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The surface superstructures in niobium disulfide and diselenide intercalated by Cu, Co and Fe
Surface Science 476 (2001) 71±77
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The surface superstructures in niobium disul®de and diselenide intercalated by Cu, Co and Fe A. Prodan a,*, V. Marinkovic a,1, M. Rojsek a, N. Jug a, H.J.P. van Midden a, hm d F.W. Boswell b, J.C. Bennett c, H. Bo a
Institute Jozef Stefan, Ljubljana, Jamova 39, SI-1000 Slovenia Department of Physics, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1 c Department of Physics, Acadia University, Wolfville, Nova Scotia, Canada B0P 1X0 d Institut f ur Geowissenschaften der Universitat, D-55099 Mainz, Germany
b
Received 3 November 2000; accepted for publication 14 December 2000
Abstract A series of intercalated transition-metal dichalcogenides with nominal compositions Ax NbCh2 , with A Cu, Co, Fe, Ch S, Se and x 0:25, 0.33, were studied by scanning tunneling microscopy (STM). Evidence is given that intercalation in¯uences the STM images. Intercalated Cu atoms occupy tetrahedral interstices in the van der Waals gaps, forming pairs with Nb atoms and adopting the 2Hb ±MoS2 stacking of the host structure. Eects associated with nonstoichiometry, such as formation of agglomerates and incomplete ordering of the intercalated Cu are frequently observed. Fe and Co atoms occupy octahedral interstices in the gaps, form chains with alternating Nb and stabilize the 2Ha ±NbS2 polytype. p p In case of (Fe,Co)x NbS2 2a0 2a0 and 3a0 3a0 hexagonal superstructures are formed for x 1=4 and x 1=3, respectively, while in case of Cux Nb(S,Se)2 only the x 1=4 fraction orders into a 2a0 2a0 hexagonal superstructure at room temperature. In all cases the superstructures are visible at the surfaces and are attributed to changes in the local densities of states caused by a charge transfer between the intercalated and the surface atoms. Typically these superstructures were observed to change during prolonged scanning, probably as a result of tip-induced ¯uctuations in the charge density. The experimental STM images are compared to simulated images calculated with the extended H uckel tight binding approximation. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Surface structure, morphology, roughness, and topography; Scanning tunneling microscopy
1. Introduction The compounds TCh2 (T transition metal, Ch chalcogen: Te, Se, S) belong to a family of * Corresponding author. Tel.: +386-1-47-73-552; fax: +386-125-19-385. E-mail address: [email protected] (A. Prodan). 1 In memoriam 12 October 2000.
quasi two-dimensional layered structures and exhibit many unusual physical and electronic properties, including charge density waves (CDW). As a result, these compounds have been extensively investigated over the past several years. The crystals are made up of layers consisting of two planar sheets of hexagonal close-packed Ch atoms interposed with a layer of T atoms. There is strong bonding of the atoms within the layers, while a
0039-6028/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 6 0 2 8 ( 0 0 ) 0 1 1 1 6 - X
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A. Prodan et al. / Surface Science 476 (2001) 71±77
weak van der Waals (vdW) bonding between the layers allows a wide variety of foreign atoms to be intercalated into the gaps with resultant changes in the electronic structures and physical properties of the crystals [1]. Some of these structures have been investigated by X-ray [2±4] and electron diraction [5±8] and it was shown that the intercalated atoms generally exhibit order-disorder transitions as a function of composition and temperature. More recently scanning tunneling microscopy (STM) and atomic force microscopic (AFM) techniques have been used to examine the surfaces of the intercalated crystals, with particular reference to the eects of intercalates on CDW [9±15]. Detailed structural studies of intercalated NbS2 [5,6] have shown that during intercalation with Cu, the stacking sequence of 2Ha ±NbS2 (AcA/BcB. . .) changes to that of 2Hb ±MoS2 (AbA/BaB. . .). For both of these structures there are two sets of tetrahedral interstices with opposite orientation and one set of octahedral interstices in the vdW gaps. The intercalated Cu atoms partially occupy one or both sets of the tetrahedral sites, depending on composition. In the case of Fe intercalation, the 2Ha ±NbS2 stacking of the layers is unaltered with the Fe atoms occupying the octahedral sites. In the present paper we show that atomic resolution STM provides new data on the ordered structures of the intercalation complexes of the NbS2 system. Information was also obtained concerning the nature of the local atomic deformations about occupied interstices and the variation of the electronic properties of some surface atoms due to electron transfer from the intercalate atoms.
crystals containing lower concentrations cleaving more readily. The crystals were examined in an Omicron STM-1 and under ultra-high vacuum (UHV) conditions (10 9 ±10 10 hPa) at room temperature (RT). Both tungsten tips, prepared by electropolishing, and Pt±Rh tips, prepared by cutting a 90Pt/10Rh wire, were used. Although the contrast of the images did not depend on the tip/ sample polarity, the polarity of the samples is given in the ®gure captions. In some cases, a fast Fourier transform (FFT) algorithm was applied to ®lter electronic noise and to reveal the details of the reciprocal space. The extended H uckel tight binding (EHTB) approximation [17] was used to simulate the STM images and to calculate the electronic properties. 3. Results Fig. 1, shows a (00.1) surface of a Fe0:25 NbS2 crystal with the 2a0 2a0 hexagonal superstruc-
2. Experimental The crystals were grown by chemical transport in evacuated silica tubes [16] between 1123 and 1073 K either with or without iodine as a transport agent. Large hexagonal single crystals were obtained after about a week of growth. The samples were mounted onto supporting plates using a graphite paste and cleaved with scotch-tape along their vdW gaps. The ease of cleaving depended on the amount of intercalated Cu, Fe and Co with
2 (00.1) surface of a Fe0:25 NbS2 Fig. 1. STM image of a (100 A) single crystal with the corresponding FFT in the inset. Large indices refer to the basic structure and the small ones to the superstructure. A few defects with missing S atoms are ®lled by agglomerates, mostly triplets of atoms, which remained at the surface after cleaving. Note the variable intensity of the marked 2a0 2a0 periodicity (constant height mode, t 60 ls, Ug 5 mV, It 2 nA).
A. Prodan et al. / Surface Science 476 (2001) 71±77
ture ordering and with numerous defects. The images of the superstructure in a given area usually change in detail over time but are generally stable for several repeated scans. In the images, the superstructure periodicity is usually enhanced along one of the three equivalent directions, although ordering in all three directions can easily be detected in the image as well as in the corresponding FFT, shown in the inset. Surface defects of roughly triangular shapes are generally observed and correspond to missing groups of S atoms which are often ®lled by agglomerates of disordered atoms, probably left behind after cleaving. Since they often dier in size and orientation from the surrounding S atoms, these agglomerates are assumed to be intercalated Fe atoms on the cleaved surface. One from a series of STM images recorded a few seconds apart of a crystal with nominal composition Fe0:33 NbS2 is shown in Fig. 2. Subsequently recorded p p images show the development of the 3a0 3a0 hexagonal superstructure which increases and decreases in visibility during scanning. The superstructure shown in Fig. 3 for Co0:33 NbS2
2 large (00.1) surface area of a Fig. 2. STM image of a (75 A) Fe0:33 NbS2 single crystal (constant height mode, t 29 ls, p p Ug 0:2 mV, It 0:8 nA). The marked 3a0 3a0 superstructure is composed of a central lighter atom surrounded by six darker ones.
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2 large (00.1) surface area of a cleaved crystal Fig. 3. A (100 A) of Co0:33 NbS2 (constant height mode, t 29 ls, Ug 0:2 mV, It 0:8 nA) with the corresponding FFT in the inset. Contrary p p to the image in Fig. 2 the marked 3a0 3a0 superstructure is composed of a central darker atom surrounded by six lighter ones.
is p similar, p but with inverted contrast. The 3a0 3a0 superstructure is clearly revealed by the corresponding FFT, shown in the inset. In Fig. 2 the image contrast consists of a central lighter atom surrounded by six darker ones, while in Fig. 3 the superstructure is revealed by a contrast consisting of a central darker atom surrounded by six lighter ones. In both cases, one of the three equivalent superstructure directions is usually enhanced relative to the other two. Fig. 4 shows an area of a crystal with nominal composition Cu0:25 NbS2 . The image was recorded within a series of scans under the same tunneling conditions. A 2a0 2a0 hexagonal superstructure is observed, generally becoming more pronounced during prolonged scanning. Compared to the x 1=4 Fe intercalated crystals, the superstructure is regularly weaker and is usually well developed along one of the three equivalent directions only. In addition to the superstructure, a large number of Cu agglomerates is observed. The intensity of these features changes with time in a similar way to that of the superstructure, which suggests that the
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agglomerates are located in the vdW gaps below the surface. A typical RT vdW surface of a single crystal with nominal composition Cu0:33 NbSe2 is shown in Fig. 5. The STM images show the formation of small partially ordered regions with pronounced 2a0 periodicities. Thus it appears that only small domains having a 2a0 2a0 hexagonal superstructure are formed while the rest of the observed area remains unmodulated. 4. Discussion
2 large (00.1) surface area of a Cu0:25 NbS2 Fig. 4. A (100 A) crystal (constant height mode, t 100 ls, Ug 0:2 mV, It 0:7 nA). The 2a0 2a0 hexagonal superstructure and the visibility of the agglomerates developed after a prolonged scanning.
2 large vdW surface area of a crystal with Fig. 5. A (75 A) nominal composition Cu0:33 NbSe2 (constant height mode, t 100 ls, Ug 0:2 mV, It 1:0 nA). Note the typical short range ordering with a 2a0 periodicity along one of the three equivalent directions.
Previous X-ray and electron diraction studies of 2H±NbS2 have shown that intercalated Fe and Co occupy the only available set of octahedral interstices in the vdW gaps, forming either a p 2a0 x 1=4 or a 3a0 x 1=3 hexagonal superstructure depending on concentration. In both cases, two adjacent equivalent layers are positioned exactly one above the other, forming Nb±Fe±Nb±Fe chains along the c-direction and retaining the 2Ha ±NbS2 stacking of the host crystal. The superstructures observed in the STM images of these compounds are thus consistent with the expected intercalated ordering for both concentrations. Although the observed superlattices are clearly connected with the intercalate, the origin of the contrast in the STM images remains unclear. Dai et al. [10] have previously concluded from combined AFM and STM measurements of Fe intercalated NbSe2 , TaSe2 and TaS2 that a rather large charge transfer to the surface and a less pronounced modi®cation of the DOS in the vicinity of the Fermi level take place. Although both will in¯uence the STM images, the dominant contribution in case of STM is expected from the changes in the DOS. To investigate this, STM images of Fe intercalated NbS2 were simulated within the EHTB approximation [17] and the corresponding DOS and dispersion curves were calculated. An example is presented in Fig. 6, which shows a simulated STM image of Fe0:33 NbS2 at a constant height for a tunneling voltage Ug 0:2 mV, which corresponds to the experimental settings for Fig. 2. In this case, the concentration of the intercalate is
A. Prodan et al. / Surface Science 476 (2001) 71±77
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Fig. 6. STM image of the (00.1) surface of Fe0:33 NbS2 , calculated within the EHTB model. A constant height mode scanp p ning is simulated, at Ug 0:2 mV. The 3a0 3a0 unit cell and positions of S, Nb and Fe atoms are indicated.
high enough so that all surface S atoms end up in equivalent atomic environments. The simulated images show no superstructure ordering, suggesting that the charge redistribution around the Fe atoms is likely not directly responsible for the experimental observations. Calculated DOS curves for pure NbS2 and for intercalated Fe0:33 NbS2 are shown in Fig. 7a and b, respectively. These indicate that the Fermi level is shifted in the presence of intercalated Fe from a larger, mainly Nb peak into a rather small peak with a mixed contribution. Thus the EHTB calculations support the conclusions of Dai et al. [10] that variations in the DOS are primarily responsible for the observed image contrast. It should be noted however that the present calculations do not attempt to model possible tip±specimen interactions. The instability of the images often observed during extended scanning is attributed to a redistribution of the charge between the surface S atoms. In case of Fe0:25 NbS2 , one out of four surface S atom is in a dierent atomic environment and thus would be expected to carry a dierent charge as compared to the other three. In contrast, all surface S atoms in case of Fe0:33 NbS2 should be equivalent. However, the STM images show p that in case of Fe0:33 NbS2 and Co0:33 NbS2 a 3a0 superstructure is formed, presumably due to a charge redistribution between the originally equivalent atoms. Surplus charge may be localized on one S atom or, alternatively, shared between two S atoms, possibly accounting
Fig. 7. The calculated DOS curves for NbS2 (a) and Fe0:33 NbS2 (b) with Nb, S and Fe contributions indicated. The values along the y-axis are normalized to the unit cells calculated. The Fermi levels are shown by the vertical interrupted lines.
for the two types of superstructure contrast in the STM images. This redistribution seems to be rather unstable under the in¯uence of the tip. In contrast to Fe and Co intercalation, small domains having a 2a0 2a0 hexagonal superstructure are observed for the Cu intercalation complex with x 1=3. Previous TEM results [7] have revealed that Cu intercalation is less straightforward, because two sets of tetrahedral interstices in the vdW gaps can be ®lled up by Cu atoms. Each Cu atom requires a single Nb atom exactly above or below it along the c-direction,
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forming thus Nb±Cu pairs and consequently changing the stacking of the host crystal into the 2Hb ±MoS2 type. In addition to the formation of Nb±Cu pairs and the change in stacking, the intercalated Cu will form layers in adjacent vdW gaps with the highest possible symmetry. The layers will thus attempt to achieve proper centering relative to one another, which can be easily achieved only for x 1=4. As a result, the 2a0 superstructure in two adjacent vdW gaps occupies only one of the two available tetrahedral sites, keeping the partly occupied vdW layers equidistant along the c-direction. In this way the most symmetrical situation is obtained and the symmetry of the entire structure, i.e. host plus intercalate, remains hexagonal. If more than one Cu atom per four Nb atoms is intercalated, p the simplest way would obviously be to form 3a0 hexagonal superstructures in both adjacent vdW gaps. However, such two layers cannot be centered with respect to each other without changing the type of the occupied tetrahedral interstices in alternating gaps. Instead of being equidistant the spacing of the Cu layers would alternate 12c0 D and 12c0 D, which is energetically less favorable than centering x 1=4 separately from the remaining part. According to Boswell et al. [7], the remaining Cu, corresponding to a fraction x 1=16, is centered separately and generates a 4a0 superstructure at lower temperatures. Therefore both sets of Cu atoms x 1=4 and x 1=16 have to occupy the same set of tetrahedral interstices, without being centered with respect to each other. Such ordering is energetically more favorable, in spite of reducing the entire symmetry from hexagonal to monoclinic. The observed STM images are in accord with these considerations. Small areas with pronounced 2a0 ordering along one of the three equivalent directions appear throughout the scanned areas while p neither 4a0 or 3a0 periodicities are observed. Apparently, only part of the intercalated Cu is ordered at RT, while the rest remains disordered. Several previous STM studies of intercalated transition metal dichalcogenides [11,14,18] have raised the possibility of the existence of induced surface CDWs in these compounds. Since the 2Ha polytype of NbSe2 was reported to exhibit a CDW
with a 3a0 3a0 superstructure at 33 K, a similar behavior might be expected in the isostructural 2Ha ±NbS2 . However, it was shown by Friend and Yoe [19] already that NbS2 is the only group-V compound free of any CDW distortions. It was further concluded from an electronic band structure calculation [20] that only the 2a0 superlattice might be explained on the basis of a CDW formation. Finally, Whangbo et al. [21] have shown by calculating the electronic properties within the EHTB method that not all modulation eects in the layered transition metal chalcogenides are necessarily CDW driven. In particular, they found that for the d1 compounds like 1T-TaX2 X S; Se and NbSe2 the spanning vectors associated with the partial Fermi surface p nestingp did not correspond to the observed 13a0 13a0 or 3a0 3a0 superstructures. According to these authors they should rather be attributed to metal atom clustering, which lowers the energy levels well below the Fermi level. Thus, leaving open the unclear situation in the case of Nb-rich 3Rb polytype, it can be concluded that the observed superstructures described here are not to be attributed to CDWs. 5. Conclusions It was shown that intercalation of transition metal dichalcogenides by dierent metals in¯uences the surface charge distribution, which can be detected by surface methods like STM. The observed surface superstructures are in accord with previous TEM results and con®rm that various intercalates ®ll selectively the available interstitial sites, form composition dependent superstructures and change the stacking of the host crystals. Calculations within the EHTB approximation do not account for the weak surface eects in the simulated STM images of the intercalated compounds, but do show expected changes in the DOS spectra. Acknowledgements Financial support of the Ministry of Science and Technology of the Republic of Slovenia (AP, NJ, HJPvM), of the Deutsche Forschungsgemeins-
A. Prodan et al. / Surface Science 476 (2001) 71±77
chaft (BH) and of the Natural Science and Engineering Research Council of Canada (FWB, JCB) is acknowledged. A German±Slovenian joint research program (HB, AP) and a NATO Expert Visit Grant (AP, JCB) were helpful in facilitating the collaboration. Finally, we gratefully acknowledge helpful discussions in applying the EHTB model with Prof. M.-H. Whangbo from the Department of Chemistry, North Carolina State University at Raleigh, USA.
References [1] F. Levy (Ed.), Intercalated layered materials, in: E. Mooser (Managing Ed.), Physics and Chemistry of Materials with Layered Structures, D. Reidel Publ., Dordrecht, Holland, 1979. [2] J.M. Vandenburg, P. Cossee, Inorg. Chim. Acta 2 (1968) 143. [3] K. Koerts, Acta Cryst. 16 (1963) 432. [4] J.M. Vorhoeve-Vandenburg, J. Less-Comm. Metals 26 (1972) 399. [5] F.W. Boswell, A. Prodan, J.M. Corbett, Phys. Stat. Sol. A 35 (1976) 591. [6] R. de Ridder, G. van Tendeloo, J. van Landuyt, D. van Dyck, S. Amelinckx, Phys. Stat. Sol. A 37 (1976) 591.
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[7] F.W. Boswell, A. Prodan, W.R. Vaughan, J.M. Corbett, Phys. Stat. Sol. A 45 (1978) 469. [8] L. van Goethem, R. de Ridder, D. van Dyck, S. Amelinckx, Phys. Stat. Sol. A 55 (1979) 67. [9] G.A. Scholz, R.F. Frindt, A.E. Curzon, Phys. Stat. Sol. A 71 (1982) 531. [10] Z. Dai, Q. Xue, Y. Gong, C.G. Slough, R.V. Coleman, Phys. Rev. B 48 (1993) 14543. [11] W. Han, E.R. Hunt, S.E.Ulloa, R.F. Frindt, Phys. Rev. B 45 (1992) 14415. [12] M. Remskar, V. Marinkovic, A. Prodan, Z. Skraba, Surf. Sci. 324 (1995) L367. [13] M. Remskar, V. Marinkovic, A. Prodan, Surf. Sci. 352±354 (1996) 1012. [14] B. Koslowski, W. Xu, B. Blackford, M.H. Jericho, Phys. Rev. B 54 (1996) 11706. [15] I. Ekvall, H. Brauer, E. Wahlstr om, H. Olin, Phys. Rev. B 59 (1999) 7751. [16] H. Shaer, Chemical Transport Reactions, Academic Press, New York, 1964. [17] J. Ren, W. Liang, M.-H. Whangbo, CEASAR program, PrimeColor Software, Inc., Cary, NC, 1998. [18] G.P.E.M. Van Bakel, J.Th. De Hosson, Phys. Rev. B 45 (1992) 2001. [19] R.H. Friend, A.D. Yoe, Adv. Phys. 36 (1987) 1. [20] A. Kikuchi, S. Tsuneyuki, Surf. Sci. 409 (1998) 458. [21] M.-H. Whangbo, D.-K. Seo, E. Canadell, in: C. Schlenker, et al. (Ed.), Physics and Chemistry of Low-Dimensional Inorganic Conductors, Plenum Press, NY, 1996, p. 285.
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The surface superstructures in niobium disul®de and diselenide intercalated by Cu, Co and Fe A. Prodan a,*, V. Marinkovic a,1, M. Rojsek a, N. Jug a, H.J.P. van Midden a, hm d F.W. Boswell b, J.C. Bennett c, H. Bo a
Institute Jozef Stefan, Ljubljana, Jamova 39, SI-1000 Slovenia Department of Physics, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1 c Department of Physics, Acadia University, Wolfville, Nova Scotia, Canada B0P 1X0 d Institut f ur Geowissenschaften der Universitat, D-55099 Mainz, Germany
b
Received 3 November 2000; accepted for publication 14 December 2000
Abstract A series of intercalated transition-metal dichalcogenides with nominal compositions Ax NbCh2 , with A Cu, Co, Fe, Ch S, Se and x 0:25, 0.33, were studied by scanning tunneling microscopy (STM). Evidence is given that intercalation in¯uences the STM images. Intercalated Cu atoms occupy tetrahedral interstices in the van der Waals gaps, forming pairs with Nb atoms and adopting the 2Hb ±MoS2 stacking of the host structure. Eects associated with nonstoichiometry, such as formation of agglomerates and incomplete ordering of the intercalated Cu are frequently observed. Fe and Co atoms occupy octahedral interstices in the gaps, form chains with alternating Nb and stabilize the 2Ha ±NbS2 polytype. p p In case of (Fe,Co)x NbS2 2a0 2a0 and 3a0 3a0 hexagonal superstructures are formed for x 1=4 and x 1=3, respectively, while in case of Cux Nb(S,Se)2 only the x 1=4 fraction orders into a 2a0 2a0 hexagonal superstructure at room temperature. In all cases the superstructures are visible at the surfaces and are attributed to changes in the local densities of states caused by a charge transfer between the intercalated and the surface atoms. Typically these superstructures were observed to change during prolonged scanning, probably as a result of tip-induced ¯uctuations in the charge density. The experimental STM images are compared to simulated images calculated with the extended H uckel tight binding approximation. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Surface structure, morphology, roughness, and topography; Scanning tunneling microscopy
1. Introduction The compounds TCh2 (T transition metal, Ch chalcogen: Te, Se, S) belong to a family of * Corresponding author. Tel.: +386-1-47-73-552; fax: +386-125-19-385. E-mail address: [email protected] (A. Prodan). 1 In memoriam 12 October 2000.
quasi two-dimensional layered structures and exhibit many unusual physical and electronic properties, including charge density waves (CDW). As a result, these compounds have been extensively investigated over the past several years. The crystals are made up of layers consisting of two planar sheets of hexagonal close-packed Ch atoms interposed with a layer of T atoms. There is strong bonding of the atoms within the layers, while a
0039-6028/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 6 0 2 8 ( 0 0 ) 0 1 1 1 6 - X
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A. Prodan et al. / Surface Science 476 (2001) 71±77
weak van der Waals (vdW) bonding between the layers allows a wide variety of foreign atoms to be intercalated into the gaps with resultant changes in the electronic structures and physical properties of the crystals [1]. Some of these structures have been investigated by X-ray [2±4] and electron diraction [5±8] and it was shown that the intercalated atoms generally exhibit order-disorder transitions as a function of composition and temperature. More recently scanning tunneling microscopy (STM) and atomic force microscopic (AFM) techniques have been used to examine the surfaces of the intercalated crystals, with particular reference to the eects of intercalates on CDW [9±15]. Detailed structural studies of intercalated NbS2 [5,6] have shown that during intercalation with Cu, the stacking sequence of 2Ha ±NbS2 (AcA/BcB. . .) changes to that of 2Hb ±MoS2 (AbA/BaB. . .). For both of these structures there are two sets of tetrahedral interstices with opposite orientation and one set of octahedral interstices in the vdW gaps. The intercalated Cu atoms partially occupy one or both sets of the tetrahedral sites, depending on composition. In the case of Fe intercalation, the 2Ha ±NbS2 stacking of the layers is unaltered with the Fe atoms occupying the octahedral sites. In the present paper we show that atomic resolution STM provides new data on the ordered structures of the intercalation complexes of the NbS2 system. Information was also obtained concerning the nature of the local atomic deformations about occupied interstices and the variation of the electronic properties of some surface atoms due to electron transfer from the intercalate atoms.
crystals containing lower concentrations cleaving more readily. The crystals were examined in an Omicron STM-1 and under ultra-high vacuum (UHV) conditions (10 9 ±10 10 hPa) at room temperature (RT). Both tungsten tips, prepared by electropolishing, and Pt±Rh tips, prepared by cutting a 90Pt/10Rh wire, were used. Although the contrast of the images did not depend on the tip/ sample polarity, the polarity of the samples is given in the ®gure captions. In some cases, a fast Fourier transform (FFT) algorithm was applied to ®lter electronic noise and to reveal the details of the reciprocal space. The extended H uckel tight binding (EHTB) approximation [17] was used to simulate the STM images and to calculate the electronic properties. 3. Results Fig. 1, shows a (00.1) surface of a Fe0:25 NbS2 crystal with the 2a0 2a0 hexagonal superstruc-
2. Experimental The crystals were grown by chemical transport in evacuated silica tubes [16] between 1123 and 1073 K either with or without iodine as a transport agent. Large hexagonal single crystals were obtained after about a week of growth. The samples were mounted onto supporting plates using a graphite paste and cleaved with scotch-tape along their vdW gaps. The ease of cleaving depended on the amount of intercalated Cu, Fe and Co with
2 (00.1) surface of a Fe0:25 NbS2 Fig. 1. STM image of a (100 A) single crystal with the corresponding FFT in the inset. Large indices refer to the basic structure and the small ones to the superstructure. A few defects with missing S atoms are ®lled by agglomerates, mostly triplets of atoms, which remained at the surface after cleaving. Note the variable intensity of the marked 2a0 2a0 periodicity (constant height mode, t 60 ls, Ug 5 mV, It 2 nA).
A. Prodan et al. / Surface Science 476 (2001) 71±77
ture ordering and with numerous defects. The images of the superstructure in a given area usually change in detail over time but are generally stable for several repeated scans. In the images, the superstructure periodicity is usually enhanced along one of the three equivalent directions, although ordering in all three directions can easily be detected in the image as well as in the corresponding FFT, shown in the inset. Surface defects of roughly triangular shapes are generally observed and correspond to missing groups of S atoms which are often ®lled by agglomerates of disordered atoms, probably left behind after cleaving. Since they often dier in size and orientation from the surrounding S atoms, these agglomerates are assumed to be intercalated Fe atoms on the cleaved surface. One from a series of STM images recorded a few seconds apart of a crystal with nominal composition Fe0:33 NbS2 is shown in Fig. 2. Subsequently recorded p p images show the development of the 3a0 3a0 hexagonal superstructure which increases and decreases in visibility during scanning. The superstructure shown in Fig. 3 for Co0:33 NbS2
2 large (00.1) surface area of a Fig. 2. STM image of a (75 A) Fe0:33 NbS2 single crystal (constant height mode, t 29 ls, p p Ug 0:2 mV, It 0:8 nA). The marked 3a0 3a0 superstructure is composed of a central lighter atom surrounded by six darker ones.
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2 large (00.1) surface area of a cleaved crystal Fig. 3. A (100 A) of Co0:33 NbS2 (constant height mode, t 29 ls, Ug 0:2 mV, It 0:8 nA) with the corresponding FFT in the inset. Contrary p p to the image in Fig. 2 the marked 3a0 3a0 superstructure is composed of a central darker atom surrounded by six lighter ones.
is p similar, p but with inverted contrast. The 3a0 3a0 superstructure is clearly revealed by the corresponding FFT, shown in the inset. In Fig. 2 the image contrast consists of a central lighter atom surrounded by six darker ones, while in Fig. 3 the superstructure is revealed by a contrast consisting of a central darker atom surrounded by six lighter ones. In both cases, one of the three equivalent superstructure directions is usually enhanced relative to the other two. Fig. 4 shows an area of a crystal with nominal composition Cu0:25 NbS2 . The image was recorded within a series of scans under the same tunneling conditions. A 2a0 2a0 hexagonal superstructure is observed, generally becoming more pronounced during prolonged scanning. Compared to the x 1=4 Fe intercalated crystals, the superstructure is regularly weaker and is usually well developed along one of the three equivalent directions only. In addition to the superstructure, a large number of Cu agglomerates is observed. The intensity of these features changes with time in a similar way to that of the superstructure, which suggests that the
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agglomerates are located in the vdW gaps below the surface. A typical RT vdW surface of a single crystal with nominal composition Cu0:33 NbSe2 is shown in Fig. 5. The STM images show the formation of small partially ordered regions with pronounced 2a0 periodicities. Thus it appears that only small domains having a 2a0 2a0 hexagonal superstructure are formed while the rest of the observed area remains unmodulated. 4. Discussion
2 large (00.1) surface area of a Cu0:25 NbS2 Fig. 4. A (100 A) crystal (constant height mode, t 100 ls, Ug 0:2 mV, It 0:7 nA). The 2a0 2a0 hexagonal superstructure and the visibility of the agglomerates developed after a prolonged scanning.
2 large vdW surface area of a crystal with Fig. 5. A (75 A) nominal composition Cu0:33 NbSe2 (constant height mode, t 100 ls, Ug 0:2 mV, It 1:0 nA). Note the typical short range ordering with a 2a0 periodicity along one of the three equivalent directions.
Previous X-ray and electron diraction studies of 2H±NbS2 have shown that intercalated Fe and Co occupy the only available set of octahedral interstices in the vdW gaps, forming either a p 2a0 x 1=4 or a 3a0 x 1=3 hexagonal superstructure depending on concentration. In both cases, two adjacent equivalent layers are positioned exactly one above the other, forming Nb±Fe±Nb±Fe chains along the c-direction and retaining the 2Ha ±NbS2 stacking of the host crystal. The superstructures observed in the STM images of these compounds are thus consistent with the expected intercalated ordering for both concentrations. Although the observed superlattices are clearly connected with the intercalate, the origin of the contrast in the STM images remains unclear. Dai et al. [10] have previously concluded from combined AFM and STM measurements of Fe intercalated NbSe2 , TaSe2 and TaS2 that a rather large charge transfer to the surface and a less pronounced modi®cation of the DOS in the vicinity of the Fermi level take place. Although both will in¯uence the STM images, the dominant contribution in case of STM is expected from the changes in the DOS. To investigate this, STM images of Fe intercalated NbS2 were simulated within the EHTB approximation [17] and the corresponding DOS and dispersion curves were calculated. An example is presented in Fig. 6, which shows a simulated STM image of Fe0:33 NbS2 at a constant height for a tunneling voltage Ug 0:2 mV, which corresponds to the experimental settings for Fig. 2. In this case, the concentration of the intercalate is
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Fig. 6. STM image of the (00.1) surface of Fe0:33 NbS2 , calculated within the EHTB model. A constant height mode scanp p ning is simulated, at Ug 0:2 mV. The 3a0 3a0 unit cell and positions of S, Nb and Fe atoms are indicated.
high enough so that all surface S atoms end up in equivalent atomic environments. The simulated images show no superstructure ordering, suggesting that the charge redistribution around the Fe atoms is likely not directly responsible for the experimental observations. Calculated DOS curves for pure NbS2 and for intercalated Fe0:33 NbS2 are shown in Fig. 7a and b, respectively. These indicate that the Fermi level is shifted in the presence of intercalated Fe from a larger, mainly Nb peak into a rather small peak with a mixed contribution. Thus the EHTB calculations support the conclusions of Dai et al. [10] that variations in the DOS are primarily responsible for the observed image contrast. It should be noted however that the present calculations do not attempt to model possible tip±specimen interactions. The instability of the images often observed during extended scanning is attributed to a redistribution of the charge between the surface S atoms. In case of Fe0:25 NbS2 , one out of four surface S atom is in a dierent atomic environment and thus would be expected to carry a dierent charge as compared to the other three. In contrast, all surface S atoms in case of Fe0:33 NbS2 should be equivalent. However, the STM images show p that in case of Fe0:33 NbS2 and Co0:33 NbS2 a 3a0 superstructure is formed, presumably due to a charge redistribution between the originally equivalent atoms. Surplus charge may be localized on one S atom or, alternatively, shared between two S atoms, possibly accounting
Fig. 7. The calculated DOS curves for NbS2 (a) and Fe0:33 NbS2 (b) with Nb, S and Fe contributions indicated. The values along the y-axis are normalized to the unit cells calculated. The Fermi levels are shown by the vertical interrupted lines.
for the two types of superstructure contrast in the STM images. This redistribution seems to be rather unstable under the in¯uence of the tip. In contrast to Fe and Co intercalation, small domains having a 2a0 2a0 hexagonal superstructure are observed for the Cu intercalation complex with x 1=3. Previous TEM results [7] have revealed that Cu intercalation is less straightforward, because two sets of tetrahedral interstices in the vdW gaps can be ®lled up by Cu atoms. Each Cu atom requires a single Nb atom exactly above or below it along the c-direction,
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forming thus Nb±Cu pairs and consequently changing the stacking of the host crystal into the 2Hb ±MoS2 type. In addition to the formation of Nb±Cu pairs and the change in stacking, the intercalated Cu will form layers in adjacent vdW gaps with the highest possible symmetry. The layers will thus attempt to achieve proper centering relative to one another, which can be easily achieved only for x 1=4. As a result, the 2a0 superstructure in two adjacent vdW gaps occupies only one of the two available tetrahedral sites, keeping the partly occupied vdW layers equidistant along the c-direction. In this way the most symmetrical situation is obtained and the symmetry of the entire structure, i.e. host plus intercalate, remains hexagonal. If more than one Cu atom per four Nb atoms is intercalated, p the simplest way would obviously be to form 3a0 hexagonal superstructures in both adjacent vdW gaps. However, such two layers cannot be centered with respect to each other without changing the type of the occupied tetrahedral interstices in alternating gaps. Instead of being equidistant the spacing of the Cu layers would alternate 12c0 D and 12c0 D, which is energetically less favorable than centering x 1=4 separately from the remaining part. According to Boswell et al. [7], the remaining Cu, corresponding to a fraction x 1=16, is centered separately and generates a 4a0 superstructure at lower temperatures. Therefore both sets of Cu atoms x 1=4 and x 1=16 have to occupy the same set of tetrahedral interstices, without being centered with respect to each other. Such ordering is energetically more favorable, in spite of reducing the entire symmetry from hexagonal to monoclinic. The observed STM images are in accord with these considerations. Small areas with pronounced 2a0 ordering along one of the three equivalent directions appear throughout the scanned areas while p neither 4a0 or 3a0 periodicities are observed. Apparently, only part of the intercalated Cu is ordered at RT, while the rest remains disordered. Several previous STM studies of intercalated transition metal dichalcogenides [11,14,18] have raised the possibility of the existence of induced surface CDWs in these compounds. Since the 2Ha polytype of NbSe2 was reported to exhibit a CDW
with a 3a0 3a0 superstructure at 33 K, a similar behavior might be expected in the isostructural 2Ha ±NbS2 . However, it was shown by Friend and Yoe [19] already that NbS2 is the only group-V compound free of any CDW distortions. It was further concluded from an electronic band structure calculation [20] that only the 2a0 superlattice might be explained on the basis of a CDW formation. Finally, Whangbo et al. [21] have shown by calculating the electronic properties within the EHTB method that not all modulation eects in the layered transition metal chalcogenides are necessarily CDW driven. In particular, they found that for the d1 compounds like 1T-TaX2 X S; Se and NbSe2 the spanning vectors associated with the partial Fermi surface p nestingp did not correspond to the observed 13a0 13a0 or 3a0 3a0 superstructures. According to these authors they should rather be attributed to metal atom clustering, which lowers the energy levels well below the Fermi level. Thus, leaving open the unclear situation in the case of Nb-rich 3Rb polytype, it can be concluded that the observed superstructures described here are not to be attributed to CDWs. 5. Conclusions It was shown that intercalation of transition metal dichalcogenides by dierent metals in¯uences the surface charge distribution, which can be detected by surface methods like STM. The observed surface superstructures are in accord with previous TEM results and con®rm that various intercalates ®ll selectively the available interstitial sites, form composition dependent superstructures and change the stacking of the host crystals. Calculations within the EHTB approximation do not account for the weak surface eects in the simulated STM images of the intercalated compounds, but do show expected changes in the DOS spectra. Acknowledgements Financial support of the Ministry of Science and Technology of the Republic of Slovenia (AP, NJ, HJPvM), of the Deutsche Forschungsgemeins-
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chaft (BH) and of the Natural Science and Engineering Research Council of Canada (FWB, JCB) is acknowledged. A German±Slovenian joint research program (HB, AP) and a NATO Expert Visit Grant (AP, JCB) were helpful in facilitating the collaboration. Finally, we gratefully acknowledge helpful discussions in applying the EHTB model with Prof. M.-H. Whangbo from the Department of Chemistry, North Carolina State University at Raleigh, USA.
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