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Photoelectron spectroscopy of gold and silver ultra-thin films on V(100)
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surface science >.. .:.... :,‘::,,:,.,;::. ,..: ;,:iy ” .’:.:,.Y:‘..::.. ‘.:)::.‘.j ..f....:.:: .. ::,:_,:’ ‘y
ELSEVTER
Photoelectron
Surface
j:j::
Science 307-3OY (lYY4) 576-581
spectroscopy of gold and silver ultra-thin films on V( 100) Tonica Valla, Petar Pervan, (Received
20 August
Milorad
Milun
*
1903)
Abstract Gold ultra-thin films deposited on V(100) surface at 120 and 300 K were characterized by AES, UPS and XPS at different stages of film growth and after an annealing treatment. The work function (WF) changed from 4.3 CV at zero Au coverage to 5.6 eV at a bulk-like film. The AuSd splitting at low covet-ages was 1.4 eV. At the same time, the Au 4f peaks shifted to lower BE, increased their splitting by 0.2 eV and kept their FWHM constant. Heating the samples to 1500 K induced no Au desorption but rather alloying started at a moderate temperature which depended on the film thickness. Upon alloying the core levels and valence band of gold shifted to higher BE. The Au/V(lOO) is compared with the Ag/V(lOO) system represented here with the UP spectra of several selected coverages and annealing experiments of the 5 ML Ag film
1. Introduction In many binary alloys containing Ag or Au, the two noble-metal valence band spectra exhibit similar behaviour, namely, the d-band shifts to lower binding energy (BE) and the d-band width increases linearly with increasing noble-metal concentration [1,2]. The Au and Ag d-band behaviour is independent of the secondary constituent metal, as may be judged from a number of investigated systems [l-6]. Mason [7] studied the valence bands and core-level spectra of noble-metal clusters deposited on both weakly and strongly interacting supports and found great sim-
* Corresponding
author.
0039-6028/94/$07.00 0 1994 Elsevier SSDl 0039-6028(93)E0832-F
Science
ilarity with the spectra of corresponding noblcmetal alloys. In the case of weakly interacting systems, the net inter-atomic charge transfer was very small [4] but the intra-atomic charge transfer was very significant. The initial-state properties were found [71 to dominate the weakly interacting noble-metal-substrate systems, while the finalstate properties were more important for strongly interacting systems. The similarity between gold clusters and gold alloys was explained [7] in terms of coordination number of like nearest neighbours, whose importance for the d-band characteristics was recognized by Shevchik [6]. The valence band and the core-level properties of thin and ultra-thin films of gold were addressed by Liang et al. [8] who found significantly narrowed Au d-band splitting when compared to the bulk value; also the Au4f peaks were found
B.V. All rights resewed
T. Valla et al. /Surface
Science 307-309
to move to higher BE with decreasing gold film thickness. Citrin et al. [9] reported a shift of 0.4 eV to lower BE of the Au4f surface-atom level, and around 8% narrowing of the surface density of states accompanied with a 0.51 eV shift to lower BE, when compared to the bulk values. Salmeron et al. [lo] studied two-dimensional gold islands (0 < 1) deposited on Pt(100) in the coverage range of 0 =0.1-l and ascribed the observed 0.6 eV shift of both the Au d-band and core-level spectra to the increasing proportion of the gold islands’ edges with decreasing gold concentration. Jacobi and Althainz [ll] studied silver and gold ultra-thin films on Al(111) and found that the two noble metals behave quite differently, namely, while Ag showed a continuous transformation from monolayer (ML) to bulk silver, gold underwent alloying in the first layer producing the AuAl, alloy. In this paper we present another striking example of the difference between these two noble metals. We present UPS and XPS results of the investigation of ultra-thin films of gold and silver on a V(100) surface. To our best knowledge, there are no published data on these systems. The V(100) surface is a particularly convenient substrate for the purpose of studying the valence band development of the noble metals in two extreme situations: alloying (Au/V 1121)and total immiscibility (Ag/V [ 121). At this point it should be noted that Ag and Au are isoelectronic, have almost equal atomic radii and fee lattice constants (differing by 0.2%) [13], and exhibit very similar electronic properties. The surface energy of silver is slightly smaller (20%) [14] than that of gold.
577
(1994) 576-581
hemispherical analyzer. All work function data were measured from UP spectra widths. The sample was cleaned by a procedure described in Ref. [16], which produced a completely clean V(100) surface. The noble metals were evaporated from resistively heated tungsten baskets. During evaporation the pressure never exceeded 1 x lo-’ Pa.
3. Results and discussion 3.1. Gold / V(100)
Fig. 1 shows a set of UP spectra obtained after stepwise deposition of gold on the V(100) surface at 120 K. The bottom spectrum is taken from the clean V(100) surface with a work function (WF) of 4.3 & 0.1 eV. Gold was deposited in equal doses except for the two top spectra which were obtained after doubled doses. The final coverage corresponded to AES A~(66 eV)/V(473 eV> peak-to-peak intensity ratio (later in the text referred to as AES Au/V ratio) 3.9 (AES ratio 1.1 16000 -
AuN( 100)
UPS Hel
Tad. = 120 K
2. Experimental Since the experimental set-up is described in detail elsewhere L1.51we give here only the most important information. The experiments were performed in an ultra-high vacuum chamber at the base pressure of 3 X lop9 Pa, equipped with Mg Ka and He I photon sources, an electron gun, a quadrupole mass spectrometer and a 180
Fig. 1. UP spectra of Au/V(lOO). The bottom spectrum is of the clean WlOO) surface. Au coverage increases from zero (bottom) to AES Au/V ratio 3.9 (top). The coverage was increased in steps of 30 s deposition at constant rate and at the sample temperature of 120 K. The two top spectra are taken after 60 s deposition.
should correspond approximately to 1 ML in the case of a homogeneous and completed first gold layer, as calculated by the use of a V(469) Auger electron mean free path of 7 A>. The work function increase, as estimated from the widths of the UP spectra, from zero Au coverage to a several layers thick film is 1.3 eV. This large value reflects the work function difference between the clean V(100) surface (4.3 eV) and Au, e.g. polycrystalline (5.3 eV), Au(ll1) (5.4 eV) [ 171 or Au/Al( 111) (5.35 eV) [ll]. After the first dose. the WF increased by 0.4 eV and after the second one by an additional 0.2 eV. At the same time. Au5ds,, and 5d,,, P eaks at 4.8 and 6.2 eV BE. respectively, retain their atomic-like splitting. The same values were measured at a very small Au coverage at which no WF change could be observed. The splitting of 1.4 _t 0.1 eV is very close to the ones reported for both free Au ions and neutral Au atoms [18]. However, with increasing coverage, these atomic-like features are affected by the development of a peak at the lower BE side of the Au5d 5,2 peak. This spectral feature, due to the increased d-d orbital interaction of Au atoms, grew rapidly producing apparent splitting of 3 eV in the topmost 5d J,Z-5ds,2 spectrum of Fig. 1, which is very similar to the spectra of thick gold films. At the same time, the its position; in contrast to, 5d .1,2 peak retained e.g., the Au/Al( 111) system [ 111, where both 5d,,? and 5d s,2 peaks were found to shift towards the Fermi level as the Au coverage increased. Vanadium valence band features are located between the Fermi level and 3 eV BE [16,19,20] and at a low coverage do not overlap with the Au 5d band. As this band takes on the line shape and position of the Sd-band spectra of the Au3D system, the two bands are partly overlapping. According to the nomenclature introduced by Mason [7] this system may present a case of transformation of a weakly (no overlap) to strongly (overlap) interacting system, although care should be taken in our case as this nomenclature has been adopted for substrates with localized p and/or d states. The development of the Au5d band features with the film thickness is very similar to the case of gold containing binary alloys [1,2], where the shift of the 5d band to higher BE has been found, accom-
20000 00 ~
AuN(100) -
1 min
UPS Hel
annealing
Fig. 2. UP spectra of Au film of AES top
spectrum)
taken
after
I
Au/V
min annealing
ratio 3.0 (big.
I.
at the indicated
trmperxture.
panied with a linear band width decrease as the concentration of the second constituent has been increased. In order to see the effects of thermal treatment on the Au/V(lOO) system we performed annealing experiments on the Au film presented by the topmost spectrum in Fig. 1. Fig. 2 shows the UP spectra of this film recorded after every second step of 1 min annealing at increasing temperature. The spectra in Fig. 2 do not show large differences up to 420 K: the WF slightly increases at 210 K and remains constant up to that temperature. The first large changes are seen at 470 K: the WF drops by 0.2 eV while the density of states at the Fermi level increases. Further annealing at higher temperatures induces drastic changes. The Au 5di,., peak shifts to higher BE by losing the intensity at the lower BE side while the djj7 peak shifts by 0.2 eV to higher BE with respect to the 120 K spectrum. At 710 K, the V-band features arc clearly seen. Above 800 K the spectra are very similar to those of the very low coverage spectra displayed in Fig. 1. The reversed intensities of the Au5d peaks in the 1110 K spectrum are due to 2p derived emission of oxygen which segregated at this temperature. During the annealing steps,
T. Vallaetal./SurfaceScience307-309 (1994)576-581
Au/V( 100) XPS 1’ annealing
Au 4f(5/2)
4f(7/2)
4000
? m z c a 0
3000
2000
1000
1890K
00
84 0
01
1140Y
880
92 0
960
Blnding energy (eV)
Fig. 3. XP spectra of Au4f level taken of the high-coverage Au film (Au/V= temperature.
after 3 min annealing 11) at the indicated
the mass spectrometer was tuned to monitor gold desorption. Up to a 1500 K sample temperature no gold desorption was recorded. Therefore, the Au5d peaks’ intensity decrease seen in Fig. 2, as the annealing temperature is increased, is due solely to alloying of vanadium substrate and the gold overlayer. The XP spectra of the Au 4f levels may also be used to monitor the film growth and effects of thermal treatment. With increasing gold coverage (Tad = 300 K) from zero to an AES Au/V ratio of 11, the following changes occur: the 4f,,, peak moves from 87.8 to 87.5 eV (by 0.3 eV), the 4f,,, peak moves from 84.2 to 83.8 eV (by 0.5 eV) BE, which increases their splitting from 3.5 to 3.7 eV. At the same time the FWHM of the 4f,,, peak remains constant (1.86 eV). Annealing of the highest coverage film (Au/V = 11) induced reversed trends. The spectra are shown in Fig. 3. Annealing at 420 K induced no changes in Au peak positions but reduced the 4f s,2 FWHM to 1.74 eV. This value remains throughout the whole set of spectra in Fig. 3b. At 580 K, both peaks move by 0.3 eV to higher BE.
579
Annealing at 620 K induces an additional shift by 0.1 eV of both peaks in the same direction. Annealing at 780 and 890 K do not bring any changes. At 1140 K one observes a 0.5 eV shift of both peaks to higher BE with respect to the 890 K spectrum. Note that in all these spectra the 4f level splitting remains constant (3.65 ) 0.05 eV>. The large shift at 1140 K may be ascribed to the large drop of gold concentration in the alloy but equally well to the oxygen segregation which at this temperature takes place and may largely influence the electronic properties of the surface slab probed by XPS. The spectra at 660, 780 and 890 K (at these temperatures there is no oxygen segregation) differ mutually in their peak intensities while their position remains constant. It is possible to explain this finding in terms of a fixed Au-V stoichiometry of the surface alloy as the gold-vanadium phase diagram suggests [12]. The amount of Au atoms which diffuses into deeper V layers is replaced by the gold overlayer. This constant composition keeps the Au4f peak position constant, while the thinning of the Au overlayer induces the 4f peak intensity decrease. It should be noted here that although very similar at first glance, the spectral changes induced by the Au ultra-thin film growth and the increase of Au concentration in the Au-V alloy are different: alloying induces larger shifts of both peaks without changing their splitting and FWHM while the film growth induces different shifts for each of the 4f peaks, changing at the same time their splitting and FWHM. 3.2.
Ag/V(lOO)
A full report on the Ag/V(lOO) system studied with a wide range of coverage and temperature is given in Ref. [21]. It suffices to summarize the results only briefly. Growth modes and properties of ultra-thin films of silver on a V(100) surface were studied by means of AES, XPS, UPS, TDS and LEED techniques. The experiments were carried out in a temperature range of 50-1100 K. Above room temperature the films grow following a Stranski-Krastanov mode. The cluster formation rate strongly depends on the adsorption (annealing) temperature. Below RT the films grow
T. Vulluet al./Surfrtce Science307-309 (1994) 576-5X1
580
00
00
Blndl”y?nergy
80
(ev)
Fig. 4. He I UP spectra of 0.5, 1.O, deposited on WlOO) at 250 K.
1.5and 5 ML films of silver
in an ordered way though not clearly layer-bylayer. At 50 K the growth is statistical. Silver and vanadium do not mix at their interface at any studied temperature. Small silver coverage (0 < 0.5 ML) desorption produces single peak spectra at 1000 K. Higher coverages show two peak spectra, the more dominant one being of zeroth-order kinetics with onset at 950 K. The XPS measurements of the Ag3d levels showed no shifts throughout large Ag-coverage range of a few percent of a ML to a 20 ML film. Annealing of any of the films within this range induced Ag3d shifts smaller than 0.1 eV. The UP spectra provided more information. Fig. 4 shows He1 spectra of four Ag films on the V(100) surface deposited at 250 K, the temperature at which most ordered Ag films grow [21]. As judged from the LEED and AES measurements [21] a 1 ML film presents a homogeneous, pseudomorphic, ordered and complete silver layer on the V(100) surface. Therefore, the UP spectrum of a 1 ML film shows a 2D silver 4d-band located at 4-7.5 eV BE, split by 2 eV. The 0.5 ML spectrum differs significantly from the 1 ML one. The Ag 4d derived emission is located at 4.5-7 eV and is split by 0.75 eV. This clearly reflects the atomic-like character of this film, since at cover-
ages as low as 0.05 ML two Ag4d peaks arc clearly seen located at 6.1 eV (3/2) and 5.5 eV (5/2) BE. Their splitting of 0.6 eV is slightly larger than reported for the free Ag ion (0.57 cV) [18]. It should be mentioned here that no significant changes (< 0.1 eV1 in the width of UP spectra were observed up to 1 ML Ag coverage. The 5 ML spectrum shows an almost fully developed 3D silver 4d-band and no V-derived cmission. The peak at higher BE (Ag4d,,,, derived emission) is shifted to higher BE as the Ag concentration increases. Annealing of the 5 ML film induces large changes. The 3D Ag4d band transforms gradually into the 2D band of the 1 ML film. At the same time the vanadium spectral features emerge with increasing annealing temperature. At 800 K one obtains the same spectrum as in the cast of I ML at 250 K. Since 800 K is below the Ag desorption onset (900 K), the only possible explanation for this finding is that at elevated temperatures the silver multilayers transform into a one monolayer film with voluminous clusters dispersed across it. These clusters must be large in volume and small in area exposed to a UV photon beam so that their total contribution to the spectral line shape and intensity is negligible. At this point we may conclude that the two noble metals show completely different behaviour when deposited on a V(100) surface. Silver shows no miscibility with vanadium while gold alloys very efficiently at already moderate temperatures. Silver desorbs from the V(100) surface: at coverages up to 0.5 ML, a first-order desorption peak is obtained at 980 K; above this coverage, the zeroth-order desorption peak has an onset at 900 K. Gold does not desorb up to 1500 K. Both Ag and Au show atomic-like UP spectra at low and moderate coverages, e.g., Ag at 13= 0.5 ML. However, at high coverages the two systems differ. Namely, while the most prominent shifts in gold films are displayed by the lower, i.e. 5d,,, derived peak (the 5d,,, shifts only slightly), in the Ag case both 4d levels move but in opposite directions: the 4d,,, to lower and the 4d,,, to higher BE. The shift of the 4d,,, is somewhat larger and the peak is split at high coverages (3D films).
T. Valla et al. /Surface
Science 307-309
4. References [l] J.A. Nicholson, J.D. Riley, R.C.G. Lechey, J.G. Jenkin, J. Liesegang and J. Azoulay, Phys. Rev. B 18 (1978) 2561. [2] J.A. Nicholson, J.D. Riley, R.C.G. Lechey, J.G. Jenkin, J. Liesegang and J.G. Jenkin, J. Phys. F 7 (1977) 351. [3] R.M. Friedman, J. Hudis, M.L. Perlman and R.E. Watson, Phys. Rev. B 8 (1973) 2433. [4] T.K. Sham, M.L. Perlman and R.E. Watson, Phys. Rev. B 19 (1979) 539. [5] J.C. Fuggle, L.M. Watson, D.J. Fabian and P.R. Norris, Solid State Commun. 13 (1973) 507. [6] N.J. Shevchik, J. Phys. F 5 (1975) 1860. [7] M.G. Mason, Phys. Rev. B 27 (1983) 748. [8] K.S. Liang, W.R. Salaneck and LA. Aksay, Solid State Commun. 19 (1976) 329. [9] P.H. Citrin, G.K. Wertheim and Y. Baer, Phys. Rev. Lett. 41 (1978) 1425. [lo] M. Salmeron, S. Ferrer, M. Jazzar and G.A. Somorjai, Phys. Rev. B 28 (1983) 1158.
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[ll] K. Jacobi and P. Ahhainz, Surf. Sci. 211/212 (1989) 456. [12] M. Hansen, Constitution of Binary Alloys (McGraw-Hill, New York, 1958); T.B. Massalski, J.L. Muttay, L.H. Bennett and H. Baker, Eds., Binary Ahoy Phase Diagrams (American Society of Metals, Metals Park, OH, 1987) p. 81. [13] U. Lipphardt, H. Engelhard, J. Westhof, A. Goldman and S. Witzel, Surf. Sci. 294 (1993) 84. [14] J.H. van der Merwe and E. Bauer, Phys. Rev. B 39 (1989) 3632. [15] P. Pervan and M. Milun, Surf. Sci. 264 (1992) 135. [16] T. Valla, P. Pervan and M. Milun, Surf. Sci. 307-309 (1994) 843. [17] H.B. Michaelson, J. Appl. Phys. 48 (1977) 4729. [18] C.E. Moore, Atomic Energy Levels, Natl. Bur. Stands. Circular No. 467 (U.S. GPO, Washington, DC., 1958). [19] L. Ley, 0. Dabbousi, S.P. Kowalzyk, F.R. McFeely and D.A. Shirley, Phys. Rev. B 16 (1977) 5372. [20] L. Siller, P. Pervan and M. Milun, Fizika 23 (1991) 221. [21] T. Valla and M. Milun, Surf. Sci. to be published.
L!S$L
surface science >.. .:.... :,‘::,,:,.,;::. ,..: ;,:iy ” .’:.:,.Y:‘..::.. ‘.:)::.‘.j ..f....:.:: .. ::,:_,:’ ‘y
ELSEVTER
Photoelectron
Surface
j:j::
Science 307-3OY (lYY4) 576-581
spectroscopy of gold and silver ultra-thin films on V( 100) Tonica Valla, Petar Pervan, (Received
20 August
Milorad
Milun
*
1903)
Abstract Gold ultra-thin films deposited on V(100) surface at 120 and 300 K were characterized by AES, UPS and XPS at different stages of film growth and after an annealing treatment. The work function (WF) changed from 4.3 CV at zero Au coverage to 5.6 eV at a bulk-like film. The AuSd splitting at low covet-ages was 1.4 eV. At the same time, the Au 4f peaks shifted to lower BE, increased their splitting by 0.2 eV and kept their FWHM constant. Heating the samples to 1500 K induced no Au desorption but rather alloying started at a moderate temperature which depended on the film thickness. Upon alloying the core levels and valence band of gold shifted to higher BE. The Au/V(lOO) is compared with the Ag/V(lOO) system represented here with the UP spectra of several selected coverages and annealing experiments of the 5 ML Ag film
1. Introduction In many binary alloys containing Ag or Au, the two noble-metal valence band spectra exhibit similar behaviour, namely, the d-band shifts to lower binding energy (BE) and the d-band width increases linearly with increasing noble-metal concentration [1,2]. The Au and Ag d-band behaviour is independent of the secondary constituent metal, as may be judged from a number of investigated systems [l-6]. Mason [7] studied the valence bands and core-level spectra of noble-metal clusters deposited on both weakly and strongly interacting supports and found great sim-
* Corresponding
author.
0039-6028/94/$07.00 0 1994 Elsevier SSDl 0039-6028(93)E0832-F
Science
ilarity with the spectra of corresponding noblcmetal alloys. In the case of weakly interacting systems, the net inter-atomic charge transfer was very small [4] but the intra-atomic charge transfer was very significant. The initial-state properties were found [71 to dominate the weakly interacting noble-metal-substrate systems, while the finalstate properties were more important for strongly interacting systems. The similarity between gold clusters and gold alloys was explained [7] in terms of coordination number of like nearest neighbours, whose importance for the d-band characteristics was recognized by Shevchik [6]. The valence band and the core-level properties of thin and ultra-thin films of gold were addressed by Liang et al. [8] who found significantly narrowed Au d-band splitting when compared to the bulk value; also the Au4f peaks were found
B.V. All rights resewed
T. Valla et al. /Surface
Science 307-309
to move to higher BE with decreasing gold film thickness. Citrin et al. [9] reported a shift of 0.4 eV to lower BE of the Au4f surface-atom level, and around 8% narrowing of the surface density of states accompanied with a 0.51 eV shift to lower BE, when compared to the bulk values. Salmeron et al. [lo] studied two-dimensional gold islands (0 < 1) deposited on Pt(100) in the coverage range of 0 =0.1-l and ascribed the observed 0.6 eV shift of both the Au d-band and core-level spectra to the increasing proportion of the gold islands’ edges with decreasing gold concentration. Jacobi and Althainz [ll] studied silver and gold ultra-thin films on Al(111) and found that the two noble metals behave quite differently, namely, while Ag showed a continuous transformation from monolayer (ML) to bulk silver, gold underwent alloying in the first layer producing the AuAl, alloy. In this paper we present another striking example of the difference between these two noble metals. We present UPS and XPS results of the investigation of ultra-thin films of gold and silver on a V(100) surface. To our best knowledge, there are no published data on these systems. The V(100) surface is a particularly convenient substrate for the purpose of studying the valence band development of the noble metals in two extreme situations: alloying (Au/V 1121)and total immiscibility (Ag/V [ 121). At this point it should be noted that Ag and Au are isoelectronic, have almost equal atomic radii and fee lattice constants (differing by 0.2%) [13], and exhibit very similar electronic properties. The surface energy of silver is slightly smaller (20%) [14] than that of gold.
577
(1994) 576-581
hemispherical analyzer. All work function data were measured from UP spectra widths. The sample was cleaned by a procedure described in Ref. [16], which produced a completely clean V(100) surface. The noble metals were evaporated from resistively heated tungsten baskets. During evaporation the pressure never exceeded 1 x lo-’ Pa.
3. Results and discussion 3.1. Gold / V(100)
Fig. 1 shows a set of UP spectra obtained after stepwise deposition of gold on the V(100) surface at 120 K. The bottom spectrum is taken from the clean V(100) surface with a work function (WF) of 4.3 & 0.1 eV. Gold was deposited in equal doses except for the two top spectra which were obtained after doubled doses. The final coverage corresponded to AES A~(66 eV)/V(473 eV> peak-to-peak intensity ratio (later in the text referred to as AES Au/V ratio) 3.9 (AES ratio 1.1 16000 -
AuN( 100)
UPS Hel
Tad. = 120 K
2. Experimental Since the experimental set-up is described in detail elsewhere L1.51we give here only the most important information. The experiments were performed in an ultra-high vacuum chamber at the base pressure of 3 X lop9 Pa, equipped with Mg Ka and He I photon sources, an electron gun, a quadrupole mass spectrometer and a 180
Fig. 1. UP spectra of Au/V(lOO). The bottom spectrum is of the clean WlOO) surface. Au coverage increases from zero (bottom) to AES Au/V ratio 3.9 (top). The coverage was increased in steps of 30 s deposition at constant rate and at the sample temperature of 120 K. The two top spectra are taken after 60 s deposition.
should correspond approximately to 1 ML in the case of a homogeneous and completed first gold layer, as calculated by the use of a V(469) Auger electron mean free path of 7 A>. The work function increase, as estimated from the widths of the UP spectra, from zero Au coverage to a several layers thick film is 1.3 eV. This large value reflects the work function difference between the clean V(100) surface (4.3 eV) and Au, e.g. polycrystalline (5.3 eV), Au(ll1) (5.4 eV) [ 171 or Au/Al( 111) (5.35 eV) [ll]. After the first dose. the WF increased by 0.4 eV and after the second one by an additional 0.2 eV. At the same time. Au5ds,, and 5d,,, P eaks at 4.8 and 6.2 eV BE. respectively, retain their atomic-like splitting. The same values were measured at a very small Au coverage at which no WF change could be observed. The splitting of 1.4 _t 0.1 eV is very close to the ones reported for both free Au ions and neutral Au atoms [18]. However, with increasing coverage, these atomic-like features are affected by the development of a peak at the lower BE side of the Au5d 5,2 peak. This spectral feature, due to the increased d-d orbital interaction of Au atoms, grew rapidly producing apparent splitting of 3 eV in the topmost 5d J,Z-5ds,2 spectrum of Fig. 1, which is very similar to the spectra of thick gold films. At the same time, the its position; in contrast to, 5d .1,2 peak retained e.g., the Au/Al( 111) system [ 111, where both 5d,,? and 5d s,2 peaks were found to shift towards the Fermi level as the Au coverage increased. Vanadium valence band features are located between the Fermi level and 3 eV BE [16,19,20] and at a low coverage do not overlap with the Au 5d band. As this band takes on the line shape and position of the Sd-band spectra of the Au3D system, the two bands are partly overlapping. According to the nomenclature introduced by Mason [7] this system may present a case of transformation of a weakly (no overlap) to strongly (overlap) interacting system, although care should be taken in our case as this nomenclature has been adopted for substrates with localized p and/or d states. The development of the Au5d band features with the film thickness is very similar to the case of gold containing binary alloys [1,2], where the shift of the 5d band to higher BE has been found, accom-
20000 00 ~
AuN(100) -
1 min
UPS Hel
annealing
Fig. 2. UP spectra of Au film of AES top
spectrum)
taken
after
I
Au/V
min annealing
ratio 3.0 (big.
I.
at the indicated
trmperxture.
panied with a linear band width decrease as the concentration of the second constituent has been increased. In order to see the effects of thermal treatment on the Au/V(lOO) system we performed annealing experiments on the Au film presented by the topmost spectrum in Fig. 1. Fig. 2 shows the UP spectra of this film recorded after every second step of 1 min annealing at increasing temperature. The spectra in Fig. 2 do not show large differences up to 420 K: the WF slightly increases at 210 K and remains constant up to that temperature. The first large changes are seen at 470 K: the WF drops by 0.2 eV while the density of states at the Fermi level increases. Further annealing at higher temperatures induces drastic changes. The Au 5di,., peak shifts to higher BE by losing the intensity at the lower BE side while the djj7 peak shifts by 0.2 eV to higher BE with respect to the 120 K spectrum. At 710 K, the V-band features arc clearly seen. Above 800 K the spectra are very similar to those of the very low coverage spectra displayed in Fig. 1. The reversed intensities of the Au5d peaks in the 1110 K spectrum are due to 2p derived emission of oxygen which segregated at this temperature. During the annealing steps,
T. Vallaetal./SurfaceScience307-309 (1994)576-581
Au/V( 100) XPS 1’ annealing
Au 4f(5/2)
4f(7/2)
4000
? m z c a 0
3000
2000
1000
1890K
00
84 0
01
1140Y
880
92 0
960
Blnding energy (eV)
Fig. 3. XP spectra of Au4f level taken of the high-coverage Au film (Au/V= temperature.
after 3 min annealing 11) at the indicated
the mass spectrometer was tuned to monitor gold desorption. Up to a 1500 K sample temperature no gold desorption was recorded. Therefore, the Au5d peaks’ intensity decrease seen in Fig. 2, as the annealing temperature is increased, is due solely to alloying of vanadium substrate and the gold overlayer. The XP spectra of the Au 4f levels may also be used to monitor the film growth and effects of thermal treatment. With increasing gold coverage (Tad = 300 K) from zero to an AES Au/V ratio of 11, the following changes occur: the 4f,,, peak moves from 87.8 to 87.5 eV (by 0.3 eV), the 4f,,, peak moves from 84.2 to 83.8 eV (by 0.5 eV) BE, which increases their splitting from 3.5 to 3.7 eV. At the same time the FWHM of the 4f,,, peak remains constant (1.86 eV). Annealing of the highest coverage film (Au/V = 11) induced reversed trends. The spectra are shown in Fig. 3. Annealing at 420 K induced no changes in Au peak positions but reduced the 4f s,2 FWHM to 1.74 eV. This value remains throughout the whole set of spectra in Fig. 3b. At 580 K, both peaks move by 0.3 eV to higher BE.
579
Annealing at 620 K induces an additional shift by 0.1 eV of both peaks in the same direction. Annealing at 780 and 890 K do not bring any changes. At 1140 K one observes a 0.5 eV shift of both peaks to higher BE with respect to the 890 K spectrum. Note that in all these spectra the 4f level splitting remains constant (3.65 ) 0.05 eV>. The large shift at 1140 K may be ascribed to the large drop of gold concentration in the alloy but equally well to the oxygen segregation which at this temperature takes place and may largely influence the electronic properties of the surface slab probed by XPS. The spectra at 660, 780 and 890 K (at these temperatures there is no oxygen segregation) differ mutually in their peak intensities while their position remains constant. It is possible to explain this finding in terms of a fixed Au-V stoichiometry of the surface alloy as the gold-vanadium phase diagram suggests [12]. The amount of Au atoms which diffuses into deeper V layers is replaced by the gold overlayer. This constant composition keeps the Au4f peak position constant, while the thinning of the Au overlayer induces the 4f peak intensity decrease. It should be noted here that although very similar at first glance, the spectral changes induced by the Au ultra-thin film growth and the increase of Au concentration in the Au-V alloy are different: alloying induces larger shifts of both peaks without changing their splitting and FWHM while the film growth induces different shifts for each of the 4f peaks, changing at the same time their splitting and FWHM. 3.2.
Ag/V(lOO)
A full report on the Ag/V(lOO) system studied with a wide range of coverage and temperature is given in Ref. [21]. It suffices to summarize the results only briefly. Growth modes and properties of ultra-thin films of silver on a V(100) surface were studied by means of AES, XPS, UPS, TDS and LEED techniques. The experiments were carried out in a temperature range of 50-1100 K. Above room temperature the films grow following a Stranski-Krastanov mode. The cluster formation rate strongly depends on the adsorption (annealing) temperature. Below RT the films grow
T. Vulluet al./Surfrtce Science307-309 (1994) 576-5X1
580
00
00
Blndl”y?nergy
80
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Fig. 4. He I UP spectra of 0.5, 1.O, deposited on WlOO) at 250 K.
1.5and 5 ML films of silver
in an ordered way though not clearly layer-bylayer. At 50 K the growth is statistical. Silver and vanadium do not mix at their interface at any studied temperature. Small silver coverage (0 < 0.5 ML) desorption produces single peak spectra at 1000 K. Higher coverages show two peak spectra, the more dominant one being of zeroth-order kinetics with onset at 950 K. The XPS measurements of the Ag3d levels showed no shifts throughout large Ag-coverage range of a few percent of a ML to a 20 ML film. Annealing of any of the films within this range induced Ag3d shifts smaller than 0.1 eV. The UP spectra provided more information. Fig. 4 shows He1 spectra of four Ag films on the V(100) surface deposited at 250 K, the temperature at which most ordered Ag films grow [21]. As judged from the LEED and AES measurements [21] a 1 ML film presents a homogeneous, pseudomorphic, ordered and complete silver layer on the V(100) surface. Therefore, the UP spectrum of a 1 ML film shows a 2D silver 4d-band located at 4-7.5 eV BE, split by 2 eV. The 0.5 ML spectrum differs significantly from the 1 ML one. The Ag 4d derived emission is located at 4.5-7 eV and is split by 0.75 eV. This clearly reflects the atomic-like character of this film, since at cover-
ages as low as 0.05 ML two Ag4d peaks arc clearly seen located at 6.1 eV (3/2) and 5.5 eV (5/2) BE. Their splitting of 0.6 eV is slightly larger than reported for the free Ag ion (0.57 cV) [18]. It should be mentioned here that no significant changes (< 0.1 eV1 in the width of UP spectra were observed up to 1 ML Ag coverage. The 5 ML spectrum shows an almost fully developed 3D silver 4d-band and no V-derived cmission. The peak at higher BE (Ag4d,,,, derived emission) is shifted to higher BE as the Ag concentration increases. Annealing of the 5 ML film induces large changes. The 3D Ag4d band transforms gradually into the 2D band of the 1 ML film. At the same time the vanadium spectral features emerge with increasing annealing temperature. At 800 K one obtains the same spectrum as in the cast of I ML at 250 K. Since 800 K is below the Ag desorption onset (900 K), the only possible explanation for this finding is that at elevated temperatures the silver multilayers transform into a one monolayer film with voluminous clusters dispersed across it. These clusters must be large in volume and small in area exposed to a UV photon beam so that their total contribution to the spectral line shape and intensity is negligible. At this point we may conclude that the two noble metals show completely different behaviour when deposited on a V(100) surface. Silver shows no miscibility with vanadium while gold alloys very efficiently at already moderate temperatures. Silver desorbs from the V(100) surface: at coverages up to 0.5 ML, a first-order desorption peak is obtained at 980 K; above this coverage, the zeroth-order desorption peak has an onset at 900 K. Gold does not desorb up to 1500 K. Both Ag and Au show atomic-like UP spectra at low and moderate coverages, e.g., Ag at 13= 0.5 ML. However, at high coverages the two systems differ. Namely, while the most prominent shifts in gold films are displayed by the lower, i.e. 5d,,, derived peak (the 5d,,, shifts only slightly), in the Ag case both 4d levels move but in opposite directions: the 4d,,, to lower and the 4d,,, to higher BE. The shift of the 4d,,, is somewhat larger and the peak is split at high coverages (3D films).
T. Valla et al. /Surface
Science 307-309
4. References [l] J.A. Nicholson, J.D. Riley, R.C.G. Lechey, J.G. Jenkin, J. Liesegang and J. Azoulay, Phys. Rev. B 18 (1978) 2561. [2] J.A. Nicholson, J.D. Riley, R.C.G. Lechey, J.G. Jenkin, J. Liesegang and J.G. Jenkin, J. Phys. F 7 (1977) 351. [3] R.M. Friedman, J. Hudis, M.L. Perlman and R.E. Watson, Phys. Rev. B 8 (1973) 2433. [4] T.K. Sham, M.L. Perlman and R.E. Watson, Phys. Rev. B 19 (1979) 539. [5] J.C. Fuggle, L.M. Watson, D.J. Fabian and P.R. Norris, Solid State Commun. 13 (1973) 507. [6] N.J. Shevchik, J. Phys. F 5 (1975) 1860. [7] M.G. Mason, Phys. Rev. B 27 (1983) 748. [8] K.S. Liang, W.R. Salaneck and LA. Aksay, Solid State Commun. 19 (1976) 329. [9] P.H. Citrin, G.K. Wertheim and Y. Baer, Phys. Rev. Lett. 41 (1978) 1425. [lo] M. Salmeron, S. Ferrer, M. Jazzar and G.A. Somorjai, Phys. Rev. B 28 (1983) 1158.
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[ll] K. Jacobi and P. Ahhainz, Surf. Sci. 211/212 (1989) 456. [12] M. Hansen, Constitution of Binary Alloys (McGraw-Hill, New York, 1958); T.B. Massalski, J.L. Muttay, L.H. Bennett and H. Baker, Eds., Binary Ahoy Phase Diagrams (American Society of Metals, Metals Park, OH, 1987) p. 81. [13] U. Lipphardt, H. Engelhard, J. Westhof, A. Goldman and S. Witzel, Surf. Sci. 294 (1993) 84. [14] J.H. van der Merwe and E. Bauer, Phys. Rev. B 39 (1989) 3632. [15] P. Pervan and M. Milun, Surf. Sci. 264 (1992) 135. [16] T. Valla, P. Pervan and M. Milun, Surf. Sci. 307-309 (1994) 843. [17] H.B. Michaelson, J. Appl. Phys. 48 (1977) 4729. [18] C.E. Moore, Atomic Energy Levels, Natl. Bur. Stands. Circular No. 467 (U.S. GPO, Washington, DC., 1958). [19] L. Ley, 0. Dabbousi, S.P. Kowalzyk, F.R. McFeely and D.A. Shirley, Phys. Rev. B 16 (1977) 5372. [20] L. Siller, P. Pervan and M. Milun, Fizika 23 (1991) 221. [21] T. Valla and M. Milun, Surf. Sci. to be published.