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Investigation on electronic structure of Cu clusters on graphite by EELS and XPS studies
Solid State Communications, Vol. 74, No. 2, pp. 115-118, 1990. Printed in Great Britain.
0038-1098/90 $3.00 + .00 Pergamon Press plc
INVESTIGATION ON ELECTRONIC STRUCTURE OF Cu CLUSTERS ON GRAPHITE BY EELS AND XPS STUDIES M. De Crescenzi*, M. Diociaiuti, L. Lozzi, P. Picozzi and S. Santucci Dipartimento di Fisica, Universita' dell'Aquila, 67100 L'Aquila, Italy * Dipartimento di Fisica, Universita' di Roma II, Tor Vergata, 00173 Roma, Italy (Received 22 December 1988 by E. Tosatti) We have applied electron energy loss (EELS) and X-ray photoemission (XPS) spectroscopies to study the electronic structure of copper clusters deposited in situ on clean graphite. The XPS results show an increase in the binding energy of both valence band and core levels when the cluster size decreases. Furthermore the EELS results show a sizeable increase in energy loss for the feature located at about 4 eV involving 3d ~ E/states when the cluster size decreases. The observed shifts in the EELS spectrum are the same as those measured for the 3d valence band shifts in the XPS spectra. Since the electrostatic charging effects, suggested in the past as the explanation for the variation in electronic properties occurring in the XPS of small clusters, cannot be invoked in EELS measurements we interpret the reported shifts as a genuine effect caused by band structure variation due to a redistribution of the states within the cluster d-band.
INTRODUCTION
At the moment the problem of the predominance of one effect over the other is still open from both an experimental and theoretical point of view although cluster ionization potential recently computed by Bagus et aL [10] supported the initial state effect. In order to contribute to the discussion of the previous point of view we have performed EELS spectra on Cu in an energy loss region involving excitations from the 3d electrons towards the Fermi level. The EELS spectra show a sizeable shift of the structure located at 4 eV, towards higher energy losses, of the same order of magnitude as the valence band XPS shift. The EELS features involve single particle electronic transitions without photoelectron emission ruling out any charging effect on the cluster. As a consequence we conclude that the observed shifts in the XPS valence band may be a genuine effect of the electronic structure variation occurring in the smallest clusters.
COPPER clusters have been widely studied in the last few years in order to investigate variations in structural and electronic properties when the cluster size decreases [1-5]. Our previous studies have dealt with optical properties [6], Auger and XPS .(core level and valence band) [7] and extended electron energy loss fine structure (EELFS) [8] spectroscopies. The most significant result is a sizeable variation of both valence and core level binding energies when the cluster diameter decreases. These variations, of the order of 1 eV, also product shifts in the corresponding Auger spectra [2. 7, 9]. Generally the shifts observed in the XPS spectra have been attributed in the past either to initial or to final state effects [1-4]. Initial state effects are generally attributed to the intrinsic change in the electronic structure when passing from the isolated atom to the bulk. On the contrary the electrostatic charging of clusters not completely neutralized by the free elecEXPERIMENT trons supplied by the substrate has been invoked to justify final state effects. Copper aggregates were deposited in ultra high In order to emphasize latter effect Wertheim et al. vacuum by evaporation from a tungsten wire on both [4] reported careful XPS spectra of A g clusters show- a polycrystalline graphite (Goodfellow 99.8%) and a ing a shift of the same order of the 3d5/2 core electrons, grid for transmission electron microscopy. The grid of the 4d band centroid and of the Fermi cut off. was used to determine the cluster mean diameter 115
116
INVESTIGATION ON E L E C T R O N I C S T R U C T U R E
distribution. The nominal thicknesses were monitored by an Inficon quartz microbalance. The VG-3MKII-ESCA chamber was equipped with an X-ray (AIK~) source for XPS measurements and with an electron gun for EELS measurements. More details on the experimental apparatus are reported elsewhere [11]. RESULTS AND DISCUSSION Figure 1 shows the XPS spectra of different copper nominal thicknesses on graphite. We note a continuous shift of the valence band towards higher binding energies decreasing the cluster size. The shift of the valence band centroid amounts to 1 eV for a 1 A nominal deposition constituted by clusters with mean diameter of about 6 A. No shift has been observed for depositions with nominal thickness greater than 20 A (mean diameter about 60 A). These results are !n good agreement with previous investigations of Cu on graphite reported by different authors [1, 2, 12]. On the other hand the same qualitative results have been reported for Cu deposited on clean metal surfaces like Zn [13] and AI [14]. Shifts of the same order of magnitude have also been observed for core levels and Auger spectra [7, 9]. BINDING 0
2
4
(eV)
ENERGY 6
8
I0
C
IZ IM IZ
KINETIC ENERGY (eV)
Fig. I. X-ray photoelectron spectra of the valence band of bulk and Cu clusters deposited on polycrystalline graphite. The nominal thicknesses are reported.
Vol. 74, No. 2
Two different points of view, in the last few years. have been suggested to explain the shifts observed in the photoemission spectra. The first deals with th,: change in the intrinsic electronic properties (so called "initial state effect"), which occurs in the small clusters, due to the level hybridization and/or the change in the nearest neighbours coordination number. According to this interpretation the observed shift has been attributed to a lowering of the d band portion of the valence band with respect to the Fermi level [1, 2]. The second point of view interprets the observed shift as an "apparent shift" caused by the cluster charging, during the photoemission process, not being fully screened by the flowing of the free electrons supplied by the substrate. In particular, this so called "final state effect" should shift all energies (valence d band, Fermi level and core structures) by an amount of energy of the order of e2/2r where r is the cluster radius [3, 4]. Furthermore a d band narrowing, decreasing the cluster size, should increase the threshold excitation energy while the valence band centroid remains fixed relative to the Fermi level [4, 14]. This consideration is ruled out because our XPS spectra show an important shift of the centroid of the d band without a corresponding shift of the Fermi level for all investigated clusters. Due to our limited experimental resolution (~>leV) no sizeable narrowing of the 3 d-band can be observed. In order to discriminate initial state from the final state effect we have performed electron energy loss measurements of the same Cu evaporation. The EELS results are shown in Fig. 2. The electron beam impinged on the surface at an angle of 50 ° in order to produce a greater surface (Cu cluster) to volume (graphite) ratio. The energy of the primary electron beam was 1000 eV and a typical beam current density of 1 #A mm -2 was used. The evaporations with nominal thicknesses 20 and 30 ~, (which correspond to cluster mean diameters of about 60 and 90A respectively) show the same features labelled with A, B, C, D and E reported in the EELS spectrum of the Cu bulk [15]. Decreasing the nominal thickness we note a continuous shift towards higher energy losses in the structures A and D. The structures B, C and E, however, do not show a sizeable shift, probably because of the presence of the strong rr* graphite substrate structures. For this reason we have focused our attention on the structure A because it can be interpreted in terms of electronic single particle energy band and, moreover, because it is not influenced at all by graphite absorption. According to the dielectric theory [15-17] each structure in the energy loss spectra should correspond to the hollows of the imaginary part of the complex dielectric function g = e~ + ig2 because of
INVESTIGATION ON ELECTRONIC STRUCTURE
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Energy Loss (eV) Fig. 2. Low electron energy loss spectra for clean polycrystalline graphite and different Cu nominal thicknesses. The primary beam energy was E~ = 1000 eV. the behaviour of the energy loss function [15] N(E) oc - I r a
=
+ ~ + F;
Figure 3(a) shows the contribution to e2 coming from the transitions between initial d-band states (i = 1-5) below the Fermi level and the first two final empty band states (.f = 6, 7) above the Fermi level for copper bulk as reported by Lfisser et al. [18]. Figure 3(b) shows the experimental energy loss structures A and B for the Cu bulk. Variations in the electronic band structure and, in particular its position with respect to the Fermi level of the d band, should produce an energy shift in the e_,(i ~ f ) features.
Fig. 3. (a) Partial e2(i-f) spectra of Cu bulk transitions from the initial bands i (1-5) to the final bands f ( 6 - 7 ) above Ef as reported by L/isser et al. [18], (b) the energy positions of the Cu bulk EELS excitation (thick lines) are reported together with the first EELS structures A and B. Since each structure in the N(E) energy loss measurements follows very closely the variations occurring in e2, an increase of energy loss indicates an increase in binding energy of the 3d band with respect to the Fermi level. The shifts observed of the EELS feature A of the different clusters are in good agreement with the XPS experiments reported in Fig. 2. Since the electrostatic charging effect should play a minor role in the energy loss spectra (because the loss energies refer to the primary beam energy) we can exclude the final state effect as responsible for the observed shift also in the XPS data. On the other hand one should note that a rigid shift of both the valence band and Fermi level [4] should not produce a detectable shift in the electron energy loss features. A more complete knowledge of the electronic structure variation occurring in the cluster should also take into account the other energy loss features B, C, D, E superimposed upon the energy loss
118
INVESTIGATION ON ELECTRONIC STRUCTURE
structure of graphite substrate. This requires a complete calculation of the dielectric function based on the cluster electronic density of states following the computation of Delley et aL [19]. In conclusion we have reported the energy loss measurements and XPS results confirming that the changes in the electronic structure of small Cu clusters are sizeable and evolve continuously towards higher binding energies with respect to EI decreasing cluster diameters.
Acknowledgements - The XPS and EELS measurements have been performed at the Istituto di Teoria e Struttura Elettronica del CNR Monterotondo Roma. The authors are grateful to C. Battistoni and G. Mattogno for their kind hospitality during the measurements. REFERENCES 1. 2. 3. 4.
5. 6. 7. 8.
W.F. Egelhoff, Jr & G.G. Tibbets, Phys. Rev. BIg, 5028 (1979). M.G. Mason, Phys. Rev. B27, 748 (1983). P.H. Citrin & G.K. Wertheim, Phys. Rev. B27, 3176 (1983). G.K. Wertheim, S.B. Di Cenzo, D.N.E. Buchanan & P.A. Bennet, Solid State Commun. 53, 377 (1985); G.K. Wertheim, S.B. Di Cenzo & D.N.E. Buchanan, Phys. Rev. !!33, 5384 (1986). P.A. Montano, J.K. Shenoy, E.E. Alp, W. Schulze & J. Urban, Phys. Rev. Lett. 56, 2076 (1986). P. Picozzi, S. Santucci, M. De Crescenzi, A. Antonangeli & M. Piacentini, Phys. Rev, 1331, 4023 (1985). M. De Crescenzi, N. Diociaiuti, L. Lozzi, P. Picozzi, S. Santucci, C. Battistoni & G. Mattogno, Surf. Sci. 178, 282 (1986). M. De Crescenzi, M. Diociaiuti, L. Lozzi,
9. 10.
11. 12. 13. 14. 15. 16. 17.
18. 19.
Vol. 74, No. 2
P. Picozzi & S. Santucci, Phys. Rev. B35, 5997 (1988). L. Oberli, R. Monot, H.J. Mathieu, D. Landolt & J. Buttet, Surf. Sci. 106, 301 (1981). P.S. Bagus, in "Elemental and molecular clus-. ters" (Edited by G. Benedek, T.P. Martin and G. Pacchioni), Springer Series in Material Science p. 286 (1988). M. De Crescenzi, P. Picozzi, S. Santucci, C. Battistoni & G. Mattogno, Surf. Sci. 156, 352 (1985). R.C. Baetzold, J. Amer. Chem. Soc. 103, 6116 (1981). I. Abbati, L. Braicovich, C.M. Bertoni, C. Calandra & F. Manghi, Phys. Rev. Lett. 40, 469 (1978). W.F. Egelhoff, Jr., J. Vac. Sci. Technol. 20, 668 (1982). G. Chiarello, E. Colavita, M. De Crescenzi & S. Nannarone, Phys. Rev. B29, 4878 (1984). H. Raether, Excitation of Plasmon and Interband Transitions by Electrons (Springer-Verlag, Berlin Heidelberg New York 1980). In principle the electron energy loss function N(E), measured using the reflection geometry, contains both bulk ( - I m 1/~) and surface ( - I m 1/(~ + I))contributions [15]. The relative weight of these two functions to the observed N(E) strictly depends on the penetration depth of the impinging electrons. As in our experiment we are interested to the changes of the electronic structures of the metallic clusters compared to those of the bulk we have used a relatively high primary beam energy (1000 eV) in order to minimize the contribution of the surface plasmon loss function. R. L/isser, N.V. Smith & R.U Benbow, Phys. Rev. B24, 1895 (1981). B. Delley, E. Ellis, A.J. Freeman, E.J. Baerends & D. Post, Phys. Rev. B27, 2132 (1983).
0038-1098/90 $3.00 + .00 Pergamon Press plc
INVESTIGATION ON ELECTRONIC STRUCTURE OF Cu CLUSTERS ON GRAPHITE BY EELS AND XPS STUDIES M. De Crescenzi*, M. Diociaiuti, L. Lozzi, P. Picozzi and S. Santucci Dipartimento di Fisica, Universita' dell'Aquila, 67100 L'Aquila, Italy * Dipartimento di Fisica, Universita' di Roma II, Tor Vergata, 00173 Roma, Italy (Received 22 December 1988 by E. Tosatti) We have applied electron energy loss (EELS) and X-ray photoemission (XPS) spectroscopies to study the electronic structure of copper clusters deposited in situ on clean graphite. The XPS results show an increase in the binding energy of both valence band and core levels when the cluster size decreases. Furthermore the EELS results show a sizeable increase in energy loss for the feature located at about 4 eV involving 3d ~ E/states when the cluster size decreases. The observed shifts in the EELS spectrum are the same as those measured for the 3d valence band shifts in the XPS spectra. Since the electrostatic charging effects, suggested in the past as the explanation for the variation in electronic properties occurring in the XPS of small clusters, cannot be invoked in EELS measurements we interpret the reported shifts as a genuine effect caused by band structure variation due to a redistribution of the states within the cluster d-band.
INTRODUCTION
At the moment the problem of the predominance of one effect over the other is still open from both an experimental and theoretical point of view although cluster ionization potential recently computed by Bagus et aL [10] supported the initial state effect. In order to contribute to the discussion of the previous point of view we have performed EELS spectra on Cu in an energy loss region involving excitations from the 3d electrons towards the Fermi level. The EELS spectra show a sizeable shift of the structure located at 4 eV, towards higher energy losses, of the same order of magnitude as the valence band XPS shift. The EELS features involve single particle electronic transitions without photoelectron emission ruling out any charging effect on the cluster. As a consequence we conclude that the observed shifts in the XPS valence band may be a genuine effect of the electronic structure variation occurring in the smallest clusters.
COPPER clusters have been widely studied in the last few years in order to investigate variations in structural and electronic properties when the cluster size decreases [1-5]. Our previous studies have dealt with optical properties [6], Auger and XPS .(core level and valence band) [7] and extended electron energy loss fine structure (EELFS) [8] spectroscopies. The most significant result is a sizeable variation of both valence and core level binding energies when the cluster diameter decreases. These variations, of the order of 1 eV, also product shifts in the corresponding Auger spectra [2. 7, 9]. Generally the shifts observed in the XPS spectra have been attributed in the past either to initial or to final state effects [1-4]. Initial state effects are generally attributed to the intrinsic change in the electronic structure when passing from the isolated atom to the bulk. On the contrary the electrostatic charging of clusters not completely neutralized by the free elecEXPERIMENT trons supplied by the substrate has been invoked to justify final state effects. Copper aggregates were deposited in ultra high In order to emphasize latter effect Wertheim et al. vacuum by evaporation from a tungsten wire on both [4] reported careful XPS spectra of A g clusters show- a polycrystalline graphite (Goodfellow 99.8%) and a ing a shift of the same order of the 3d5/2 core electrons, grid for transmission electron microscopy. The grid of the 4d band centroid and of the Fermi cut off. was used to determine the cluster mean diameter 115
116
INVESTIGATION ON E L E C T R O N I C S T R U C T U R E
distribution. The nominal thicknesses were monitored by an Inficon quartz microbalance. The VG-3MKII-ESCA chamber was equipped with an X-ray (AIK~) source for XPS measurements and with an electron gun for EELS measurements. More details on the experimental apparatus are reported elsewhere [11]. RESULTS AND DISCUSSION Figure 1 shows the XPS spectra of different copper nominal thicknesses on graphite. We note a continuous shift of the valence band towards higher binding energies decreasing the cluster size. The shift of the valence band centroid amounts to 1 eV for a 1 A nominal deposition constituted by clusters with mean diameter of about 6 A. No shift has been observed for depositions with nominal thickness greater than 20 A (mean diameter about 60 A). These results are !n good agreement with previous investigations of Cu on graphite reported by different authors [1, 2, 12]. On the other hand the same qualitative results have been reported for Cu deposited on clean metal surfaces like Zn [13] and AI [14]. Shifts of the same order of magnitude have also been observed for core levels and Auger spectra [7, 9]. BINDING 0
2
4
(eV)
ENERGY 6
8
I0
C
IZ IM IZ
KINETIC ENERGY (eV)
Fig. I. X-ray photoelectron spectra of the valence band of bulk and Cu clusters deposited on polycrystalline graphite. The nominal thicknesses are reported.
Vol. 74, No. 2
Two different points of view, in the last few years. have been suggested to explain the shifts observed in the photoemission spectra. The first deals with th,: change in the intrinsic electronic properties (so called "initial state effect"), which occurs in the small clusters, due to the level hybridization and/or the change in the nearest neighbours coordination number. According to this interpretation the observed shift has been attributed to a lowering of the d band portion of the valence band with respect to the Fermi level [1, 2]. The second point of view interprets the observed shift as an "apparent shift" caused by the cluster charging, during the photoemission process, not being fully screened by the flowing of the free electrons supplied by the substrate. In particular, this so called "final state effect" should shift all energies (valence d band, Fermi level and core structures) by an amount of energy of the order of e2/2r where r is the cluster radius [3, 4]. Furthermore a d band narrowing, decreasing the cluster size, should increase the threshold excitation energy while the valence band centroid remains fixed relative to the Fermi level [4, 14]. This consideration is ruled out because our XPS spectra show an important shift of the centroid of the d band without a corresponding shift of the Fermi level for all investigated clusters. Due to our limited experimental resolution (~>leV) no sizeable narrowing of the 3 d-band can be observed. In order to discriminate initial state from the final state effect we have performed electron energy loss measurements of the same Cu evaporation. The EELS results are shown in Fig. 2. The electron beam impinged on the surface at an angle of 50 ° in order to produce a greater surface (Cu cluster) to volume (graphite) ratio. The energy of the primary electron beam was 1000 eV and a typical beam current density of 1 #A mm -2 was used. The evaporations with nominal thicknesses 20 and 30 ~, (which correspond to cluster mean diameters of about 60 and 90A respectively) show the same features labelled with A, B, C, D and E reported in the EELS spectrum of the Cu bulk [15]. Decreasing the nominal thickness we note a continuous shift towards higher energy losses in the structures A and D. The structures B, C and E, however, do not show a sizeable shift, probably because of the presence of the strong rr* graphite substrate structures. For this reason we have focused our attention on the structure A because it can be interpreted in terms of electronic single particle energy band and, moreover, because it is not influenced at all by graphite absorption. According to the dielectric theory [15-17] each structure in the energy loss spectra should correspond to the hollows of the imaginary part of the complex dielectric function g = e~ + ig2 because of
INVESTIGATION ON ELECTRONIC STRUCTURE
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Energy Loss (eV) Fig. 2. Low electron energy loss spectra for clean polycrystalline graphite and different Cu nominal thicknesses. The primary beam energy was E~ = 1000 eV. the behaviour of the energy loss function [15] N(E) oc - I r a
=
+ ~ + F;
Figure 3(a) shows the contribution to e2 coming from the transitions between initial d-band states (i = 1-5) below the Fermi level and the first two final empty band states (.f = 6, 7) above the Fermi level for copper bulk as reported by Lfisser et al. [18]. Figure 3(b) shows the experimental energy loss structures A and B for the Cu bulk. Variations in the electronic band structure and, in particular its position with respect to the Fermi level of the d band, should produce an energy shift in the e_,(i ~ f ) features.
Fig. 3. (a) Partial e2(i-f) spectra of Cu bulk transitions from the initial bands i (1-5) to the final bands f ( 6 - 7 ) above Ef as reported by L/isser et al. [18], (b) the energy positions of the Cu bulk EELS excitation (thick lines) are reported together with the first EELS structures A and B. Since each structure in the N(E) energy loss measurements follows very closely the variations occurring in e2, an increase of energy loss indicates an increase in binding energy of the 3d band with respect to the Fermi level. The shifts observed of the EELS feature A of the different clusters are in good agreement with the XPS experiments reported in Fig. 2. Since the electrostatic charging effect should play a minor role in the energy loss spectra (because the loss energies refer to the primary beam energy) we can exclude the final state effect as responsible for the observed shift also in the XPS data. On the other hand one should note that a rigid shift of both the valence band and Fermi level [4] should not produce a detectable shift in the electron energy loss features. A more complete knowledge of the electronic structure variation occurring in the cluster should also take into account the other energy loss features B, C, D, E superimposed upon the energy loss
118
INVESTIGATION ON ELECTRONIC STRUCTURE
structure of graphite substrate. This requires a complete calculation of the dielectric function based on the cluster electronic density of states following the computation of Delley et aL [19]. In conclusion we have reported the energy loss measurements and XPS results confirming that the changes in the electronic structure of small Cu clusters are sizeable and evolve continuously towards higher binding energies with respect to EI decreasing cluster diameters.
Acknowledgements - The XPS and EELS measurements have been performed at the Istituto di Teoria e Struttura Elettronica del CNR Monterotondo Roma. The authors are grateful to C. Battistoni and G. Mattogno for their kind hospitality during the measurements. REFERENCES 1. 2. 3. 4.
5. 6. 7. 8.
W.F. Egelhoff, Jr & G.G. Tibbets, Phys. Rev. BIg, 5028 (1979). M.G. Mason, Phys. Rev. B27, 748 (1983). P.H. Citrin & G.K. Wertheim, Phys. Rev. B27, 3176 (1983). G.K. Wertheim, S.B. Di Cenzo, D.N.E. Buchanan & P.A. Bennet, Solid State Commun. 53, 377 (1985); G.K. Wertheim, S.B. Di Cenzo & D.N.E. Buchanan, Phys. Rev. !!33, 5384 (1986). P.A. Montano, J.K. Shenoy, E.E. Alp, W. Schulze & J. Urban, Phys. Rev. Lett. 56, 2076 (1986). P. Picozzi, S. Santucci, M. De Crescenzi, A. Antonangeli & M. Piacentini, Phys. Rev, 1331, 4023 (1985). M. De Crescenzi, N. Diociaiuti, L. Lozzi, P. Picozzi, S. Santucci, C. Battistoni & G. Mattogno, Surf. Sci. 178, 282 (1986). M. De Crescenzi, M. Diociaiuti, L. Lozzi,
9. 10.
11. 12. 13. 14. 15. 16. 17.
18. 19.
Vol. 74, No. 2
P. Picozzi & S. Santucci, Phys. Rev. B35, 5997 (1988). L. Oberli, R. Monot, H.J. Mathieu, D. Landolt & J. Buttet, Surf. Sci. 106, 301 (1981). P.S. Bagus, in "Elemental and molecular clus-. ters" (Edited by G. Benedek, T.P. Martin and G. Pacchioni), Springer Series in Material Science p. 286 (1988). M. De Crescenzi, P. Picozzi, S. Santucci, C. Battistoni & G. Mattogno, Surf. Sci. 156, 352 (1985). R.C. Baetzold, J. Amer. Chem. Soc. 103, 6116 (1981). I. Abbati, L. Braicovich, C.M. Bertoni, C. Calandra & F. Manghi, Phys. Rev. Lett. 40, 469 (1978). W.F. Egelhoff, Jr., J. Vac. Sci. Technol. 20, 668 (1982). G. Chiarello, E. Colavita, M. De Crescenzi & S. Nannarone, Phys. Rev. B29, 4878 (1984). H. Raether, Excitation of Plasmon and Interband Transitions by Electrons (Springer-Verlag, Berlin Heidelberg New York 1980). In principle the electron energy loss function N(E), measured using the reflection geometry, contains both bulk ( - I m 1/~) and surface ( - I m 1/(~ + I))contributions [15]. The relative weight of these two functions to the observed N(E) strictly depends on the penetration depth of the impinging electrons. As in our experiment we are interested to the changes of the electronic structures of the metallic clusters compared to those of the bulk we have used a relatively high primary beam energy (1000 eV) in order to minimize the contribution of the surface plasmon loss function. R. L/isser, N.V. Smith & R.U Benbow, Phys. Rev. B24, 1895 (1981). B. Delley, E. Ellis, A.J. Freeman, E.J. Baerends & D. Post, Phys. Rev. B27, 2132 (1983).