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Localization and charge density wave transformation in Cs intercalated 1T–TaSe2
Surface Science 465 (2000) 301–309 www.elsevier.nl/locate/susc
Localization and charge density wave transformation in Cs intercalated 1T–TaSe 2 H.J. Crawack, Y. Tomm, C. Pettenkofer * Hahn-Meitner-Institut, Abteilung SE-6: Elektronische Struktur, Glienicker Straße 100, 14109 Berlin, Germany Received 17 March 2000; accepted for publication 29 June 2000
Abstract Cs was adsorbed at room temperature onto the (0001) cleavage planes of 1T–TaSe . The deposited Cs forms no 2 metallic overlayer, but intercalates after an initial adsorption stage into the van der Waals gap of the layered crystal. A limit of the relative concentration ratio c /c of ca. 60% is observed. Due to the electron transfer associated with Cs Ta the intercalation the charge density wave induced p(E13×E13)R13.89° superstructure exhibits a phase transition to a c(2E3×4)rect. structure. As a further consequence of the intercalation a metal to semiconductor transition occurs. The change in the electronic structure could not be interpreted within the rigid band model, but is tentatively explained by an localization effect, caused by the charge density wave phase transition. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Alkali metals; Low energy electron diffraction (LEED); X-ray photoelectron spectroscopy
1. Introduction Layered transition metal dichalcogenides ( TMDCs) are regarded as prototypes of low dimensional solids. They are formed by covalently bound XMMMX sandwich layers (M=transition metal, X=S, Se, Te), which are connected to each other by a weak so-called van der Waals like interaction [1,2]. The TMDCs have been investigated intensively during the recent decades due to of a wide range of exceptional properties. For example they serve as cathodes for rechargeable intercalation batteries [3] and they are proposed for an application in thin film solar cells [4,5]. Most of the TMDCs are known to form intercalation complexes by inserting electron donating * Corresponding author. Fax: +49-30-80622434. E-mail address: [email protected] (C. Pettenkofer)
species (e.g. alkalis) between the layers into their ‘van der Waals gap’ [6 ]. As with electrochemical treatment or intercalation during the crystal growth it was recently shown that in situ intercalation takes place by depositing the intercalate on the (0001) cleavage plane in ultrahigh vacuum ( UHV ) [7–11]. Deintercalation is also possible by additionally adsorbing electronegative species (e.g. oxygen or halogens) onto the surface of an intercalated TMDC [12,13]. In a first approximation the reversible redox reaction maybe described in terms of the rigid band model, which assumes that electronic charge from the intercalated species is transferred to the lowest unoccupied band of the host material [14,15]. The charge transfer hardly modifies the hosts band structure (rigid band) but raises only the position of the Fermi level and so if the host is a metal it changes the shape of the Fermi surface [16 ].
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Among the metallic layer crystals of group Vb the octahedrally coordinated 1T–TaS and 2 1T–TaSe show a strong anomaly of the conduc2 tion band electrons, a so-called charge density wave (CDW ), which was first predicted for lowdimensional metals by Peierls [17,18]. A CDW is always accompanied by a periodic lattice distortion (PLD), which shows up as a superstructure in low energy electron diffraction (LEED). The nearly commensurate (nc) p(E13×E13)R13.89° superstructure of pure 1T–TaS , present at room temper2 ature, has been explained by the ‘Star of David model’ introduced by Fazekas and Tosatti [19]. For the nc superstructure the CDW is arranged in a regular hexagonal lattice in circular domains of ˚ diameter [20]. 1T–TaSe exhibits a comca. 73 A 2 mensurate (c) p(E13×E13)R13.89° CDW. Moreover the CDW, which concerns only the Ta-derived d 2 level forming the conduction band, z causes a splitting of the Ta 4f core level, due to the inequivalent charge at different Ta sites [21]. The spanning vector q=2k , which determines F direction and periodicity of the CDW phase is closely related to the shape of the Fermi surface. Only if q fulfils the so-called nesting condition can the respective CDW occur [22,23]. Intercalation will change the Fermi surface in a controlled way and thus drives the crystal into different CDW phases, as it was shown for Na/1T–TaS [11], Cu/1T–TaS /TaSe [8] and 2 2 2 Li/1T–TaS [24,25]. Especially for the latter system 2 a variety of CDW phase transitions has been observed. Here we present a photoemission and LEED study of the system Cs/1T–TaSe . Varying 2 amounts of Cs were deposited at room temperature onto the (0001) van der Waals plane of 1T–TaSe 2 and the electronic and morphologic changes are studied simultaneously by photoelectron spectroscopy and LEED.
range 10–120 eV ( TGM 7 beamline) and an angle integrating Leybold EA II MCD spectrometer with HeI and Mg Ka radiation. The spectra for the Cs 3d and 4d emissions were exclusively taken at the latter system. All spectra were recorded in normal emission and at room temperature. They are calibrated to the photon flux for SXPS and scaled in counts/point for HeI/Mg Ka radiation. For the valence band spectra a bias voltage of −6 V (and −2 V, respectively) was applied to the sample. Furthermore all spectra are given in binding energy (BE ) referred to the Fermi level of a metallic sample holder (Cu). The energy resolution is ca. 200 meV for the SXPS measurements and ca. 50 meV for the HeI–UPS valence band measurements. At BESSY-1 we always measured in CAE-modus with 15 eV pass energy, which results in an broadening of 150 meV due to the analysator (DE/E=1/100). The HeI–UPS measurements are taken with 6 eV pass energy (DE/E=1/130), but here the angle resolution is ±30°, which results in an additional broadening for the dispersing valence bands. The crystals were prepared by chemical vapour transport. They were attached to the sample holder by a conductive Ag epoxy and cleaved in UHV to achieve a clean, mirror-like (0001) surface. Cs was deposited in situ by SAES dispensers. The base pressure of the system was ca. 5×10−11 mbar in the main chamber and below 3×10−9 mbar during Cs deposition in the preparation chamber (at 7 A dispenser heating current with 30 s preheat period before deposition). Derived from the stoichiometry data we estimate a deposition rate of at least a monolayer of Cs for a 4 min exposure. No contamination caused by the Cs evaporation is detected on the surface by XPS.
3. Results
2. Experimental Photoemission experiments ( UPS, XPS, SXPS ) were performed in two different UHV systems: an angle resolving (Dk=±1°) VG ADES 500 spectrometer at BESSY-1 with photon energies in the
Figs. 1–4 displays the valence band and core level spectra of 1T–TaSe (sample I ) in the course 2 of Cs deposition taken at BESSY. After 1 min Cs deposition time in the valence band spectra (Fig. 1) the Cs 5p doublet grows in at 10.6 eV. With ongoing Cs exposure up to 10 min the doublet shifts (altogether) by 200 meV to higher BEs. The high
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Fig. 1. Valence band spectra of 1T–TaSe (sample I ) taken at 2 BESSY I (hv=21 eV, RT ) in the course of Cs deposition. After 0.5 min Cs deposition the Cs 5p doublet grows in, which shifts by 200 meV to higher BEs after the following steps.
Fig. 2. The Ta d 2 derived valence band (expanded from Fig. 1) z in the course of Cs deposition. After 2 min Cs deposition a bandgap of ca. 300 meV opens. At the same stage a CDW transition occurs (see Fig. 5).
BE component of the Cs 5p doublet shows a noticeable asymmetric lineshape, which we will address later. The changes in the Ta d 2 derived z valence band in between the first eV BE are shown in Fig. 2 on an extended scale. For the clean sample the CDW already causes a splitting of this Ta d 2 z band into three peaks ( labelled ‘1’, ‘2’ and ‘3’). This is also the case for 1T–TaS [26,27]. After 2 0.5 min Cs exposure peak 1 broadens and grows slightly in intensity. Additionally it shifts away from the Fermi level by >50 meV. After the next deposition step (+1 min Cs) a new emission ( labelled ‘4’) is observed at 650 meV BE, which exceeds peaks 1–3 in intensity. After the following deposition step (+2 min Cs) this peak ‘4’ is dominant and a bandgap of ca. 300 meV has opened: a metal to semiconductor transition has occurred. At the same stage (+2 min Cs) the LEED picture (Fig. 5) reveals an CDW phase transition from
the commensurate p(E13×E13)R13.89° structure ( Fig. 5a) to the c(2E3×4)rect. structure (Fig. 5b and c). A similar superstructure was described for N H intercalation which is labelled as a 2 4 E7×E7 structure [16 ]. The primitive translation of the superstructure is in fact E7a, where a is the lattice constant of the original hexagonal lattice. However, the vectors of the rectangular centred unit cell have the lengths 2E3a and 4a, so we prefer to label it as a c(2E3×4)rect. structure [28]. This superstructure is presented elsewhere in more detail [29]. For other intercalation systems the appearance of a number of different superstructures is reported [11,24,25], but for the system Cs/1T–TaSe we find only one CDW phase 2 transition. In Fig. 3 the Se 3d emission broadens slightly and shifts by 200 meV to higher BEs in the course of Cs deposition. In Fig. 4 the Ta 4f core level
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Fig. 3. Se 3d core level spectra (hv=80 eV ) with increasing total Cs deposition time. After 10 min Cs deposition the Se 3d emission shifts by 200 meV to higher BEs and broadens slightly.
doublet shows that there is for the clean sample already an additional so called ‘CDW splitting’. After 10 min Cs exposure this splitting increases from 600 to 850 meV, by shifting the high BE components by 250 meV to higher BEs, whereas the low BE component remains unchanged. As a supplement to these SXPS data we present in Figs. 6–8 XPS and valence band spectra of another 1T–TaSe sample (sample II ), taken at the 2 angle integrating spectrometer. In Fig. 6 the Ta d 2 derived valence band inbetween the first eV BE z shows the equivalent behaviour as sample I in Fig. 1. The CDW phase transition (checked by LEED) occurs after 50 s Cs deposition. But for the Cs 5p emission we find some differences between sample I and II. After 10 s Cs deposition the 5p doublet grows in at 12 eV BE (solid vertical lines) and it is only after 50 s Cs deposition that a second doublet appears at 10.6 eV BE (dotted vertical lines). Therefore the above mentioned
Fig. 4. Ta 4f core level spectra (hv=80 eV ) with increasing total Cs deposition time. The spectra of the clean sample shows already a ‘CDW splitting’. After 10 min Cs deposition this splitting increases by 250 meV.
asymmetric lineshape of the high BE Cs 5p component, observed in Fig. 1, is provoked by two overlapping doublet components. Also the C 3d 5/2 emission in Fig. 7 shows a splitting into two components. For the first Cs deposition steps a weak emission at 726 eV BE ( labelled ‘S’) and a second emission at 724.55 eV BE ( labelled ‘I’) are observed after 50 s Cs exposure. In particular the low BE emission grows in intensity for further Cs deposition. A total of 13 h after a Cs exposure of 7 min we observe a slight decrease of the ‘S’ component intensity, which increases again after an additional 7 min deposition step (+14 min Cs). In Fig. 8 the splitting of the Cs core levels can be seen in the Cs 4d emission, taken from a third 1T–TaSe sample (sample III ). The ‘first visible’ 2 doublet (again labelled ‘S’) has a BE of 77 eV. After 27 min Cs deposition the second doublet
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Fig. 5. The LEED picture of the clean sample (a) shows the p(E13×E13)R13.89° structure typical for the commensurate CDW phase at RT. After 2 min Cs deposition the picture switches to the c(2E3×4)rect. structure (b), indicating a CDW phase transition. For clarification an additional LEED picture of the rather complex c(2E3×4)rect. structure (from another sample) is shown in (c).
Fig. 6. Valence band spectra of 1T–TaSe (sample II ) taken at 2 the angle integrating system (hv=HeI, RT ). The Cs 5p emission consists from two doublets.
grows in at 75.45 eV BE and after the following step it shifts by 250 meV to higher BEs. When 11 h have elapsed the Cs 4d ‘S’ doublet is shifted
Fig. 7. Cs 3d core level spectra (hv=1253.6 eV ) of sample II 5/2 in the course of Cs deposition. The fits of two Cs 3d compo5/2 nents are also shown. The high BE component is labelled ‘S’, because it results from little Cs adatoms at the surface. The low BE component originates from intercalated Cs and is labelled ‘I’.
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Fig. 8. The splitting in the ‘S’-surface and the ‘I’-intercalated Cs components is mainly seen for the Cs 4d core level spectra (hv=1253.6 eV ).
gression of the relative concentration ratio C /C , determined from the fitted low BE compoCs Ta nent (‘I’) of the Cs 3d emission ( Fig. 7). Within the first minutes of Cs deposition the work function W is lowered by 1.2 eV. During further deposition W saturates at 3.8 eV. We also investigated the intercalation system Cs/1T–TaS , which shows no significant deviating 2 behaviour compared to the Cs/1T–TaSe system 2 presented here. The Ta 4f CDW splitting is smaller for the nc-p(E13×E13)R13.89° phase in clean 1T–TaS [20,21]. The intercalation induced 2 increase in the CDW splitting is somewhat larger. The S 2p core level shows a shift of 200 meV to higher BEs in the course of Cs deposition and the maximum relative concentration ratio C /C also Cs Ta saturates below 60%.
>100 meV to lower BEs. Fig. 9 displays the change in work function W of sample II, derived from the secondary electron cutoff and the pro4. Discussion
Fig. 9. Work function and relative Cs concentration of sample II in the course of Cs deposition. The c /c ratio was deterCs Ta mined from the Cs 3d ‘I’ component in Fig. 7. 5/2
The core levels of the substrate give no evidence for a decomposition reaction as a consequence of Cs deposition, as was observed for other intercalation systems, that is, Li/InSe or Li/WX ( X=Se, 2 S) [30,31]. From the missing attenuation of the hosts core levels and the absent minimum in the work function progression in the course of Cs deposition, we deduce, that there is no compact Cs overlayer or metallic multilayer formed as it is reported for alkali adsorption on metals at room temperature [32,33]. In addition the final work function of 3.8 eV differs remarkably from W for metallic polycrystalline Cs (1.95 eV ) [34]. The formation of metallic Cs clusters as it was observed for the System Cs/WSe at room temperature [35] 2 can also be excluded, because we observe no additional Fermi edge after the metal to semiconductor transition. The strong initial decrease of the work function may be explained by the existence of a very small number but nearly totally ionized Cs adatoms, that form strong dipoles at the surface. We attribute the first visible high BE Cs 5p doublet (‘S’) at 12 eV to these ionized adatoms as is reported for other metallic systems, where intercalation can be excluded: for example 11.7 eV for Cs/W(100), 12 eV for Cs/Ta(100) [36 ]
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and 11.7 eV for Cs/Al(111) [37]. The low BE doublet (‘I’) we assign to intercalated Cs+ is in agreement with Ref. [9]. Therefore we observe no emission near 10.6 eV (intercalated Cs) on the sample holder in contrast to the high BE component. Another hint is the order (sequence) of the ‘I’ and ‘S’ Cs peak appearance for all three Cs emissions. Finally angle variated PE Cs 4d measurements of the similar system Cs/VSe reveals 2 explicitly, that the ‘S’ component stems from surface adsorbed Cs, whereas the ‘I’ component is due to Cs below the surface, that is, to intercalated Cs [38]. The surface component of the Cs 5p level is better resolved at the angle integrating system (Fig. 6) than at BESSY (Fig. 1) because of the better ‘S’/‘I’ intensity ratio and the better energy resolution (50 meV ). The adsorption of this small amount of Cs results in slightly different electrostatical chalcogen sites at the surface and is so attributed to the broadening of the Se 3d emission, displayed in Fig. 3. The 6s valence electron of the Cs is assumed to be transferred to the Ta derived d 2 valence band. z The slight shift and increase in intensity at the initial stage (+0.5 min Cs Fig. 2) can be understood as a successive filling of this band as a consequence of intercalation. This band filling evokes a shift of the Fermi level (with respect to the core level positions), which is seen in the Se 3d, Cs 5p (‘I’), Cs 3d (‘I’) and Cs 4d (‘I’) core level shifts of (altogether) 200–250 meV to higher BEs. The respective Ta 4f core level shift is partly compensated by the charge transfer to the Ta atoms, which results in an inverse shift to lower BEs. The increasing of the Ta 4f CDW splitting is caused by the electrostatic interaction between the Ta 4f levels and the spatial charge distribution pattern in the new c(2E3×4)rect. CDW phase. This assumption is proved in detail by model calculations, which we present elsewhere, as well as the intercalation induced changes in the overall electronic band structure [29,39]. The Cs/Ta ratio, displayed in Fig. 9, is determined only from the fitted ‘I’ Cs 3d emission, resulting in the relative concentration of the intercalated Cs. To take the exponential depth dependence of photoemission spectra into account, an
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average correction factor of 1.7 is applied to the concentration ratio [40]. We chose for the mean ˚ and for l(Ta 4f ) 10– free paths l(Cs 3d) 7–12 A ˚ 20 A, resulting in an correction factor of 1.3–2.1. In regard to this correction range the observed concentration limit of ca. 60% correlates with the (uncorrected ) concentration of 20–30% found for the system Cs/VSe [38]. In contrast to Cu intercal2 ation [8] Cs intercalation takes place on a timescale of minutes and the system comes rapidly to equilibrium, as no pronounced time dependent changes in the spectra after Cs deposition are detected. The only observable ‘time effects’ are the slight increase of the work function after 13 h have elapsed ( Fig. 9), the small decrease in the Cs 3d (‘S’) intensity after 13 h and in the Cs 4d (‘S’) Intensity after 11 h ( Figs. 7 and 8). These effects are caused by the diffusion resp. intercalation of a small amount of surface Cs into the crystals bulk, which seems to push the already intercalated Cs deeper into the volume. After cleaving an intercalated sample again, even when several days after the last deposition step has elapsed, no Cs is detected. Cs does not diffuse deeply into the bulk, as was observed for Li intercalation [24]. The c(2E3×4)rect. superstructure is stable at least for several days. Thus the mobility of the once intercalated Cs seems to be small. Regarding the size of ˚ ) and of the van der Waals the Cs+ ion (ca. 3.4 A ˚ ), Cs must enlarge the gap of 1T–TaSe (ca. 3.06 A 2 gap for intercalation. Additionally the Coulomb energy caused by the repulsion of the Cs+ ions grows with increasing intercalation concentration. Therefore the concentration limit is reached, when the gain of ‘electronic energy’ is exceeded by the loss of Coulomb energy and the loss of energy required for widening the gap. These assumptions are confirmed by the results obtained for the system Li/1T–TaSe . There no Li/Ta concentration 2 limit has been observed, due to the smaller size and the higher charge density of the Li+ion [24]. Furthermore we observe for the system Li/1T–TaX an irreversible morphologic 1T–2H 2 transition close to but below c /c =100% which Li Ta is not reached for Cs/1T–TaX because of the 2 previously mentioned Cs concentration limit. The BE difference between the (high BE ) Cs
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‘S’ and the ( low BE ) Cs ‘I’ doublets amounts to 1.4–1.5 eV after the first deposition steps for all three displayed Cs emissions. This value correlates with the surface dipole induced change of the work function of 1.4–1.5 eV. So we assume that the core level BE shifts due to the Cs surface dipole do not affect the Cs core levels of the adatoms themselves. Additionally we take the possibility of a relaxation effect for the ‘I’ component into account, because the intercalated Cs is much better screened by conduction electrons in the bulk than the Cs surface adatoms. The time dependent shift of the Cs 4d ‘S’ emission to lower BE has been observed also for the Cs/VSe system as well as for other 2 alkali intercalants [38]. It is also possible that this shift exists for the Cs 3d ‘S’ component in Fig. 7, but to resolve it, a more meaningful fitting method is needed, for example, the ‘maximum entropy method’ [41]. The alkali adsorption on layered crystals is not fully understood until now and we guess, that several different Cs surface phases exist, which are responsible for the observed shift to lower BEs. The CDW induced splitting of the Ta 5d 2 z valence band is closely related to the CDW phase and therefore to the PLD rearrangement of the Ta atoms. For the clean crystal at room temperature the conduction band splits up into three narrow manifolds displayed in Fig. 2 due to the forming of star shaped Ta clusters[19,26 ]. The topmost level is supposed to be only a few tenths of meVs broad. This level is half filled with one of the 13 Ta 5d 2 electrons and belongs to the star centre z side, which is not involved in the reconstruction. Therefore this narrow conduction band is completely filled, when a concentration of 1/13=7.7% is reached. Simple LCAO calculations [26 ] suggest that the empty Ta derived bands above E are not F separated by gaps, but overlapping with a range of at least 500 meV. Thus no gap-opening is expected within this range, due to band filling. The CDW transition and the related gap-opening takes place at a Cs/Ta ratio of 20–30%, far below total filling of the Ta 5d 2 derived valence band. z Therefore we tentatively attribute the gap-opening to a Mott–Hubbard localization effect [42]. Altering the Fermi surface by intercalation changes the nesting condition for the CDW vector q=2k F
and drives the phase transition from the p(E13×E13)R13.89° structure to the c( 2E3×E4)rect. structure. The new superstructure is a manifestation of a different PLD arrangement of the crystal atoms with changed symmetry and therefore a different shape (splitting) of the conduction band. The splitting due to the p( E13×E13)R13.89° structure (emissions ‘1’–‘3’) is transformed into a new one, which is observed in Fig. 2 as the new emissions ‘4’ and ‘5’ (+1 min Cs). Emission ‘4’ is assigned to a lower Mott– Hubbard band, so that a Mott–Hubbard energy U (which is the energetical distance between the lower and the upper MH band) of ca. 1.4 eV is obtained. As the dimension of the spatial primitive translation in the c(2E3×4)rect. structure is reduced by ca. 27% compared to the p(E13×E13)R13.89° structure, we can only speculate about changes in the Ta d 2 overlap integrals z and in correlation energy, because the exact spatial structure of the c(2E3×4)rect. phase is not investigated until now. In future it is required to specify the magnitude and direction of the atom displacements for the c(2E3×4)rect. phase, for example, by X-ray diffraction measurements. These parameters are needed as an input for theoretical bandstructure calculations which could verify or disprove the aforementioned proposition. Summarizing the changes in the Ta d 2 conducz tion band and in the Ta 4f core levels (as well as the changes in the overall electronic band structure, which are presented elsewhere [39]) cannot be interpreted in terms of the rigid band model, except for the early stage before the CDW phase transition occurs.
Acknowledgement The authors would like to thank R. Adelung (Christian-Albrechts-Uni, Kiel ) for the decoding of the c(2E3×4)rect. structure.
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Localization and charge density wave transformation in Cs intercalated 1T–TaSe 2 H.J. Crawack, Y. Tomm, C. Pettenkofer * Hahn-Meitner-Institut, Abteilung SE-6: Elektronische Struktur, Glienicker Straße 100, 14109 Berlin, Germany Received 17 March 2000; accepted for publication 29 June 2000
Abstract Cs was adsorbed at room temperature onto the (0001) cleavage planes of 1T–TaSe . The deposited Cs forms no 2 metallic overlayer, but intercalates after an initial adsorption stage into the van der Waals gap of the layered crystal. A limit of the relative concentration ratio c /c of ca. 60% is observed. Due to the electron transfer associated with Cs Ta the intercalation the charge density wave induced p(E13×E13)R13.89° superstructure exhibits a phase transition to a c(2E3×4)rect. structure. As a further consequence of the intercalation a metal to semiconductor transition occurs. The change in the electronic structure could not be interpreted within the rigid band model, but is tentatively explained by an localization effect, caused by the charge density wave phase transition. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Alkali metals; Low energy electron diffraction (LEED); X-ray photoelectron spectroscopy
1. Introduction Layered transition metal dichalcogenides ( TMDCs) are regarded as prototypes of low dimensional solids. They are formed by covalently bound XMMMX sandwich layers (M=transition metal, X=S, Se, Te), which are connected to each other by a weak so-called van der Waals like interaction [1,2]. The TMDCs have been investigated intensively during the recent decades due to of a wide range of exceptional properties. For example they serve as cathodes for rechargeable intercalation batteries [3] and they are proposed for an application in thin film solar cells [4,5]. Most of the TMDCs are known to form intercalation complexes by inserting electron donating * Corresponding author. Fax: +49-30-80622434. E-mail address: [email protected] (C. Pettenkofer)
species (e.g. alkalis) between the layers into their ‘van der Waals gap’ [6 ]. As with electrochemical treatment or intercalation during the crystal growth it was recently shown that in situ intercalation takes place by depositing the intercalate on the (0001) cleavage plane in ultrahigh vacuum ( UHV ) [7–11]. Deintercalation is also possible by additionally adsorbing electronegative species (e.g. oxygen or halogens) onto the surface of an intercalated TMDC [12,13]. In a first approximation the reversible redox reaction maybe described in terms of the rigid band model, which assumes that electronic charge from the intercalated species is transferred to the lowest unoccupied band of the host material [14,15]. The charge transfer hardly modifies the hosts band structure (rigid band) but raises only the position of the Fermi level and so if the host is a metal it changes the shape of the Fermi surface [16 ].
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Among the metallic layer crystals of group Vb the octahedrally coordinated 1T–TaS and 2 1T–TaSe show a strong anomaly of the conduc2 tion band electrons, a so-called charge density wave (CDW ), which was first predicted for lowdimensional metals by Peierls [17,18]. A CDW is always accompanied by a periodic lattice distortion (PLD), which shows up as a superstructure in low energy electron diffraction (LEED). The nearly commensurate (nc) p(E13×E13)R13.89° superstructure of pure 1T–TaS , present at room temper2 ature, has been explained by the ‘Star of David model’ introduced by Fazekas and Tosatti [19]. For the nc superstructure the CDW is arranged in a regular hexagonal lattice in circular domains of ˚ diameter [20]. 1T–TaSe exhibits a comca. 73 A 2 mensurate (c) p(E13×E13)R13.89° CDW. Moreover the CDW, which concerns only the Ta-derived d 2 level forming the conduction band, z causes a splitting of the Ta 4f core level, due to the inequivalent charge at different Ta sites [21]. The spanning vector q=2k , which determines F direction and periodicity of the CDW phase is closely related to the shape of the Fermi surface. Only if q fulfils the so-called nesting condition can the respective CDW occur [22,23]. Intercalation will change the Fermi surface in a controlled way and thus drives the crystal into different CDW phases, as it was shown for Na/1T–TaS [11], Cu/1T–TaS /TaSe [8] and 2 2 2 Li/1T–TaS [24,25]. Especially for the latter system 2 a variety of CDW phase transitions has been observed. Here we present a photoemission and LEED study of the system Cs/1T–TaSe . Varying 2 amounts of Cs were deposited at room temperature onto the (0001) van der Waals plane of 1T–TaSe 2 and the electronic and morphologic changes are studied simultaneously by photoelectron spectroscopy and LEED.
range 10–120 eV ( TGM 7 beamline) and an angle integrating Leybold EA II MCD spectrometer with HeI and Mg Ka radiation. The spectra for the Cs 3d and 4d emissions were exclusively taken at the latter system. All spectra were recorded in normal emission and at room temperature. They are calibrated to the photon flux for SXPS and scaled in counts/point for HeI/Mg Ka radiation. For the valence band spectra a bias voltage of −6 V (and −2 V, respectively) was applied to the sample. Furthermore all spectra are given in binding energy (BE ) referred to the Fermi level of a metallic sample holder (Cu). The energy resolution is ca. 200 meV for the SXPS measurements and ca. 50 meV for the HeI–UPS valence band measurements. At BESSY-1 we always measured in CAE-modus with 15 eV pass energy, which results in an broadening of 150 meV due to the analysator (DE/E=1/100). The HeI–UPS measurements are taken with 6 eV pass energy (DE/E=1/130), but here the angle resolution is ±30°, which results in an additional broadening for the dispersing valence bands. The crystals were prepared by chemical vapour transport. They were attached to the sample holder by a conductive Ag epoxy and cleaved in UHV to achieve a clean, mirror-like (0001) surface. Cs was deposited in situ by SAES dispensers. The base pressure of the system was ca. 5×10−11 mbar in the main chamber and below 3×10−9 mbar during Cs deposition in the preparation chamber (at 7 A dispenser heating current with 30 s preheat period before deposition). Derived from the stoichiometry data we estimate a deposition rate of at least a monolayer of Cs for a 4 min exposure. No contamination caused by the Cs evaporation is detected on the surface by XPS.
3. Results
2. Experimental Photoemission experiments ( UPS, XPS, SXPS ) were performed in two different UHV systems: an angle resolving (Dk=±1°) VG ADES 500 spectrometer at BESSY-1 with photon energies in the
Figs. 1–4 displays the valence band and core level spectra of 1T–TaSe (sample I ) in the course 2 of Cs deposition taken at BESSY. After 1 min Cs deposition time in the valence band spectra (Fig. 1) the Cs 5p doublet grows in at 10.6 eV. With ongoing Cs exposure up to 10 min the doublet shifts (altogether) by 200 meV to higher BEs. The high
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Fig. 1. Valence band spectra of 1T–TaSe (sample I ) taken at 2 BESSY I (hv=21 eV, RT ) in the course of Cs deposition. After 0.5 min Cs deposition the Cs 5p doublet grows in, which shifts by 200 meV to higher BEs after the following steps.
Fig. 2. The Ta d 2 derived valence band (expanded from Fig. 1) z in the course of Cs deposition. After 2 min Cs deposition a bandgap of ca. 300 meV opens. At the same stage a CDW transition occurs (see Fig. 5).
BE component of the Cs 5p doublet shows a noticeable asymmetric lineshape, which we will address later. The changes in the Ta d 2 derived z valence band in between the first eV BE are shown in Fig. 2 on an extended scale. For the clean sample the CDW already causes a splitting of this Ta d 2 z band into three peaks ( labelled ‘1’, ‘2’ and ‘3’). This is also the case for 1T–TaS [26,27]. After 2 0.5 min Cs exposure peak 1 broadens and grows slightly in intensity. Additionally it shifts away from the Fermi level by >50 meV. After the next deposition step (+1 min Cs) a new emission ( labelled ‘4’) is observed at 650 meV BE, which exceeds peaks 1–3 in intensity. After the following deposition step (+2 min Cs) this peak ‘4’ is dominant and a bandgap of ca. 300 meV has opened: a metal to semiconductor transition has occurred. At the same stage (+2 min Cs) the LEED picture (Fig. 5) reveals an CDW phase transition from
the commensurate p(E13×E13)R13.89° structure ( Fig. 5a) to the c(2E3×4)rect. structure (Fig. 5b and c). A similar superstructure was described for N H intercalation which is labelled as a 2 4 E7×E7 structure [16 ]. The primitive translation of the superstructure is in fact E7a, where a is the lattice constant of the original hexagonal lattice. However, the vectors of the rectangular centred unit cell have the lengths 2E3a and 4a, so we prefer to label it as a c(2E3×4)rect. structure [28]. This superstructure is presented elsewhere in more detail [29]. For other intercalation systems the appearance of a number of different superstructures is reported [11,24,25], but for the system Cs/1T–TaSe we find only one CDW phase 2 transition. In Fig. 3 the Se 3d emission broadens slightly and shifts by 200 meV to higher BEs in the course of Cs deposition. In Fig. 4 the Ta 4f core level
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Fig. 3. Se 3d core level spectra (hv=80 eV ) with increasing total Cs deposition time. After 10 min Cs deposition the Se 3d emission shifts by 200 meV to higher BEs and broadens slightly.
doublet shows that there is for the clean sample already an additional so called ‘CDW splitting’. After 10 min Cs exposure this splitting increases from 600 to 850 meV, by shifting the high BE components by 250 meV to higher BEs, whereas the low BE component remains unchanged. As a supplement to these SXPS data we present in Figs. 6–8 XPS and valence band spectra of another 1T–TaSe sample (sample II ), taken at the 2 angle integrating spectrometer. In Fig. 6 the Ta d 2 derived valence band inbetween the first eV BE z shows the equivalent behaviour as sample I in Fig. 1. The CDW phase transition (checked by LEED) occurs after 50 s Cs deposition. But for the Cs 5p emission we find some differences between sample I and II. After 10 s Cs deposition the 5p doublet grows in at 12 eV BE (solid vertical lines) and it is only after 50 s Cs deposition that a second doublet appears at 10.6 eV BE (dotted vertical lines). Therefore the above mentioned
Fig. 4. Ta 4f core level spectra (hv=80 eV ) with increasing total Cs deposition time. The spectra of the clean sample shows already a ‘CDW splitting’. After 10 min Cs deposition this splitting increases by 250 meV.
asymmetric lineshape of the high BE Cs 5p component, observed in Fig. 1, is provoked by two overlapping doublet components. Also the C 3d 5/2 emission in Fig. 7 shows a splitting into two components. For the first Cs deposition steps a weak emission at 726 eV BE ( labelled ‘S’) and a second emission at 724.55 eV BE ( labelled ‘I’) are observed after 50 s Cs exposure. In particular the low BE emission grows in intensity for further Cs deposition. A total of 13 h after a Cs exposure of 7 min we observe a slight decrease of the ‘S’ component intensity, which increases again after an additional 7 min deposition step (+14 min Cs). In Fig. 8 the splitting of the Cs core levels can be seen in the Cs 4d emission, taken from a third 1T–TaSe sample (sample III ). The ‘first visible’ 2 doublet (again labelled ‘S’) has a BE of 77 eV. After 27 min Cs deposition the second doublet
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Fig. 5. The LEED picture of the clean sample (a) shows the p(E13×E13)R13.89° structure typical for the commensurate CDW phase at RT. After 2 min Cs deposition the picture switches to the c(2E3×4)rect. structure (b), indicating a CDW phase transition. For clarification an additional LEED picture of the rather complex c(2E3×4)rect. structure (from another sample) is shown in (c).
Fig. 6. Valence band spectra of 1T–TaSe (sample II ) taken at 2 the angle integrating system (hv=HeI, RT ). The Cs 5p emission consists from two doublets.
grows in at 75.45 eV BE and after the following step it shifts by 250 meV to higher BEs. When 11 h have elapsed the Cs 4d ‘S’ doublet is shifted
Fig. 7. Cs 3d core level spectra (hv=1253.6 eV ) of sample II 5/2 in the course of Cs deposition. The fits of two Cs 3d compo5/2 nents are also shown. The high BE component is labelled ‘S’, because it results from little Cs adatoms at the surface. The low BE component originates from intercalated Cs and is labelled ‘I’.
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Fig. 8. The splitting in the ‘S’-surface and the ‘I’-intercalated Cs components is mainly seen for the Cs 4d core level spectra (hv=1253.6 eV ).
gression of the relative concentration ratio C /C , determined from the fitted low BE compoCs Ta nent (‘I’) of the Cs 3d emission ( Fig. 7). Within the first minutes of Cs deposition the work function W is lowered by 1.2 eV. During further deposition W saturates at 3.8 eV. We also investigated the intercalation system Cs/1T–TaS , which shows no significant deviating 2 behaviour compared to the Cs/1T–TaSe system 2 presented here. The Ta 4f CDW splitting is smaller for the nc-p(E13×E13)R13.89° phase in clean 1T–TaS [20,21]. The intercalation induced 2 increase in the CDW splitting is somewhat larger. The S 2p core level shows a shift of 200 meV to higher BEs in the course of Cs deposition and the maximum relative concentration ratio C /C also Cs Ta saturates below 60%.
>100 meV to lower BEs. Fig. 9 displays the change in work function W of sample II, derived from the secondary electron cutoff and the pro4. Discussion
Fig. 9. Work function and relative Cs concentration of sample II in the course of Cs deposition. The c /c ratio was deterCs Ta mined from the Cs 3d ‘I’ component in Fig. 7. 5/2
The core levels of the substrate give no evidence for a decomposition reaction as a consequence of Cs deposition, as was observed for other intercalation systems, that is, Li/InSe or Li/WX ( X=Se, 2 S) [30,31]. From the missing attenuation of the hosts core levels and the absent minimum in the work function progression in the course of Cs deposition, we deduce, that there is no compact Cs overlayer or metallic multilayer formed as it is reported for alkali adsorption on metals at room temperature [32,33]. In addition the final work function of 3.8 eV differs remarkably from W for metallic polycrystalline Cs (1.95 eV ) [34]. The formation of metallic Cs clusters as it was observed for the System Cs/WSe at room temperature [35] 2 can also be excluded, because we observe no additional Fermi edge after the metal to semiconductor transition. The strong initial decrease of the work function may be explained by the existence of a very small number but nearly totally ionized Cs adatoms, that form strong dipoles at the surface. We attribute the first visible high BE Cs 5p doublet (‘S’) at 12 eV to these ionized adatoms as is reported for other metallic systems, where intercalation can be excluded: for example 11.7 eV for Cs/W(100), 12 eV for Cs/Ta(100) [36 ]
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and 11.7 eV for Cs/Al(111) [37]. The low BE doublet (‘I’) we assign to intercalated Cs+ is in agreement with Ref. [9]. Therefore we observe no emission near 10.6 eV (intercalated Cs) on the sample holder in contrast to the high BE component. Another hint is the order (sequence) of the ‘I’ and ‘S’ Cs peak appearance for all three Cs emissions. Finally angle variated PE Cs 4d measurements of the similar system Cs/VSe reveals 2 explicitly, that the ‘S’ component stems from surface adsorbed Cs, whereas the ‘I’ component is due to Cs below the surface, that is, to intercalated Cs [38]. The surface component of the Cs 5p level is better resolved at the angle integrating system (Fig. 6) than at BESSY (Fig. 1) because of the better ‘S’/‘I’ intensity ratio and the better energy resolution (50 meV ). The adsorption of this small amount of Cs results in slightly different electrostatical chalcogen sites at the surface and is so attributed to the broadening of the Se 3d emission, displayed in Fig. 3. The 6s valence electron of the Cs is assumed to be transferred to the Ta derived d 2 valence band. z The slight shift and increase in intensity at the initial stage (+0.5 min Cs Fig. 2) can be understood as a successive filling of this band as a consequence of intercalation. This band filling evokes a shift of the Fermi level (with respect to the core level positions), which is seen in the Se 3d, Cs 5p (‘I’), Cs 3d (‘I’) and Cs 4d (‘I’) core level shifts of (altogether) 200–250 meV to higher BEs. The respective Ta 4f core level shift is partly compensated by the charge transfer to the Ta atoms, which results in an inverse shift to lower BEs. The increasing of the Ta 4f CDW splitting is caused by the electrostatic interaction between the Ta 4f levels and the spatial charge distribution pattern in the new c(2E3×4)rect. CDW phase. This assumption is proved in detail by model calculations, which we present elsewhere, as well as the intercalation induced changes in the overall electronic band structure [29,39]. The Cs/Ta ratio, displayed in Fig. 9, is determined only from the fitted ‘I’ Cs 3d emission, resulting in the relative concentration of the intercalated Cs. To take the exponential depth dependence of photoemission spectra into account, an
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average correction factor of 1.7 is applied to the concentration ratio [40]. We chose for the mean ˚ and for l(Ta 4f ) 10– free paths l(Cs 3d) 7–12 A ˚ 20 A, resulting in an correction factor of 1.3–2.1. In regard to this correction range the observed concentration limit of ca. 60% correlates with the (uncorrected ) concentration of 20–30% found for the system Cs/VSe [38]. In contrast to Cu intercal2 ation [8] Cs intercalation takes place on a timescale of minutes and the system comes rapidly to equilibrium, as no pronounced time dependent changes in the spectra after Cs deposition are detected. The only observable ‘time effects’ are the slight increase of the work function after 13 h have elapsed ( Fig. 9), the small decrease in the Cs 3d (‘S’) intensity after 13 h and in the Cs 4d (‘S’) Intensity after 11 h ( Figs. 7 and 8). These effects are caused by the diffusion resp. intercalation of a small amount of surface Cs into the crystals bulk, which seems to push the already intercalated Cs deeper into the volume. After cleaving an intercalated sample again, even when several days after the last deposition step has elapsed, no Cs is detected. Cs does not diffuse deeply into the bulk, as was observed for Li intercalation [24]. The c(2E3×4)rect. superstructure is stable at least for several days. Thus the mobility of the once intercalated Cs seems to be small. Regarding the size of ˚ ) and of the van der Waals the Cs+ ion (ca. 3.4 A ˚ ), Cs must enlarge the gap of 1T–TaSe (ca. 3.06 A 2 gap for intercalation. Additionally the Coulomb energy caused by the repulsion of the Cs+ ions grows with increasing intercalation concentration. Therefore the concentration limit is reached, when the gain of ‘electronic energy’ is exceeded by the loss of Coulomb energy and the loss of energy required for widening the gap. These assumptions are confirmed by the results obtained for the system Li/1T–TaSe . There no Li/Ta concentration 2 limit has been observed, due to the smaller size and the higher charge density of the Li+ion [24]. Furthermore we observe for the system Li/1T–TaX an irreversible morphologic 1T–2H 2 transition close to but below c /c =100% which Li Ta is not reached for Cs/1T–TaX because of the 2 previously mentioned Cs concentration limit. The BE difference between the (high BE ) Cs
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‘S’ and the ( low BE ) Cs ‘I’ doublets amounts to 1.4–1.5 eV after the first deposition steps for all three displayed Cs emissions. This value correlates with the surface dipole induced change of the work function of 1.4–1.5 eV. So we assume that the core level BE shifts due to the Cs surface dipole do not affect the Cs core levels of the adatoms themselves. Additionally we take the possibility of a relaxation effect for the ‘I’ component into account, because the intercalated Cs is much better screened by conduction electrons in the bulk than the Cs surface adatoms. The time dependent shift of the Cs 4d ‘S’ emission to lower BE has been observed also for the Cs/VSe system as well as for other 2 alkali intercalants [38]. It is also possible that this shift exists for the Cs 3d ‘S’ component in Fig. 7, but to resolve it, a more meaningful fitting method is needed, for example, the ‘maximum entropy method’ [41]. The alkali adsorption on layered crystals is not fully understood until now and we guess, that several different Cs surface phases exist, which are responsible for the observed shift to lower BEs. The CDW induced splitting of the Ta 5d 2 z valence band is closely related to the CDW phase and therefore to the PLD rearrangement of the Ta atoms. For the clean crystal at room temperature the conduction band splits up into three narrow manifolds displayed in Fig. 2 due to the forming of star shaped Ta clusters[19,26 ]. The topmost level is supposed to be only a few tenths of meVs broad. This level is half filled with one of the 13 Ta 5d 2 electrons and belongs to the star centre z side, which is not involved in the reconstruction. Therefore this narrow conduction band is completely filled, when a concentration of 1/13=7.7% is reached. Simple LCAO calculations [26 ] suggest that the empty Ta derived bands above E are not F separated by gaps, but overlapping with a range of at least 500 meV. Thus no gap-opening is expected within this range, due to band filling. The CDW transition and the related gap-opening takes place at a Cs/Ta ratio of 20–30%, far below total filling of the Ta 5d 2 derived valence band. z Therefore we tentatively attribute the gap-opening to a Mott–Hubbard localization effect [42]. Altering the Fermi surface by intercalation changes the nesting condition for the CDW vector q=2k F
and drives the phase transition from the p(E13×E13)R13.89° structure to the c( 2E3×E4)rect. structure. The new superstructure is a manifestation of a different PLD arrangement of the crystal atoms with changed symmetry and therefore a different shape (splitting) of the conduction band. The splitting due to the p( E13×E13)R13.89° structure (emissions ‘1’–‘3’) is transformed into a new one, which is observed in Fig. 2 as the new emissions ‘4’ and ‘5’ (+1 min Cs). Emission ‘4’ is assigned to a lower Mott– Hubbard band, so that a Mott–Hubbard energy U (which is the energetical distance between the lower and the upper MH band) of ca. 1.4 eV is obtained. As the dimension of the spatial primitive translation in the c(2E3×4)rect. structure is reduced by ca. 27% compared to the p(E13×E13)R13.89° structure, we can only speculate about changes in the Ta d 2 overlap integrals z and in correlation energy, because the exact spatial structure of the c(2E3×4)rect. phase is not investigated until now. In future it is required to specify the magnitude and direction of the atom displacements for the c(2E3×4)rect. phase, for example, by X-ray diffraction measurements. These parameters are needed as an input for theoretical bandstructure calculations which could verify or disprove the aforementioned proposition. Summarizing the changes in the Ta d 2 conducz tion band and in the Ta 4f core levels (as well as the changes in the overall electronic band structure, which are presented elsewhere [39]) cannot be interpreted in terms of the rigid band model, except for the early stage before the CDW phase transition occurs.
Acknowledgement The authors would like to thank R. Adelung (Christian-Albrechts-Uni, Kiel ) for the decoding of the c(2E3×4)rect. structure.
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