J . Phys. Chem. 1992, 96, 8679-8682
property. As shown in Figure 4, the electric junction starts emitting light at an applied voltage of 10 V. A similar electroluminescence diode using poly(3-alkylthiophene) spincoated on I T 0 glass has been reported? An Al/vd-PPP/Au electric junction also shows the rectification effect with a rectification ratio of about loo.
As described above, the vacuum-deposited poly(ary1ene)s are electrochemically active and usable in making devices such as electrochromic devices, diodes, and electroluminescent devices.
References and Notes (1) (a) Yamamoto, T.; Kanbara, T.; Mori, C. Synrh. Mer. 1990,38, 399. (b) Yamamoto, T.;Mori, C.; Wakayama, H.; Zhou, Z.-H.; Maruyama, T.; Kanbara, T.; Ohki, R. Chem. Lett. 1991, 1483. The polymers were prepared by polycondensation (e.g., Yamamoto, T.; Morita, A.; Miyazaki, Y.; MaruNakamura, Y.; Kanbara, T.;Sasaki, yama, T.; Wakayama, H.;Zhou, Z.-H.; S.; Kubota, K. Macromolecules 1992, 25, 1214) based on organonickel chemistry (e.g., Yamamoto, T.; Yamamoto, A,; Ikeda, S. J . Am. Chem. Soc. 1971, 93, 3350). (2) E.g.: (a) Yashima, H.;Kobayashi, M.; Lee,K.-B.; Chung, D.; Heeger, A. J.; Wudl, F. J. Electrochem. Soc. 1987, 134,46. (b) Etemad, S.; Heeger,
8679
A. J.; MacDiarmid, A. G. Annu. Rev. Phys. Chem. 1982, 33, 443. (c) Tourillon, G. In Handbook of Conductive Polymers; Skotheim, T. A., Ed.; Marcel Dekker: New York, 1986; Vol. 2, p 330. (3) E.g.: (a) Yamamoto, T. J . Chem. Soc., Chem. Commun. 1981, 187. (b) Kaneto, K.; Yoshino, K.; Inubushi, Y. J . Appl. Phys. 1981,22, L567. (c) Yamamoto, T.; Zama, M.; Hishinuma, M.; Yamamoto, A. J . Appl. Electrochem. 1987, 17, 607. (4) E.g.; (a) Aizawa, M.; Yamada, T.; Shinohara, H.; Akagi, K.; Shirakawa, H.J.Chem.Soc., Chem. Commun. 1986,1315. (b) Koezuka, H.;Etoh, S. J . Appl. Phys. 1983, 54, 2511. ( 5 ) (a) Tsumura, A.; Koezuka, H.; Ando, T. Appl. Phys. Lett. 1986,49, 1210. (b) Chao, S.; Wrighton, M. S. J . Am. Chem. Soc. 1987, 109, 2197. (6) (a) Skotheim, T. A., Ed. Handbook of Conducting Polymers; Marcel Dekker: New York, 1986; Vols. 1 and 2. (b) Salaneck, W. R., Clark, D. T., Samuelsen, E. J., Eds. Science and Applications of Conducting Polymers; Adam Hilger: Bristol, 1991. (c) Kuzmany, H., Mehring, M., Roth, S., Eds. Electronic Properties of Conjugated Polymers; Springer-Verlag: Berlin, 1987. (d) Bredas, J. L., Sibbey, R., Eds. Conjugated Polymers; Kluwer Academic Publishers; Dordrecht, 1991. (7) Horowitz, G.; Fichou, D.; Peng, X.; Xu, Z.; Garnier, F. Solid State Commun. 1989, 72, 381 and references therein. (8) Waltman, R. J.; Bargon, J.; Diaz, A. F. J. Phys. Chem. 1983.87, 1459. (9) Ohmori, Y.; Uchida, M.; Muro, K.; Yoshino, K. Jpn. J . Appl. Phys. 1991, L1938.
Metal-Insulator Transltlons In Metal Clusters: A High-Energy Spectroscopy Study of Pd and Ag Clusters V. Vijayakrishnan, A. Cbainani, D. D. Sarma, and C. N. R. Rao* CSIR Centre of Excellence in Chemistry and Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560 012, India (Received: April 15, 1992; In Final Form: July 28, 1992)
Bremsstrahlung isochromat spectroscopy (BIS) along with ultraviolet and X-ray photoelectron spectroscopy (UPS and XPS) has been employed to investigate the electron states of Pd and Ag deposited on amorphous graphite at different coverages. The metal core level binding energies increase with decreasing cluster size while the UPS valence bands show a decrease in the 4d states at EF accompanied by a shift in the intensity maximum to higher binding energies. BIS measurements show the emergence of new states closer to EF with increase in the cluster size. It is pointed out that the observed spectral shifts cannot be accounted for by final-state effects alone and that initial-state effects have a significant role. It therefore appears that a decrease in cluster size is accompanied by a metal-insulator transition.
Metal clusters constitute a frontline area of research today.' Metal clusters can be of varying sizes starting with molecular clusters containing a few atoms to colloidal particles just preceding the bulk state. One of the fundamental questions pertaining to metal clusters therefore relates to the change in their properties with the cluster size which can be anywhere between those of the bulk metal and of an isolated metal atom. Properties of metal clusters could change periodically with cluster size due to the formation of electron shells and atom shells, the transition between the two types of shells occurring around 1500 atoms in alkali and alkaline earth metals.' Of special interest in the study of metal clusters is their metallicity itself. We would expect a transition from the metallic state to the insulating state to occur on decreasing the cluster size. We considered it important to obtain direct evidence for the Occurrence of such a transition and have therefore investigated Ag and Pd metal clusters deposited on amorphous graphite substrates by use of Bremsstrahlung isochromat spectroscopy (BIS)along with ultraviolet and X-ray photoelectron spectroscopy (UPS and XPS). There have been several studies of metal clusters, in particular of Pd and Ag clusters, supported on graphite by photoelectron and Auger ~pectroscopies~-~ to probe the occupied part of the density of states (DOS). Some work has also been reported based on X-ray absorption spectroscopy and electron energy loss spectroscopy (EELS)to probe the unoccupied part of the DOS. However, these techniques usc transitions between the unoccupied and the occupied states and do not allow direct probing of the To whom correspondence should be addressed.
unoccupied DOS. Cluster-sizedependent shifts in the metal core level binding energiesz4 have been related to the metal-nonmetal transition; direct shifts of the spectral features at the Fermi energy have also been observed for small sized clusters. Yet there is still some doubt conceming the origin of these shifts and whether they arise from metal-nonmetal transitions. Thus, it has been s u g gested' that the Fermi cutoff in small clusters does not coincide with that of the substrate due to the Coulomb energy of the uncompensated hole in the final state, rather than being an initial-state effect. We have therefore used the core level binding energy shifts to characterize the clusters and UPS valence bands to probe the shifts in the leading edge as a function of cluster size. More importantly, we have employed BIS to probe the unoccupied part of the DOS directly and to show that the small clusters show a corresponding spectral shift to higher energy. While the uncompensated charge in the final state of the small clusters certainly shifts the spectra, our observations are not consistent with an explanation based on the final-state effect alone. At least a part of the observed changes seem to be related to the initial state of the cluster. That the initial-state effects play an important role in determining the spectral shape and shifts of small clusters is borne out by an EELS stud9 of Pd clusters supported on graphite. The observed EELS shifts cannot be attributed to final-state effects since no uncompensated charge is created in the process. The present study of the metallicity of metal clusters is also of relevance to the properties of thin films. It has been shown that ultrathin metal films exhibit insulatorsuperconductor transitions as a function of thickness.1° While electrical resistivity measurements depend on the formation of homogeneous thin films and would
0022-3654/92/2096-8619~03.00~00 1992 American Chemical Society
Letters
8680 The Journal of Physical Chemistry, Vol. 96, No. 22, 1992
I?
(a)
BEIeV
'Pd"C
I
1
I 1
I
I
1
2'
6
10 10
0 0
0
0
1
O OO
2
6
4
E
10
IAgI I C
Figure 1. Variation of the shift in the 3d?/*binding energy of Pd (a) and Ag (b) deposited on amorphous graphite substrates with the average ( I M / I c ) . AE is the shift in the binding energy in the cluster relative to the value in the bulk metal (Pd, 335.1 eV; Ag, 368.2 eV). Typical uncertainties in AE are indicated.
be severely limited if island growth occurs due to discontinuities in the film, high-energy spectrapcopic measurements can effectively probe metal-insulator transitions even in inhomogeneous films or clusters. Pd and Ag were deposited in situ at room temperature under ultrahigh-vacuum conditions' I on amorphous graphite substrates in a preparation chamber of the electron spectrometer by means of resistive evaporation of high-purity (99.9%), degassed metals wound around a tungsten filament. In order to prepare the graphite substrate,I2polycrystalline graphite was annealed at 600 K and sputtered by Ar ions (6 kV, 15 min). It is known that, at low coverages of the metal, small clusters are formed at the nucleation sites.I3 We have quantified the sizes of Ag and Pd clusters by measuring the relative intensity of the metal 3d5/2 core level, I,, relative to that of the C( Is) level of the support, I C ; the ratio of IM/Ic is a measure of the amount of the metal deposited or the cluster size. We have ensured that there is no carbide formation by means of C( 1s) spectra. We could calculate the surface coverage, 8, by employing the method of Carley and Roberts.14 The surface coverage estimated by this method for the lowest IM/Ic (0.12) was 9.1 X 1013at 7.7 X I O l 3 atoms cm-2 respectively for Pd and Ag. At the smallest IM/IC studied by us, we estimate the mean radius of the cluster to be 5-9 A (with 50 f 30 atoms), assuming that the nucleation sites are 3 X 10I2/cm2 and that the deposited clusters are hemispherical.13J5UPS and XPS measurements were carried out with a VG-ESCA3 MK I1 spectrometer fitted with a sample preparation chamber at a base pressure of - 4 X Torr by employing He I1 (40.8 eV) and Mg Ka (1253.6 eV) radiation, respectively. BIS measurements were carried out with a VSW XPS-BIS spectrometer by making use of an electron gun operating at a beam current of 200 PA. In order to ensure that the observed shifts in the core level binding energy and in the valence band were genuine, repeated measurements were made on surfaces with comparable metal coverages. Core level (3d5/2) binding energies of Pd and Ag deposited on the graphite substrate show marked changes with the coverage or the ZM/Zc ratio, similar to the earlier reports.24 The shifts in
L:
I
I
EF
5
10
I
BEleV
Figure 2. He I1 UP spectra of Pd clusters deposited on a amorphous graphite substrate at various coverages. Inset shows the difference spectra after subtraction of the substrate background.
the binding energies AE of Pd and Ag clusters relative to the bulk values are plotted against IM/Ic in Figure 1. We find that AE for Pd is as large as 1.2 eV at the smallest coverage. With the increasing coverage, leading to the formation of larger sized Pd clusters, the AE decreases rapidly to about 0.7 eV for Ipd/Ic= 0.4; the AE decreases gradually, approaching the binding energy of the bulk metal at larger Iw/Ic We note however that AE does not reach zero even at the largest IM/Ic examined by us. The dependence of AE on the coverage of Ag exhibits a behavior similar to that of Pd (Figure I), showing a rapid decrease with increase in IAg/Ic when IAg/Ic < 1 and a gradual variation at higher IA$Ic values. For Ih/Ic 2 3, AE is nearly constant around 0.2 eV. The AE value for the lowest coverage of Ag is smaller (-0.6 eV) than that for Pd (1.2 ev). At IM/Ic 1, the surface coverage corresponds to 6.4 X I O l 4 and 7.68 X loi4atoms cm-2 for Ag and Pd, respectively. The variation of AE(3dS12).with cluster size depicted in Figure 1 arising from the decrease in the core-hole screening with the decrease in cluster size has been considered to be a manifestation of the occurrence of the metal-insulator transition as the cluster size decreases. In order to understand the nature of the metal-insulator transition, we have investigated the states near the Fermi level, EF,by employing BIS as well as UPS. Ultraviolet photoelectron spectra of Pd deposited to different extents on amorphous graphite are shown in Figure 2, where we also show the spectrum of the substrate for comparison. The UP spectra of these clusters are similar to those reported by other workers.24 The spectrum of the substrate has a broad peak around 7.5 eV binding energy. On depositing Pd on the substrate, we see the emergence of a feature between 0.6 and 6 eV arising from the d emission of the Pd clusters. We have subtracted out the contribution from the substrate by normalizing the spectra at 16 eV binding energy (corresponding to the maximum in the graphite signal) to obtain
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The Journal of Physical Chemistry, Vol. 96, No. 22, 1992 8681
Letters
(E-EF),eV
Figure 3. (a) BI spectra of Pd clusters deposited on amorphous graphite at various coverages. (b) Difference BI spectra after subtraction of the substrate background.
the difference spectra (see inset of Figure 2) reflecting the 4d spectral features. At the highest deposition, the difference spectrum is similar to that of the bulk metal. In particular, there is considerable spectral intensity at EF due to the 4d states, indicating that the clusters possess metallicd states. With decreasing Ipd/Icor cluster size, the intensity of the Pd 4d states at EF decreases rapidly and becomes negligible at the lowest Iw/Ic values examined by us. This decrease in intensity is accompanied by a shift of the intensity maximum to higher binding energies and a narrowing of the 4d-related spectral feature. It seems that while a part of the shift of the d band centroid from Pd clusters may be due to the final-state effects, the spectral narrowing is an initial-state effect. Furthermore, the leading edges (lower binding energy side) of the spectra from the smallest clusters do not exhibit clearly defined Fermi cutoff. There are also indications that small sized Pd clusters do not exhibit metallic 4d states; instead, the 4d states are localized well below EF. Although the UPS features of Pd clusters appear to be consistent with the Occurrence of a metal-insulator transition with the decrease in cluster size, we should consider the possibility that the shifts in the centroid of the d band intensity accompanying the shift of the signal away from the Fermi energy are due to a final-state e f f e ~ trather , ~ than an initial-state effect suggested here. According to the final-state interpretation,' the hole created by photoemission is not compensated for by the transfer of a screening electron from the substrate to the cluster due to the poor conductivity and coupling of the substrate with the cluster within the time scale of the photoemission process. This leads to a lowering of the kinetic energy of the escaping electron due to the Coulomb interaction between the remaining hole and the escaping photoelectron and leads to the observed shifts in the binding energy. Results of EELS," where there is an electronic transition between the two states rather than the ejection and removal of an electron, are not consistent with this viewpoint, since the final-state effect7 is not expected to be present. Bremsstrahlung isochromat spectra of Pd deposited on the amorphous graphite substrate (Figure 3a) clearly suggest the Occurrence of a metal-insulator transition with the variation in cluster size. The difference spectra obtained after normalizing each spectrum 10 eV above EF are shown in Figure 3b. From these spectra we find that the empty 4d states of Pd have negligible intensity at EF for small sized clusters (at low values of Ipd/Zc), showing only a broad feature away from EF. With increasing cluster size, however, new states emerge closer to (and eventually at) EF for larger clusters, resembling the bulk Pd spectrum at the highest Zpd/Icvalue. This observation is consistent with the Occurrence of a metal-insulator transition with decreasing cluster size, since changes in spectral shape cannot be accounted for by a final-state interpretation.
5
0. 0
IPd/lC
.
10.0
0
5
A
1.67
x
0.44
10
15
( € - E F ),eV
Figure 4. Comparison of the difference BI spectra of Pd clusters on amorphous graphite substrate.
BIS provides an extra electron into the system in the final state, and thus an effect opposite to that in photoemission should be expected. In this sense, it may appear that the shift of the signal intensity away from EF to higher energies with decreasing cluster size is a confirmation of the final-state interpretation7of the origin of the energy shifts. However, a closer comparison of the spectral shapes as a function of cluster size (Figure 4) suggests that the spectra do not merely exhibit a rigid shift as would be expected on the basis of final-state effects alone. From this comparison we find that the spectral shape matches rather well on the high-energy side, suggestingthat new states grow in intensity with increasing cluster size near EF. Such an observation is not consistent with the suggested final-state effect.7 We therefore believe that there is indeed a metal-nonmetal transition in these clusters as a function of cluster size. Evidence for a cluster-controlled metal-insulator transition is also provided by the BI spectra of Ag clusters (Figure 5a) where also we see the emergence of new states at and around EF with increasing cluster size while the higher energy features remain relatively unaltered. Since Ag has a nearly filled 4d state, the states near the E, are primarily due to the 5s state. In order to understand the changes in the 5s spectral features, we have obtained the difference spectra by subtracting out the substrate signal. The difference spectra (Figure 5b) exhibit a feature around
8682 The Journal of Physical Chemistry, Vol. 96, No. 22, 1992
Letters
( E - E F ), eV
Figure 5. (a) BI spectra of Ag clusters deposited on amorphous graphite at various coverages. (b) Difference spectra after removal of substrate
+/
background.
1.2
w m
1
the energy shifts of the valence-band centroids for the same clusters. In the figure we have also included the data on gold clusters reported by Costanzo et While the final-state effect alone would give rise to a 45' line for this plot, we find considerable deviation, especially for the small clusters. This observation gives evidence to the conclusion that the ftnal-state effect cannot account alone for the observed spectral shifts for the small clusters and that the initial-state effect plays a significant role. Accordingly, our recent measurements on small Ni clusters deposited on graphite have shown core level shifts to be significantly large although the BI spectra show negligible shifts of the position of EF or of the empty d band centroid.
0.8
Q
References and Notes I
I
I
I
I
I
0.2
0.4
0.6
I
0.8
1.0
12
1.4
I
1.6
AVB,eV
i 6. Plot of the core level binding energy shifts Al?against the shifts of the valence-band centroids, AVB. In the case of Au, AVB corresponds
F
to shift in ,EF.Uncertainties are shown in certain experimental points to illustrate deviations from the theoretical 45O line.
5.1 eV above E p If the final-state effects' were to dominate, we would expect this feature also to shift by the same amount. However, the first feature closer to EF around 2.1 eV shown by the smallest clusters shifts to 1.2 eV above EF for the large clusters. The apparent shift of this feature toward EF in the large clusters is due to the creation of new states close to E p In particular, we find that this effect leads to considerable intensity of the 5s states at EFwhich we consider as due to the metal-insulator transition occurring in Ag clusters with decreasing cluster size. If the final-state effects were solely responsible for the observed shifts in the core levels as well as in the valence band spectral features of small metal clusters, we would expect the two shifts to be identical. We have plotted in Figure 6 the m e level binding energy shifts of different metal clusters obtained by us against
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(I) Martin, T. P.; Bergmann, T.; Lange, T. J. Phys. Chem. 1991,95,6421. (2) Mason, M. G. Phys. Reo. E 1983, 27, 748. (3) Wertheim, G. K.; DiCenzo, S.B.;Buchanan, D. N. E. Phys. Reo. B 1986, 33, 5384. DiCenzo, S. B.; Wertheim, G. K. Commenrs Solid Srute Phys. 1985,11,203. Wertheim, G. K. Phase Transitions 1990.24-26,203, Wertheim, G. K. Z . Phys. D 1989, 12, 319. (4) DeCrescenzi, M.; Lozzi, L.; Picozzi, P.; Santucci, S.Z . Phys. D 1989, 12, 417. (5) Eberhardt, W.; Fayet, P.; Cox, D. M.; Fu, Z.; Kaldor, A,; Shewood, R.; Sondericker, D. Phys. Reo. Lett. 1990, 64, 780. (6) Henry, C. R.; Poppa, H. Z . Phys. D 1989, 12,421. (7) Wertheim, G. K.; DiCenzo, S.B.; Youngquist, S.E . Phys. Reu. Leu. 1983,51,2310. Wertheim, G. K.; DiCenzo, S.B. Phys. Rco. E 1988,37,844. (8) DiCenzo, S.B.; Berry, S.D.; Hartford Jr., E. H. Phys. Reo. E 1988, 38, 8465. (9) Lozzi, L.; Paswantando, M.; Picozzi, P.; Santucci, S.;DeCrescenzi, M. 2.Phys. D 1991,20, 387. (10) Missert, N.; Beasky, M. R. Phys. Reo. Leu. 1989,63,672. Haviland, D. B.; Lu, Y.; Goldman, A. M. Phys. Reu. Lett. 1989, 62, 2180. (11) Vijayakrishnan, V.; Rao, C. N. R. Surf.Sci. Len. 1991, 255, L516. (12) Evans, S.; Thomas, J. M. Proc. R. Soc. London,A 1917,353, 103. (13) Wertheim, G. K.; DiCenzo, S . B. Phys. Reo. B 1988, 37, 374. (14) Carley, A. F.; Roberts, M. W. Proc. R . Soc. London, A 1978,363, 403. (15) Hamilton, J. F.; Logel, P. C. Thin Solid Films 1973, 16, 49; 1974, 23, 89. (16) Costanzo, E.; Faraci, G.; Pennisi, A. R.; Ravesi, S.;Terrasi, A.; Margaritondo, 0. Solid State Commun. 1992, 81, 155.
