a corrugated surface. These results caution the assigning of STM features to atomic positions in a complex system with highly covalent and metalmetal bonding such as WTe,. Spatial positions of the charge density due to states near the Fermi
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S. L. Tang et al. / Surface valence charge distributions and STM of WTe,
level (from and to which tunneling occurs) can be considerably shifted from the atomic positions. Very subtle changes in tip heights and tip geometries may then produce rather dramatic changes in the STM images due to variable contributions from these states.
Acknowledgements We would like to thank Dr. F. Ohuchi and Dr. L.E. Firment for performing the XPS measurements, Professor J. Dow , Dr. W.Y. Hsu, and Dr. T.R. Albrecht for helpful discussions. The technical assistance of J. Koston is gratefully acknowledged.
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