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WTe2 surfaces in UHV-STM image formation and analysis of point defect structures
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surface science ELSEVIER
WYe 2
Surface Science 380 (1997) 568 575
surfaces in UHV-STM image formation and analysis of point defect structures J.A.A. C r o s s l e y a,,, C.J. Sofield a, S. M y h r a ~,b ~ AEA Technology, 551 Harwell Laboratory, Oxon 0X11 ORA, UK b Faculty of Science and Technology, Griffith University, Nathan, Queensland 4111, Australia Received 2 February 1996; accepted for publication 16 December 1996
Abstract
The layered semi-metallic T d phase of WTe2 has been examined by STM in UHV. The effects of transient transfer of tip apex atom from/to the surface ( W ~ T e exchange) on the imaging conditions have been observed: the effects demonstrate the inadequacy of the Tersoff-Hamann approximation to the description of the tunnelling process. Two distinctly different point-defect configurations have been observed, assigned tentatively to vacancies in either "top" or "'bottom" Te sites. Anomalous image conditions were observed frequently; these have been ascribed to delamination of the structure, and consequential transfer of the tunnelling to a delocalised internal gap, with the image representing the averaged tunnel current during relative displacements of two neighbouring Te planes. © 1997 Elsevier Science B.V. Keywords: Defects; Dichalcogenide; Tunneling microscopy; WTe 2
1. Introduction The layered transition-metal dichalcogenides have formula unit building blocks which are joined along the c-axis by van der Waals interactions; the stacking sequence is (]re W Te)-(Te-W-Te)-, where the van der Waals gap of 0.29 nm is between adjacent Te layers. Thus atomically flat surfaces of macroscopic dimensions can readily be prepared by cleavage. The resultant surfaces are relatively inert in air and even in aqueous fluids, due to the favourable surface energy and lack of dangling bonds. Therein lies much of the appeal that these compounds have had for the practitioners of scanned probe microscopy, who have been able to
* Corresponding author. Fax: +44 1235 434606.
obtain atomically resolved images in air, UHV and under water [1-4]. The electronic properties of the compounds are highly anisotropic, with in-plane conductivity, alp, ranging from semimetallic (e.g., WTe2) to that of a small band-gap insulator (e.g., WSe2) [5], while or± is low. The T~ phase of WTe 2 is a special member of the family; it has an orthorhombic unit cell with nearest neighbour Te chains being offset by 0.06 nm in the c-direction. The W atoms in the central layer (in the c-direction) of the stacking sequence form a zig-zag pattern since one of the W atoms in the unit cell has been displaced horizontally by 0.095 nm toward the "top" Te chain, and vertically by 0.021 nm vis-'fi-vis the "bottom" W atom [6]. The details of the structure are illustrated in Fig. 1. In a previous paper [7] good agreement was demonstrated between STM data obtained in
0039-6028/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved PH S0039-6028 ( 9 7 ) 0 0 0 4 6 - 0
J.A.A. Crosslev et al. ~ Surface Science 380 (1997) 568 575
(a)
a-- 6.3A b = 3.5 .~ a' = 2.2
Ic
(b)
~- c = o.61 ,~ 0"
..... 0"
B o t t o m Te
Top Te
0
3_ c' -- 0.21 t
Bottom W
Top W
Fig. 1. A schematic depiction of the WTe2 Td structure in (a) (001) projection and (b) (010) projection. The "top" and "'bottom" Te atoms are indicated by large light and dark spheres, respectively, while the "top" and "bottom" W atoms are similarly drawn as small spheres. Unit cell vectors are drawn, as are some of the most relevant dimensions and displacements.
UHV and the spatial maps of the density of states predicted from pseudofunction calculations [8]. This study is concerned with exploring some apparent scanning artefacts and with relating reversible tip changes to the image formation process(es). In addition, new results are reported for point defect structures.
2. Experimental details
The W Y e 2 crystals were grown by vapour transport from the elements in an evacuated sealed quartz ampoule. Tungsten powder (ESP! 45 to 75 #m size fraction, 99.99% pure by mass) was heated to redness on a tungsten boat in order to
569
remove surface oxides, and then mixed with a stoichiometric amount of tellurium (Aldrich 99.99% pure by mass) in the ampoule. The crystal growth procedure has been reported elsewhere [9]. A platey crystal was cleaved in air immediately before entry into the UHV environment of the STM instrument. The exposed surface of typical dimensions 3 x 6 mm 2 was then allowed to outgas for several days. Similar procedures were used during XPS analyses, which showed that surface contamination was below the level of detectability. The relative cleanliness of the surfaces presumably was due to the absence of dangling bonds. The scanning tunnelling images were obtained in UHV ( 2 x 10 l°Torr) with an Omicron UHV STM. The STM was operated in the constantcurrent mode with tunnel currents in the range 2-5 nA. The tunnel voltage was in the range 1 50mV, with positive or negative polarity. Spectroscopic I V measurements were not attempted due to lack of bit-size resolution over the _+ 1-10 mV range; also, thermal drift precluded obtaining I V data with adequate signal-to-noise ratios while maintaining single atom spatial stability. The tips were made from electrochemically etched tungsten wire; the tips were cleaned by field emission/desorption in UHV immediately prior to each tunnelling session. Some imaging was also carried out with etched P t - l r tips. The images were levelled and corrected for thermal drift. The noisiest images were smoothed with an 8-point routine, but not otherwise processed.
3. Results and discussion
3.1. Reversible tip alteration Tip instabilities may be thought of as falling into two broad categories - t h o s e that are due to a sudden change in the geometry of the tip (e.g. the location of the minitip and its sharpness), and those that are due to a sudden change in the electronic structure of the tip. The former may give rise to a rigid shift in space of the surface map, and may alter the spatial resolution, but the underlying features of the surface map will remain invariant. The latter is more interesting, in the sense that
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J.A.A. Crossley et al. / SurJace Science 380 (1997) 568 575
the change in electronic structure may alter the convolution of tip and surface structures in such a way as to produce a recognisably different map of the surface. The second category will be relevant to the present study. The simplest approximation to the tunnel process is that of a planar junction, which predicts a current density of [10]:
,IT = Cl(llT ko/s) e x p ( - 2kos),
( 1)
where C1 =3eZ/(4nh), ko is the decay constant of the wavefunction in the barrier and s is the tunnel gap. This approach with minor modification, was used initially to describe the STM process (see e.g., Ref. [ 11 ]). A more physically useful expression has been derived by Tersoff and Hamann [12] using the independent electrode approximation, but describing the electronic structure of the surface by Bloch wavefunctions. They showed that the tip could be modelled by an s-wave function centered in the tip. Adoption of the Bardeen transfer matrix formalism resulted in the following expression [13]: Ix = C2Vx~2Ntip(EFR2p(R -k- s,EF) e x p ( 2 k o R )
relevant factor is that the apex atom is a constituent of the specimen. It should also be noted that the dwell-time of the tip at a particular location (10 -4 s) in the raster is a factor of 106 108 longer than the likely electronic relaxation time. The simple, and relatively uninteresting, case of instability of an original minitip, and the sudden creation of an equivalent minitip is illustrated in a previous paper [7]. Line profiles along closepacked axes (not shown in the interest of brevity) revealed that the shifts along the z-axis at points of discontinuity were consistent with this interpretation. In addition, the underlying structure of the surface map remained invariant. Two images obtained successively (Fig. 2) demonstrate a more complex sequence of reversible tip changes. Several point defects were located within the field of view. In the context of the discussion in this section the defects may be considered merely as spatial markers. For instance, it is
(2)
where C2=32n3e2/h, cb is the work function, Nti p is the density of states in the tip, p(R+s,Ev) is the electron density in the surface at a location of R + s with respect to the centre of the tip, and R is the radius of curvature of the tip. Since p ~ exp(-2ko(R + s)) the tunnel current is yet again proportional to exp(-2kos). Eqs. (1) and (2) applied to typical metals or semiconductors show that values for s=0.5-0.6 nm are consistent with Ix ~ 1 nA and Vx ~ 1 V, thus justifying the independent electrode approximation. Adopting the same formalism in the case of WTe2 with I x = 2 - 5 nA and V-r= 1-10 mV it can be shown that the tunnel gap should be less than 0.3 nm. The implication is that the W (or Te) apex atom at the minitip will be scanning at a distance comparable to the Te-W (or Te-Te) bond lengths of the structure; these are 0.271-0.282 and 0.350-0.368 nm, respectively [ 14]. Thus the present system should be described better by an approach along the lines adopted by Batra and Ciraci for graphite [ 15 ]; this picture represents a kind of a tip-to-surface bond. The failure of the Tersoff and Hamann approach has been noted elsewhere; see, e.g., Ref. [16]. Another potentially
Fig. 2. The two images over an identical field of view were acquired over a period of some 10 min, with Vt= 1.7 mV and I t = 3 . 5 nA. Reversible tip change produces different image conditions, types A and B in the lower and upper sections, respectively, of the top image and reversed in the image below.
,LA.A. Crossley et al. Smjace Science 380 (1997; 568 575
apparent from the locations of the defects in the images that the tip alterations induced lateral spatial shifts in the slow scan direction of ca. 0.2 nm, and negligible shift along the z-axis. Thus the reversible imaging conditions must be due to transfer of a single atom to (top image), and from (bottom image), the same minitip location. Indeed, it is apparent that the transient atom must have been attached along the side of the original apex atom approximately in the direction of the slow scan. The fields of view in Fig. 2 reflect the outcomes of two imaging conditions. One (upper and lower sections in top and bottom images, respectively), is characterised by relatively large spatial amplitudes (0.17 nm) along the z-axis, and high and low intensities in locations that are likely to correspond to the two non-equivalent Te sites in the cell, respectively. The other image condition differs mainly by having lesser amplitudes along the zaxis (0.08 nm) and having comparable intensities associated with the two non-equivalent sites. In the present context it is useful to consider the tip apex atom as a surface adatom. Also it should be noted that eVt is less than kBT by up to an order of magnitude; thus both occupied and unoccupied states will be available over an energy range which exceeds that corresponding to the optimum values of Vt. The proximity of the tip apex to the surface is another important factor. Of the two candidate adatom species W can occupy two nonequivalent bonding sites at comparable z-heights (the displacement is only 0.02 nm in the completed block), Fig. lb. Thus it is plausible to ascribe the second set of image conditions (type A) to a tipto-surface configuration with a W atom at the apex. The other condition (type B) is likely to occur when a Te atom is transferred from the surface to the apex; the Te species will be nonbonding as an adatom, and will therefore trace out density of states contours as in the case of "normal" tunnelling (i.e., trace out "top" and "bottom" Te sites). It can also be seen that the characteristic "clover leaf" appearance of the defects, Fig. 3b, is conditioned by the state of the tip. Type B condition produces lobes along neighbouring "top" rows spaced 0.63 nm apart, while in the case of type A condition the lobes have most of the intensities
571
Fig. 3. Types of vacancy defects observed in the surfaces of WTe> The images are reminiscent of (a! a butterfly (type II, and (b) a cloverleaf ttype II). respectively. The imaging conditions were similar, although not identical.
along neighbouring "top" and "bottom" rows spaced ca. 0.4 nm apart. (The "bottom" Te site is shifted in the a-direction from the central diagonal position in the rectangle defined by four "'top" Te sites). Scanning was also carried out with a P t - l r tip, but resulted in noisy and uninterpretable images. The latter result is ascribed to Pt-Ir having less affinity for Te adatoms mobile on the surface; the time constant for tip alteration by exchange of apex atom is therefore mis-matched with the scan rate. The most significant implication of the description above is that, for this particular system, the images are conditioned by the state of the electronic structure of the tip. A full description will therefore require a formalism beyond that of Tersoff and Hamann [12]. It may be that the particular merit of the WTe 2 Td phase will be as a convenient and sensitive test-bed for theories describing STM image formation as a travelling state of surface-apex/adatom interaction.
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3.2. Point-defect surface structures Point defects in the present surfaces of WTe2 are ubiquitous; it is not uncommon to find ten defect sites within a field of view containing 103 Te surface sites. Only vacancy defects have been observed. Associated with the defects are changes in the local density of states as well as redistribution of charge, due to the resultant dangling bonds. Two general types of images are observed. One is of the "butterfly" type (I) characterised by two identical lobes of higher tunnel probability along the b-direction in a single "top" row. The other is of the "clover leaf" variety (II) (see also Fig. 2), and is characterised by four asymmetrical lobes arranged around the defect site in two b-direction rows. The two types are shown in Figs. 3a and 3b, respectively. It is plausible to assign the type I variety to a vacancy in a "top" Te site. Transfer of density of states to nearest neighbours along the b-axis is implied by this interpretation. In a similar vein the type II variety can be ascribed to a vacancy in the "bottom" Te site. This configuration is more likely to transfer density of states to sites in two neighbouring "top" chains, possibly by an indirect linkage through W sites in the next layer down. There is asymmetry in the intensities in the lobes around the mirror plane containing the b- and c-axes, in accord with the underlying asymmetry of the unit cell. There is additional asymmetry around the mirror plane containing the a- and c-axes; this observation suggests reconstruction at the defect site. The distinctly different signatures of the two types of defects are independent of tip instabilities, although type II can often occur with smaller spacing between the two lobes (see discussion in the previous section). Definitive assignments and detailed correlations with the underlying electronic structure must await theoretical calculations, however. Both varieties of defects are dynamic at room temperature in the UHV-STM environment. The two images in Fig. 4 show a defect (butterfly, as in Fig. 3a) being annihilated, presumably by a mobile Te surface species. The image quality and contrast are inferior to those of Fig. 3a due to the much higher scan rate than was used in order to overcome thermal drift, and in order to capture the
Fig. 4. Annihilation [(a) to (b)] of defects within an identical field of view. The two images were acquired over a period of some 3 min. Tunnel voltage and current were - 2 . 5 rnV and 3.2 nA, respectively.
dynamic processes. Annihilation of the second kind (cloverleaf) also has been observed, but the sequence of images is not shown in the interest of brevity. Creation of defects was also observed frequently during repetitive scanning over a particular field of view. Finally, close inspection of Fig. 4 will reveal that one of the point defects has migrated by one or two steps in the b-direction. These observations show that point defects are generated and annihilated under the present STM conditions; thus mobile Te surface species must be present. These conclusions support our contention in a previous section that (at least some of) the tip anomalies are associated with transient and reversible transfer of mobile surface species to (and from) the tip. Also, the relatively rapid dynamics is in accord with the assignments of the structures as vacancies in the top surface layer, and with the image being correlatable with the position of Te sites (rather than the underlying W sites). The
J.A.A. Crossley et al. SutJace Science 380 (1997) 568 575
observation of rapid dynamics cannot support alternative interpretations since these would require that there be high mobility at laboratory ambient temperature of W species in buried sites. The present experiments did not reveal any correlations between scan conditions and defect dynamics (aside from the observation that tip instabilities appeared to be correlated with the presence of defects; e.g., results reported previously [7] were obtained for specimens with much lower defect concentrations, and were thus less affected by tip instabilities). An earlier STM investigation of WTe2 in air [17] resulted in stable images, which only rarely revealed defects.
3.3. Delamination ~2,cts
internal tunnel gap
Images with excellent signal-to-noise ratio and good contrast, but with symmetries and relative tunnelling intensities that were demonstrably different from those of WTe2, were obtained frequently. An example is shown in Fig. 5; the intensities at primary and secondary maxima were comparable and thus different to those for a
Fig. 5. A 3 x 3 n m 2 field of view o b t a i n e d tor V~= - 1.7 mV a n d I~=3.9 nA, is s h o w n i l l u s t r a t i n g the a n o m a l o u s structures which were o b t a i n e d frequently due to d e l a m i n a t i o n effects. See Fig. 3 for an e x a m p l e of a " n o r m a l " image o b t a i n e d under n o m i n a l l y identical conditions.
573
"normal" image, and the position and structure of the secondary maximum were different to the "normal" signatures of the "bottom" Te site. The amplitudes in the z-direction of the spatial corrugations of the electronic structure were smaller by factors of two, or more, than those of "'normal" images (typically 0.04 versus 0.16 nm); nevertheless, the signal-to-noise ratio was considerably better. As well as exhibiting structural anomalies, images of this type never exhibited the defects prevalent in other fields of view, and tip hop anomalies were entirely absent. The most likely explanation of these observations is that delamination occurred so that adjacent slabs in the stacking sequence along the c-axis were being displaced vis&-vis each other: i.e., the top layer had become fixed in the tip and was being rastered with respect to a fixed layer. Thus the tunnelling process has been transferred from the external tip-to-top-layer gap to an internal layer-to-layer gap. Moreover, the tunnel current will now be delocalised, due to the relatively high in-plane electrical conductivity, all, in comparison with that perpendicular to the slabs. Indeed the inter-layer conductivity perpendicular to the van der Waals junctions is most likely best described by a hopping mechanism (i.e., a tunnelling process). Since the tunnelling is at an internal junction, and is delocalised, the tip anomalies will be absent, and the effects of local structural irregularities will be averaged out. in addition, the constant tunnel current map will represent a convolution of the two slabs being displaced vis-a-vis each other. The image in Fig. 5 was "'modelled" by displacing two slabs with respect to each other (in analogy with two surfaces of "bubble wrap" sliding across each other). Adjacent Te planes defining a van der Waals gap are not separated by a mirror plane; a "top" Te site on one side is translated by (b + at/2 with respect to an equivalent site on the other side. Also, the repeat distance along the c-axis is 1.41 rim; two Te-W Te blocks. The best fit was obtained when the fast-scan direction of the raster made an angle of ca. 65 (approximately [140] direction) with the a-axis of the stationary net. For simplicity the corrugations were taken to reflect the lattice sites rather than the contours of constant
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ZA.A. Crossley et al. / Surface Science 380 (1997) 568-575
density of states. The additional apparent structure in the image is due to the periodic T-T, B-B and T-B Te site match-up along the scan directions (T and B refer to "top" and "bottom", respectively, in the _+z-direction from the centre of the gap between two adjacent slabs). The relative intensities in the image show that the tunnelling rate per unit area of the (T-T+B-B) site configuration is approximately equivalent to that of the (T-B+B-T) configuration. Assuming that the tunnel current is only weakly dependent on the pre-exponential factors in Eq. (1)~ and that the decay constant remains fixed, it is straightforward to show that: IT(unit area) ~ A {exp(-- 2kod) + exp [-2ko(d+ 1.2)]} for T-T and B-B alignment Ix(unit area) ~ A {exp [ - 2ko(d+ 0.6)] +exp[-2ko(d+0.6)]}for T B ment,
and B-T align-
where A is a constant, d is an arbitrary gap distance, 0.6 is the z-axis displacement of "top" and "bottom" Te sites with respect to each other. The two values of 17 will be identical; thus showing the lower amplitudes of corrugation in the map in Fig. 5 are in qualitative accord with the model. The broadening of the maxima along the a-axis is due to the displacement in the a-b plane of the B site toward the T site, the effect of which is enhanced by the additional shift in the corresponding densities of state [8]. In addition to a tunnelling process, the tip-tosurface interaction(s) give rise to forces. Neither image charge nor van der Waals forces are likely to cause delamination. However, it is possible that an electrostatic force, due to excess charge being trapped on floating Te-W-Te sheets, can reach magnitudes comparable to the interlayer van der Waals forces. The observation of delamination of the structure has nanotribological implications. Recent calculations suggest that incommensurate and weakly interacting surfaces may undergo frictionless sliding, also known as superlubricity [18,19]. The present results support these notions and provide
additional impetus for continued work on the transition metal dichalcogenides.
4. Conclusions As well as being a layered semimetal with van der Waals gaps in the c-direction between the constituent blocks, WTe2 has the advantage, in the context of STM, that the structures of the top Te and underlying W layers are measurably different. Thus it is possible to determine the extent to which an STM map is conditioned by relative contributions to the density of states associated with the two species. The distinction cannot be made with the same ease in the case of most of the other transition metal dichalcogenides. The advantage can only be fully realised, however, if the scanning is carried out in an UHV environment. The present results demonstrate the need for the theoretical description of the image formation process(es) to be extended beyond the Tersoff-Hamann approximations. The WTe2 Td phase may also constitute a convenient and discriminating system for correlating experimental and theoretical descriptions of point defects.
Acknowledgements We are grateful to Dr. G.A. Hope for making the WTez specimens available. This project was funded, in part, by the AEA Corporate Research Programme and by the Australian Research Council. One of us (S.M.) wishes to acknowledge support and hospitality extended during a recent period of attachment to AEA Technology, Harwell. Useful discussions were held with Andrew Fisher and Chris Caulfield of University College London.
References [1] J.L. Stickney, S.D. Rosasco, B.C. Schardt, T. Solomun, A.T. Hubbard and B.A. Parkinson, Surf. Sci., 136 (1984) 15. [2] Th. Schimmel, H. Fuchs, R. Sander and M. Lux-Steiner, Ultramicroscopy, 42--44 11992) 683.
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[3] S.L. Tang, R.V. Kasowski and B.A. Parkinson, Phys. Rev. B, 39 (1982) 9987. [4] C.M. Lieber and Y. Kim, Thin Solid Films, 206 ( 1991 ) 355. [5] K.K. Kam and B.A. Parkinson, J. Phys. Chem., 86 (1982) 463. [6] G.H. Jeung, NATO ASI Ser., Ser. B, (1992) 283. [7] A. Crossley, S. Myhra and C.J. Sofield, Surf. Sci., 318 (1994) 39. [8] S.L. Tang, R.V. Kasowski, A. Suna and B.A. Parkinson. Surf. Sci., 238 (1990) 280. [9] J.E. Callanan, G.A. Hope, R.D. Weir and E.F. Westrum J. Chem. Thermodynamics, 24 (1992) 627. [10] J.G. Simmons, J. Appl. Phys., 34 (1963) 1793. [ 11 ] A. Baratoff, G. Binnig, H. Fuchs, F. Selvan and E. Stoll, Surf. Sci., 168 (1986) 734. [12] .I. Tersoff and D.R. Hamann, Phys. Rev. B, 31 (1985) 805.
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[13] L.E.C. van de Leemput and H. van Kempen, Rep. Prog. Phys., 55 (1992) 1165. [14] W.G. Dawson and D.W. Bullett, J. Phys. C: Solid State Phys., 20 (1987) 6159. [15] I.P. Batra and S. Ciraci, J. Vac. Sci. Technol. A, 6 (1988) 313. [ 16] A.R.H. Clarke, J.B. Pethica, J.A Nieminen, F. Besenbacher, I. Steensgaard and E. Laegsgaard, Phys. Rev. Lett., 76 (1996) 1276. [17] D.R. Cousens, A.P. Trost, S. Myhra and P.S. Turner, 13th Australian Conf. Electron Microsc., Perth, Western Australia ( 1992 ), abstract. [18] K. Shinjo and M. Hirano, Surf. Sci.. 283 (1993) 473. [19] M.R. Sorensen, K.W. Jacobsen and P. Stollze, Phys. Rev. B, 53 [1996) 2101.
~¸
i
¸
surface science ELSEVIER
WYe 2
Surface Science 380 (1997) 568 575
surfaces in UHV-STM image formation and analysis of point defect structures J.A.A. C r o s s l e y a,,, C.J. Sofield a, S. M y h r a ~,b ~ AEA Technology, 551 Harwell Laboratory, Oxon 0X11 ORA, UK b Faculty of Science and Technology, Griffith University, Nathan, Queensland 4111, Australia Received 2 February 1996; accepted for publication 16 December 1996
Abstract
The layered semi-metallic T d phase of WTe2 has been examined by STM in UHV. The effects of transient transfer of tip apex atom from/to the surface ( W ~ T e exchange) on the imaging conditions have been observed: the effects demonstrate the inadequacy of the Tersoff-Hamann approximation to the description of the tunnelling process. Two distinctly different point-defect configurations have been observed, assigned tentatively to vacancies in either "top" or "'bottom" Te sites. Anomalous image conditions were observed frequently; these have been ascribed to delamination of the structure, and consequential transfer of the tunnelling to a delocalised internal gap, with the image representing the averaged tunnel current during relative displacements of two neighbouring Te planes. © 1997 Elsevier Science B.V. Keywords: Defects; Dichalcogenide; Tunneling microscopy; WTe 2
1. Introduction The layered transition-metal dichalcogenides have formula unit building blocks which are joined along the c-axis by van der Waals interactions; the stacking sequence is (]re W Te)-(Te-W-Te)-, where the van der Waals gap of 0.29 nm is between adjacent Te layers. Thus atomically flat surfaces of macroscopic dimensions can readily be prepared by cleavage. The resultant surfaces are relatively inert in air and even in aqueous fluids, due to the favourable surface energy and lack of dangling bonds. Therein lies much of the appeal that these compounds have had for the practitioners of scanned probe microscopy, who have been able to
* Corresponding author. Fax: +44 1235 434606.
obtain atomically resolved images in air, UHV and under water [1-4]. The electronic properties of the compounds are highly anisotropic, with in-plane conductivity, alp, ranging from semimetallic (e.g., WTe2) to that of a small band-gap insulator (e.g., WSe2) [5], while or± is low. The T~ phase of WTe 2 is a special member of the family; it has an orthorhombic unit cell with nearest neighbour Te chains being offset by 0.06 nm in the c-direction. The W atoms in the central layer (in the c-direction) of the stacking sequence form a zig-zag pattern since one of the W atoms in the unit cell has been displaced horizontally by 0.095 nm toward the "top" Te chain, and vertically by 0.021 nm vis-'fi-vis the "bottom" W atom [6]. The details of the structure are illustrated in Fig. 1. In a previous paper [7] good agreement was demonstrated between STM data obtained in
0039-6028/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved PH S0039-6028 ( 9 7 ) 0 0 0 4 6 - 0
J.A.A. Crosslev et al. ~ Surface Science 380 (1997) 568 575
(a)
a-- 6.3A b = 3.5 .~ a' = 2.2
Ic
(b)
~- c = o.61 ,~ 0"
..... 0"
B o t t o m Te
Top Te
0
3_ c' -- 0.21 t
Bottom W
Top W
Fig. 1. A schematic depiction of the WTe2 Td structure in (a) (001) projection and (b) (010) projection. The "top" and "'bottom" Te atoms are indicated by large light and dark spheres, respectively, while the "top" and "bottom" W atoms are similarly drawn as small spheres. Unit cell vectors are drawn, as are some of the most relevant dimensions and displacements.
UHV and the spatial maps of the density of states predicted from pseudofunction calculations [8]. This study is concerned with exploring some apparent scanning artefacts and with relating reversible tip changes to the image formation process(es). In addition, new results are reported for point defect structures.
2. Experimental details
The W Y e 2 crystals were grown by vapour transport from the elements in an evacuated sealed quartz ampoule. Tungsten powder (ESP! 45 to 75 #m size fraction, 99.99% pure by mass) was heated to redness on a tungsten boat in order to
569
remove surface oxides, and then mixed with a stoichiometric amount of tellurium (Aldrich 99.99% pure by mass) in the ampoule. The crystal growth procedure has been reported elsewhere [9]. A platey crystal was cleaved in air immediately before entry into the UHV environment of the STM instrument. The exposed surface of typical dimensions 3 x 6 mm 2 was then allowed to outgas for several days. Similar procedures were used during XPS analyses, which showed that surface contamination was below the level of detectability. The relative cleanliness of the surfaces presumably was due to the absence of dangling bonds. The scanning tunnelling images were obtained in UHV ( 2 x 10 l°Torr) with an Omicron UHV STM. The STM was operated in the constantcurrent mode with tunnel currents in the range 2-5 nA. The tunnel voltage was in the range 1 50mV, with positive or negative polarity. Spectroscopic I V measurements were not attempted due to lack of bit-size resolution over the _+ 1-10 mV range; also, thermal drift precluded obtaining I V data with adequate signal-to-noise ratios while maintaining single atom spatial stability. The tips were made from electrochemically etched tungsten wire; the tips were cleaned by field emission/desorption in UHV immediately prior to each tunnelling session. Some imaging was also carried out with etched P t - l r tips. The images were levelled and corrected for thermal drift. The noisiest images were smoothed with an 8-point routine, but not otherwise processed.
3. Results and discussion
3.1. Reversible tip alteration Tip instabilities may be thought of as falling into two broad categories - t h o s e that are due to a sudden change in the geometry of the tip (e.g. the location of the minitip and its sharpness), and those that are due to a sudden change in the electronic structure of the tip. The former may give rise to a rigid shift in space of the surface map, and may alter the spatial resolution, but the underlying features of the surface map will remain invariant. The latter is more interesting, in the sense that
570
J.A.A. Crossley et al. / SurJace Science 380 (1997) 568 575
the change in electronic structure may alter the convolution of tip and surface structures in such a way as to produce a recognisably different map of the surface. The second category will be relevant to the present study. The simplest approximation to the tunnel process is that of a planar junction, which predicts a current density of [10]:
,IT = Cl(llT ko/s) e x p ( - 2kos),
( 1)
where C1 =3eZ/(4nh), ko is the decay constant of the wavefunction in the barrier and s is the tunnel gap. This approach with minor modification, was used initially to describe the STM process (see e.g., Ref. [ 11 ]). A more physically useful expression has been derived by Tersoff and Hamann [12] using the independent electrode approximation, but describing the electronic structure of the surface by Bloch wavefunctions. They showed that the tip could be modelled by an s-wave function centered in the tip. Adoption of the Bardeen transfer matrix formalism resulted in the following expression [13]: Ix = C2Vx~2Ntip(EFR2p(R -k- s,EF) e x p ( 2 k o R )
relevant factor is that the apex atom is a constituent of the specimen. It should also be noted that the dwell-time of the tip at a particular location (10 -4 s) in the raster is a factor of 106 108 longer than the likely electronic relaxation time. The simple, and relatively uninteresting, case of instability of an original minitip, and the sudden creation of an equivalent minitip is illustrated in a previous paper [7]. Line profiles along closepacked axes (not shown in the interest of brevity) revealed that the shifts along the z-axis at points of discontinuity were consistent with this interpretation. In addition, the underlying structure of the surface map remained invariant. Two images obtained successively (Fig. 2) demonstrate a more complex sequence of reversible tip changes. Several point defects were located within the field of view. In the context of the discussion in this section the defects may be considered merely as spatial markers. For instance, it is
(2)
where C2=32n3e2/h, cb is the work function, Nti p is the density of states in the tip, p(R+s,Ev) is the electron density in the surface at a location of R + s with respect to the centre of the tip, and R is the radius of curvature of the tip. Since p ~ exp(-2ko(R + s)) the tunnel current is yet again proportional to exp(-2kos). Eqs. (1) and (2) applied to typical metals or semiconductors show that values for s=0.5-0.6 nm are consistent with Ix ~ 1 nA and Vx ~ 1 V, thus justifying the independent electrode approximation. Adopting the same formalism in the case of WTe2 with I x = 2 - 5 nA and V-r= 1-10 mV it can be shown that the tunnel gap should be less than 0.3 nm. The implication is that the W (or Te) apex atom at the minitip will be scanning at a distance comparable to the Te-W (or Te-Te) bond lengths of the structure; these are 0.271-0.282 and 0.350-0.368 nm, respectively [ 14]. Thus the present system should be described better by an approach along the lines adopted by Batra and Ciraci for graphite [ 15 ]; this picture represents a kind of a tip-to-surface bond. The failure of the Tersoff and Hamann approach has been noted elsewhere; see, e.g., Ref. [16]. Another potentially
Fig. 2. The two images over an identical field of view were acquired over a period of some 10 min, with Vt= 1.7 mV and I t = 3 . 5 nA. Reversible tip change produces different image conditions, types A and B in the lower and upper sections, respectively, of the top image and reversed in the image below.
,LA.A. Crossley et al. Smjace Science 380 (1997; 568 575
apparent from the locations of the defects in the images that the tip alterations induced lateral spatial shifts in the slow scan direction of ca. 0.2 nm, and negligible shift along the z-axis. Thus the reversible imaging conditions must be due to transfer of a single atom to (top image), and from (bottom image), the same minitip location. Indeed, it is apparent that the transient atom must have been attached along the side of the original apex atom approximately in the direction of the slow scan. The fields of view in Fig. 2 reflect the outcomes of two imaging conditions. One (upper and lower sections in top and bottom images, respectively), is characterised by relatively large spatial amplitudes (0.17 nm) along the z-axis, and high and low intensities in locations that are likely to correspond to the two non-equivalent Te sites in the cell, respectively. The other image condition differs mainly by having lesser amplitudes along the zaxis (0.08 nm) and having comparable intensities associated with the two non-equivalent sites. In the present context it is useful to consider the tip apex atom as a surface adatom. Also it should be noted that eVt is less than kBT by up to an order of magnitude; thus both occupied and unoccupied states will be available over an energy range which exceeds that corresponding to the optimum values of Vt. The proximity of the tip apex to the surface is another important factor. Of the two candidate adatom species W can occupy two nonequivalent bonding sites at comparable z-heights (the displacement is only 0.02 nm in the completed block), Fig. lb. Thus it is plausible to ascribe the second set of image conditions (type A) to a tipto-surface configuration with a W atom at the apex. The other condition (type B) is likely to occur when a Te atom is transferred from the surface to the apex; the Te species will be nonbonding as an adatom, and will therefore trace out density of states contours as in the case of "normal" tunnelling (i.e., trace out "top" and "bottom" Te sites). It can also be seen that the characteristic "clover leaf" appearance of the defects, Fig. 3b, is conditioned by the state of the tip. Type B condition produces lobes along neighbouring "top" rows spaced 0.63 nm apart, while in the case of type A condition the lobes have most of the intensities
571
Fig. 3. Types of vacancy defects observed in the surfaces of WTe> The images are reminiscent of (a! a butterfly (type II, and (b) a cloverleaf ttype II). respectively. The imaging conditions were similar, although not identical.
along neighbouring "top" and "bottom" rows spaced ca. 0.4 nm apart. (The "bottom" Te site is shifted in the a-direction from the central diagonal position in the rectangle defined by four "'top" Te sites). Scanning was also carried out with a P t - l r tip, but resulted in noisy and uninterpretable images. The latter result is ascribed to Pt-Ir having less affinity for Te adatoms mobile on the surface; the time constant for tip alteration by exchange of apex atom is therefore mis-matched with the scan rate. The most significant implication of the description above is that, for this particular system, the images are conditioned by the state of the electronic structure of the tip. A full description will therefore require a formalism beyond that of Tersoff and Hamann [12]. It may be that the particular merit of the WTe 2 Td phase will be as a convenient and sensitive test-bed for theories describing STM image formation as a travelling state of surface-apex/adatom interaction.
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3.2. Point-defect surface structures Point defects in the present surfaces of WTe2 are ubiquitous; it is not uncommon to find ten defect sites within a field of view containing 103 Te surface sites. Only vacancy defects have been observed. Associated with the defects are changes in the local density of states as well as redistribution of charge, due to the resultant dangling bonds. Two general types of images are observed. One is of the "butterfly" type (I) characterised by two identical lobes of higher tunnel probability along the b-direction in a single "top" row. The other is of the "clover leaf" variety (II) (see also Fig. 2), and is characterised by four asymmetrical lobes arranged around the defect site in two b-direction rows. The two types are shown in Figs. 3a and 3b, respectively. It is plausible to assign the type I variety to a vacancy in a "top" Te site. Transfer of density of states to nearest neighbours along the b-axis is implied by this interpretation. In a similar vein the type II variety can be ascribed to a vacancy in the "bottom" Te site. This configuration is more likely to transfer density of states to sites in two neighbouring "top" chains, possibly by an indirect linkage through W sites in the next layer down. There is asymmetry in the intensities in the lobes around the mirror plane containing the b- and c-axes, in accord with the underlying asymmetry of the unit cell. There is additional asymmetry around the mirror plane containing the a- and c-axes; this observation suggests reconstruction at the defect site. The distinctly different signatures of the two types of defects are independent of tip instabilities, although type II can often occur with smaller spacing between the two lobes (see discussion in the previous section). Definitive assignments and detailed correlations with the underlying electronic structure must await theoretical calculations, however. Both varieties of defects are dynamic at room temperature in the UHV-STM environment. The two images in Fig. 4 show a defect (butterfly, as in Fig. 3a) being annihilated, presumably by a mobile Te surface species. The image quality and contrast are inferior to those of Fig. 3a due to the much higher scan rate than was used in order to overcome thermal drift, and in order to capture the
Fig. 4. Annihilation [(a) to (b)] of defects within an identical field of view. The two images were acquired over a period of some 3 min. Tunnel voltage and current were - 2 . 5 rnV and 3.2 nA, respectively.
dynamic processes. Annihilation of the second kind (cloverleaf) also has been observed, but the sequence of images is not shown in the interest of brevity. Creation of defects was also observed frequently during repetitive scanning over a particular field of view. Finally, close inspection of Fig. 4 will reveal that one of the point defects has migrated by one or two steps in the b-direction. These observations show that point defects are generated and annihilated under the present STM conditions; thus mobile Te surface species must be present. These conclusions support our contention in a previous section that (at least some of) the tip anomalies are associated with transient and reversible transfer of mobile surface species to (and from) the tip. Also, the relatively rapid dynamics is in accord with the assignments of the structures as vacancies in the top surface layer, and with the image being correlatable with the position of Te sites (rather than the underlying W sites). The
J.A.A. Crossley et al. SutJace Science 380 (1997) 568 575
observation of rapid dynamics cannot support alternative interpretations since these would require that there be high mobility at laboratory ambient temperature of W species in buried sites. The present experiments did not reveal any correlations between scan conditions and defect dynamics (aside from the observation that tip instabilities appeared to be correlated with the presence of defects; e.g., results reported previously [7] were obtained for specimens with much lower defect concentrations, and were thus less affected by tip instabilities). An earlier STM investigation of WTe2 in air [17] resulted in stable images, which only rarely revealed defects.
3.3. Delamination ~2,cts
internal tunnel gap
Images with excellent signal-to-noise ratio and good contrast, but with symmetries and relative tunnelling intensities that were demonstrably different from those of WTe2, were obtained frequently. An example is shown in Fig. 5; the intensities at primary and secondary maxima were comparable and thus different to those for a
Fig. 5. A 3 x 3 n m 2 field of view o b t a i n e d tor V~= - 1.7 mV a n d I~=3.9 nA, is s h o w n i l l u s t r a t i n g the a n o m a l o u s structures which were o b t a i n e d frequently due to d e l a m i n a t i o n effects. See Fig. 3 for an e x a m p l e of a " n o r m a l " image o b t a i n e d under n o m i n a l l y identical conditions.
573
"normal" image, and the position and structure of the secondary maximum were different to the "normal" signatures of the "bottom" Te site. The amplitudes in the z-direction of the spatial corrugations of the electronic structure were smaller by factors of two, or more, than those of "'normal" images (typically 0.04 versus 0.16 nm); nevertheless, the signal-to-noise ratio was considerably better. As well as exhibiting structural anomalies, images of this type never exhibited the defects prevalent in other fields of view, and tip hop anomalies were entirely absent. The most likely explanation of these observations is that delamination occurred so that adjacent slabs in the stacking sequence along the c-axis were being displaced vis&-vis each other: i.e., the top layer had become fixed in the tip and was being rastered with respect to a fixed layer. Thus the tunnelling process has been transferred from the external tip-to-top-layer gap to an internal layer-to-layer gap. Moreover, the tunnel current will now be delocalised, due to the relatively high in-plane electrical conductivity, all, in comparison with that perpendicular to the slabs. Indeed the inter-layer conductivity perpendicular to the van der Waals junctions is most likely best described by a hopping mechanism (i.e., a tunnelling process). Since the tunnelling is at an internal junction, and is delocalised, the tip anomalies will be absent, and the effects of local structural irregularities will be averaged out. in addition, the constant tunnel current map will represent a convolution of the two slabs being displaced vis-a-vis each other. The image in Fig. 5 was "'modelled" by displacing two slabs with respect to each other (in analogy with two surfaces of "bubble wrap" sliding across each other). Adjacent Te planes defining a van der Waals gap are not separated by a mirror plane; a "top" Te site on one side is translated by (b + at/2 with respect to an equivalent site on the other side. Also, the repeat distance along the c-axis is 1.41 rim; two Te-W Te blocks. The best fit was obtained when the fast-scan direction of the raster made an angle of ca. 65 (approximately [140] direction) with the a-axis of the stationary net. For simplicity the corrugations were taken to reflect the lattice sites rather than the contours of constant
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density of states. The additional apparent structure in the image is due to the periodic T-T, B-B and T-B Te site match-up along the scan directions (T and B refer to "top" and "bottom", respectively, in the _+z-direction from the centre of the gap between two adjacent slabs). The relative intensities in the image show that the tunnelling rate per unit area of the (T-T+B-B) site configuration is approximately equivalent to that of the (T-B+B-T) configuration. Assuming that the tunnel current is only weakly dependent on the pre-exponential factors in Eq. (1)~ and that the decay constant remains fixed, it is straightforward to show that: IT(unit area) ~ A {exp(-- 2kod) + exp [-2ko(d+ 1.2)]} for T-T and B-B alignment Ix(unit area) ~ A {exp [ - 2ko(d+ 0.6)] +exp[-2ko(d+0.6)]}for T B ment,
and B-T align-
where A is a constant, d is an arbitrary gap distance, 0.6 is the z-axis displacement of "top" and "bottom" Te sites with respect to each other. The two values of 17 will be identical; thus showing the lower amplitudes of corrugation in the map in Fig. 5 are in qualitative accord with the model. The broadening of the maxima along the a-axis is due to the displacement in the a-b plane of the B site toward the T site, the effect of which is enhanced by the additional shift in the corresponding densities of state [8]. In addition to a tunnelling process, the tip-tosurface interaction(s) give rise to forces. Neither image charge nor van der Waals forces are likely to cause delamination. However, it is possible that an electrostatic force, due to excess charge being trapped on floating Te-W-Te sheets, can reach magnitudes comparable to the interlayer van der Waals forces. The observation of delamination of the structure has nanotribological implications. Recent calculations suggest that incommensurate and weakly interacting surfaces may undergo frictionless sliding, also known as superlubricity [18,19]. The present results support these notions and provide
additional impetus for continued work on the transition metal dichalcogenides.
4. Conclusions As well as being a layered semimetal with van der Waals gaps in the c-direction between the constituent blocks, WTe2 has the advantage, in the context of STM, that the structures of the top Te and underlying W layers are measurably different. Thus it is possible to determine the extent to which an STM map is conditioned by relative contributions to the density of states associated with the two species. The distinction cannot be made with the same ease in the case of most of the other transition metal dichalcogenides. The advantage can only be fully realised, however, if the scanning is carried out in an UHV environment. The present results demonstrate the need for the theoretical description of the image formation process(es) to be extended beyond the Tersoff-Hamann approximations. The WTe2 Td phase may also constitute a convenient and discriminating system for correlating experimental and theoretical descriptions of point defects.
Acknowledgements We are grateful to Dr. G.A. Hope for making the WTez specimens available. This project was funded, in part, by the AEA Corporate Research Programme and by the Australian Research Council. One of us (S.M.) wishes to acknowledge support and hospitality extended during a recent period of attachment to AEA Technology, Harwell. Useful discussions were held with Andrew Fisher and Chris Caulfield of University College London.
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