- Email: [email protected]
Ni surface alloys on Ni(111)
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surface
science
....>...;:; :‘.“.“‘.“.~.‘.‘.‘.‘. ...::.>... ., .,.,. . ,,, ~:.:,.::,.:::::‘:::i::: ,...,........ .....,....: ,.) :.~ ,:::.:., ~::,, ., :::ji:;::.:::.:., :‘:.:“::::..:::::::.;.:,.,:.: ~,,.....: .“‘.‘.:.‘:.:::i:i~~li:~::.~::~:::~:~:~:~::~~~ ..:.‘.” Surface Science 301 (1994) 39-51
ELSEVIER
Structure and properties of samarium overlayers and Sm/Ni surface alloys on Ni( 111) Gerard
M. Roe ‘, Caio M.C. de Castilho
2, Richard
M. Lambert
*
Department of Chemistv, Unicersity of Cambridge, Lensfield Road, Cambridge CB2 lEU! UK (Received
4 July 1993; accepted
for publication
17 August
1993)
Abstract Development of the Ni(1 ll)/ Sm interface as a function of rare earth loading and substrate temperature has been studied by LEED, XPS, UPS, Aq5 and auxiliary measurements. At ambient temperature growth proceeds by the Stranski-Krastanov mechanism involving initial formation of a metastable low density, fi bilayer of Sm adatoms. Sm atoms in the bilayer are trivalent, divalent character appearing for thicker films; angle-resolved XP data suggest that the divalent Sm is surface-localised and therefore that this mixed valency is heterogeneous rather than homogeneous in nature. Thermal treatment results in the formation of a series of ordered surface alloys whose structures may be rationalised in terms of known Ni/Sm bulk alloys. The most important (most stable) of these surface alloys is based on the Ni,,Sm, structure and it appears to be capped by an overlayer of trivalent Sm.
1. Introduction The physics and chemistry of rare earth/ transition metal intermetallic systems are of interest with respect to bulk properties such as magnetism and hydrogen storage. Surface properties of such systems are also of considerable interest in connection with the formation of magnetic overlayers [l], and, in the case of alloy surfaces, with regard to catalytic behaviour [21. Although the catalytic chemistry of rare earth/
* Corresponding author. address: Comalco Research Centre, Thomastown, Melbourne, Victoria 3074, Australia. ’ Present address: Instituto de Fisica, UFBa Campus Universitsrio da Federacao, 40 210 Salvador, Bahia, Brazil.
’ Present
0039-6028/94/$07.00 0 1994 Elsevier XSDI 0039-6028(93)E0497-I
Science
transition metal alloys has received a great deal of investigation [3] there have been relatively few studies of the surface structure, composition and reactive properties of such systems under well defined conditions. Our earlier UHV/ single crystal work on rare earth/Cu systems indicates that such observations can provide important clues to the origin of the extraordinarily effective catalysts which can be produced from these materials 14-71. In the case of Sm, an additional interesting feature arises, namely the phenomenon of variable valency. Metallic Sm is trivalent in the bulk (Xe 4f5(5d6sj3), the gaseous atom is divalent (Xe4f6(5d6sj2), and the surface of trivalent metallic Sm exhibits divalent character. Intermetallic compounds of Sm often also exhibit divalent Sm in the surface layer, whatever the bulk electronic properties [8,9]. The present paper
B.V. All rights reserved
40
G.M. Roe rt ul. /Surface
deals with the Ni(ll l)/Sm system with particular reference to the nature of overlayer structures, the conditions necessary for their conversion to surface alloys, the structure and composition of such alloys, and the associated electronic properties. At ambient temperatures a metastable, low density bilayer of trivalent Sm forms on the Nit 111) surface; thicker layers contain surfacelocalised divalent Sm. Thermal treatment leads to the formation of a series of well ordered surface alloys, the most important of which (Ni,,Sm ,) appears to be capped by a layer of Sm atoms. It is of interest to compare the results reported here with earlier work on the closely related Cu(lll)/ Sm system [.5,6].
2. Experimental Experiments were carried out in two separate UHV chambers. Chamber A was a modified VG ADES 400 angle dispersive electron spectrometer system, incorporating LEED, AES, UPS and XPS facilities. Chamber B was used for LEED, AES and TDS experiments, and both chambers operated at a base pressure of - 1 X 1OW”’ Torr. The Ni(lll> single crystal wafer was cut from a 99.999% pure ingot (Metal Crystals Ltd) after orientation to within 0.5” of the (111) plane following mechanical polishing, a chromel/ alumel
1.2
thermocouple was spot welded to the back face of the specimen which was mounted on a liquid nitrogen cooled manipulator. Cleaning was achieved by Ar+ bombardment at room temperature, surface order being restored by annealing at - 900 K for 10 min. In both chambers, Sm was evaporated from collimated, shuttered sources which were heated by electron bombardment; during Sm deposition the increase in system pressure never exceeded 1 x lo-” Torr.
3. Results 3.1. Auger spectroscopy At deposition times ,< 8 min (Sm coverage < 1 ML, see below) the Sm transitions at 100 eV (N,,O,,N,,) and 129 eV (N,,N,,V) were observable, but the 138 eV (N,,VV) emission was below the detection limit; the latter only became apparent at - 1 ML coverage. For Sm coverages between 1 and - 2 ML, the 138 eV (NdsVV) transition became progressively stronger relative to the 100 eV (N,,O,,N,,) and 129 eV (N4sNh7V) transitions, while for coverages - 2 ML, the relative intensities of the three signals were constant. Typical low coverage and high coverage Sm spectra are shown in the inset to Fig. 1 which illustrates the variations in intensity of the SmNON
a
TJ
0.8
.s
.b 1.0 c/l
i
Kj 0.8
0.6
Q u
-0 a 0.6 .v, iT E 0.4
.$ 0.4 T!
E c&
b c
Science 301 (IYYJ) 39-51
0.2
0.2
deposition time/minutes 0.0
0
8
16
24
32
0.0
Fig. 1, Normalised Sm NON and Ni MVV intensities as a function of Sm deposition deviation +5%. Inset to (A) shows Auger spectra for 1.0 and 4.1 ML Sm films.
0
8
16
24
32
time at 293 K (A) and 800 K (B). Standard
GM Roe 81al. /Surface Science 301 (1994139-51
0
8
16
24
32
40
deposition time/minutes Fig. 2. Auger ratios (AR) for Sm uptake at 293 and 800 K.
eV) and NiMVV (60 eV) transitions as a function of Sm deposition time at 293 and 800 K. Signal intensities may also be expressed in terms of the ratio of Sm-to-Ni peak heights (Auger ratio: AR), 3 quantity which is independent of incident beam current. Variations in the AR as a function of Sm coverage and deposition time at 293 and 800 K are shown in Fig. 2. At 293 K the Sm intensi~ exhibits linear growth with a clear break at 16 min deposition time; beyond this point the graph is slightly curved. No clear breaks are evident in the corresponding Ni data. However, at 800 K, clear breaks occur at 16 min deposition time in both the Sm and the Ni data; the principal differences between the 293 and 800 K uptake curves emerge at deposition times > 16 min. The AR plot (Fig. 2) highIights (100
31
these differences, as reflected by the divergence of the 293 and 800 K data at 16 min deposition time. The Auger data for deposition at 293 and 800 K are characteristic of Stranski-~astanov growth: at 800 K, island formation occurs beyond 16 min deposition, whereas at 293 K simultaneous multilayer growth ensues at this point 1103. The LEED, workfunction (A+) and TDS results presented below are consistent with formation of 3 Sm bilayer structure at 293 K, completion of this bilayer occurring at 16 min deposition. (This also provides the calibration point which is used to obtain the quoted Sm coverages.) The sample current to ground measured during Auger spectroscopy experiments can provide useful information about the structural evolution of systems of this type [II], and Figs. 3A and 3B show the results obtained at 293 and 800 K. In both cases the sample current initially rose to a m~imum at the monohyer point (8 m); at 293 K, it then fell continuously, whereas at 800 K it fell and then levelled out after 16 min deposition. Note that this behaviour resembles but is not identical to that reported for Sm/Cu(lll); in that case a change in slope was observed at the monolayer point (see Section 4). Variations in the AR with temperature for a series of different initial Sm loadings are shown in Fig. 4; experiments were performed by depositing Sm at 293 K followed by annealing for 2 min at the temperatures specified. At Sm coverages
B
. A,
deposition time/minutes
deposition time/minutes
5.6 0
8
16
24
32
8
76
24
Fig. 3. Variation in crystal current with Sm deposition time at 293 K(A) and HO0K(B).
32
G.M.Roe etal./ Surface Science 301 (1994)39-53
42
G 2 ML no significant changes occurred, whereas for higher coverage films (2.2, 3.5, 4.3 ML) annealing caused an AR decrease to a limiting value between 1.6 and 1.8 at 800 K. Note that for 4.3 and 3.5 ML films a distinct plateau appears between 600 and 700 K. The ARs observed after thermally treating Sm films of a given coverage were similar, whether the overlayer was deposited at a particular elevated temperature, or deposited at 203 K and subsequently annealed to that temperature (compare Figs. 2 and 4).
293 K, indicating that the associated surface structures were not the same. The 6 structure resulting from annealing is henceforth designated as A-(& x v%)R30” in order to distinguish it from the 293 K v% structure. The 1.5 X 1.5 and (1.5 X l.5>R30° patterns occurred only in combination with other structures, and the A-t&? X fi>R30” and (2 X 2) patterns were often found to coexist. The various phases appearing, either separately or in coexistence with other structures, are summarised below, and representative diffraction patterns are shown in Fig. 5.
3.2. LEED (i)
The only overlayer LEED pattern observed during Sm deposition at 293 K corresponded to a tfi X fi)R30” surface structure (Fig. 5). This pattern appeared at a coverage of - 0.6 ML, reached its maximum intensity at - 1.5 ML, and then progressively faded. By - 3.5 ML the pattern had disappeared, although diffuse (1 X 1) beams were still risible; the latter were extinguished at Sm coverages - 4.5 ML. A variety of LEED patterns were seen upon subsequent thermal treatment of 293 K deposited films, corresponding to (26 X 2v%)R30°, tv% X fi)R30”, (2 X 21,(1.5 1.5) and (1.5 X 1.5)R30° periodicities. The intensity versus beam voltage characteristics of this second tfi X &)R30” phase were entirely different to those of the v% structure observed at
(Zfi~?fi)R30",
(iv)
A-(&
Xfi)R.?O"
+(2x2). (ii)
A-(6
x\/i)R30",
(v)
A-(fiXfi)R30 +(2x2)+(1.5x1.5) +
(iii)
(2X2),
(vi)
(26
( 1.5 x I .S)RW. x2fi)R30
+(1.5x1.5).
Occurrence of these surface structures was dependent on Sm pre-coverage and subsequent annealing temperature; results are summarised in Fig. 6, LEED observations being performed at each of the points indicated after heating to the relevant temperature for 2 min. It is apparent that the (26 x 2fi)R30” structure alone occurred only at coverages G 2 ML; above 2 ML, the A-(v% X v%)R30” or (2 X 2) phases resulted from heating at 500 K, the latter predominating at higher coverages (up to - 4.3 ML). For higher temperatures (600 and 700 K) ( 1.5 X 1.5)R30° and (1.5 X 1.5) patterns were associated with the ACJ? X &)R30” and (2 X 2) structures, while for temperatures 2 X00 K only the coexisting (2fi X 2v%)R30”) + (1.5 x 1.5) phases were produced, regardless of coverage. 3.3. Photoemission
a
0.5
0
c -
q
300
500
700
900
temperature/K Fig. 4. Dependence were
deposited
temperatures
of AR
on temperature.
at 293 K and
specified.
annealed
for
Sm overlayera 2 min
at the
3.3.1. As-deposited Sm orlerlayers: XPS Fig. 7 shows a series of representative Sm 3d,,: XP spectra as a function of Sm coverage following deposition at 293 K, quoted coverages are calibrated with respect to the Auger spectroscopy data, as noted above. The high coverage spectra are typical of those exhibited by bulk Sm, showing transitions due to both divalent Sm (SmtII), BE
GM
Roe et al. /Surface
= 1075.9 eV) and trivalent Sm (Sm(III), BE = 1083.6 eV): the lower BE associated with Sm(I1) could arise as a result of either initial state effects (higher inter-electron repulsion), final state effects (increased screening of core hole), or both. Both the divalent and trivalent emissions are broad due to their multiplet structure which arises as a result of 3d-4f coupling [12,13]. At low coverages, only the characteristic Sm(II1) emis-
Science 301 (1994) 39-51
43
sion was detectable: Sm(I1) emission did not uppear until a threshold corjerage Lsalue of two Sm monolayers had been exceeded. This behaviour of the Sm(I1) and Sm(II1) intensities is illustrated in Fig. 8A for deposition experiments carried out at 150 K; essentially similar results were obtained at 298 K. No coverage-dependent binding energy shifts were apparent. Measurements carried out as a function of photoelectron exit angle for 6.5
Fig. 5. LEED patterns. (A) (v% X fi)R30” structure, 1.8 ML Sm at 293 K, 105 eV. (B) 2(&f x fi)R30” structure, 1.4 ML Sm. 61h K anneal, 110 eV. (C) (2 x 2) structure, 4.4 ML Sm, 611 K anneal, 120 eV. (D) 2(& X &)R30” + (1.5 X 1.5) structure, 4.4 ML Sm. 796 K anneal. 100 eV.
G. M. Roe et ul. /
SwfuceScience 301 (1994) 39-51
1000
800 & L g 600 s
E s 400
u.0
1 .o
2.0
3.0
coverage
4.0
5.0
(ML)
Fig. 6. Temperature-initial Sm loading diagram tence regimes for the various structures.
showing
exis10717 10762 lOB07 IO852 10897
Binding Energy/eV ML Sm films showed that the Sm(II) emission is enhanced at grazing exit angles, indicating that the divalent Sm atoms are surface localised. Fig. 8B summarises the divalent, trivalent and (II + III) integrated intensities over a wide range of coverage. Here, as in all other cases, the data were treated by subtracting a Shirley-type background [14] and normalized with respect to the intensity. In passing, we clean surface Ni 2p,,, note that at very low coverages (5 0.07 ML) an unusual weak Sm-derived feature appeared at a BE of 1086.6 eV.
Fig. 7. Sm 3d 5,2 spectra as a function K. Divalent (II). trivalent (III) and indicated.
of Sm deposition at 293 1086.6 eV BE features
3.3.2. Annealed films: XPS The intensity variation of the Sm(II + III) emission for a series of different Sm loadings as a function of annealing temperature is shown in Fig. 9 which also presents spectra illustrating decay of the divalent state. For initial Sm covcrages _<2 ML the Sm and Ni signals were essentially invariant over the entire temperature range,
III
(ML)
coverage
0.08 0 Fig. X. Divalent,
trivalent
and total Sm 3d,,,,
intensities
as a function
’ 5
of Sm deposition
IO
15
at IS0 K(A)
20
25
30
and 293 K CR)
G.M. Roeet
al./
Surface Science 301 (1994) 39-51
45
whereas at - 5.8 ML the Sm intensity decreased for annealing temperatures > 400 K. For higher coverage films, the Ni signal increased markedly for temperatures 2 500 K. The Sm(I1 + III) intensities for 2.8 and 5.8 ML films tended rapidly to zero for temperatures in excess of 1000 K while at the same time the Ni signals returned to the clean surface value: this behaviour is entirely consistent with the Auger data reported above. Again, thermal treatment led to no detectable BE . shifts m the Sm3d,,, and Ni 2p,,, spectra.
0.1 ML t ,
0.00 200
400
f ‘. 600
-, ? 800
temperature
f 1000
.1200
(K)
Sm 3d5/2 XPS
B,
-3.0’
-I
0
1
2
3
4
5
Fig. 10. (A) He1 UP spectra of clean Ni (a) and after deposition of 1.8 ML Sm at 293 K (b) and 800 K Cc). (B) AC#Jas a function of Sm deposition at 293 and 800 K.
3.3.3. As-deposited
and annealed
films:
UPS, AI$,
TPD
1071.7 1076 2 1080 7 1085.2 IOIN 7
Binding energy/e\/ Fig. 9. (A) Total Sm3dS,, intensity as a function of annealing temperature for a series of different initial coverages. (B) Sm3dsj2 spectra for a 5.8 ML Sm film annealed at 293 K (a), 623 K (b), and 906 K (cl.
Normal emission He I spectra for 1.8 ML Sm films deposited at 298 and 800 K are shown in Fig. 10A; these spectra exemplify typical features observed in the UPS data. Sm deposition led to attenuation and broadening of the Nid-band emission accompanied by growth of an Sm-induced feature at BE = 5 eV which may be assigned to 4f emission from a trivalent initial state [131. As might be expected, Sm deposition led to
46
G.M. Roe et ul. /Surface Science 301 (1994139-51
large decreases in work function and Fig. 10B shows A+ as a function of Sm coverage for depositions covered out at 293 and 800 K. At 293 K, A$ reached a limiting value of -2.5 eV after deposition of - 2.5 ML Sm, which is in agreement with the work function difference between Ni(ll1) and Sm metal [15]. The initial work function decrease was more rapid at 800 K, yielding a limiting value of A4 = -2.0 eV at 1 ML Sm. Thermal treatment of the 4.6 ML film caused a work function increase of 0.5 eV, which corresponds to the difference in limiting values between films deposited at 293 and at 800 K. Finally it is important to note that at 293 K, deposition of 1 ML Sm led to complete suppression of the Hz TPD feature associated with clean Ni(l11). This provides strong confirmation of monolayer calibration point (8 min deposition).
4. Discussion 4.1. Room temperature (293 K) behaviour The crystal current and H, TPD data clearly indicate that an important stage in the structural evolution of the Sm/ Ni(ll1) interface was reached at 8 m deposition time. In particular, the complete quenching of H, desorption from Ni sites provides strong evidence that the contact layer is completed at 8 min deposition time. Furthermore, the appearance threshold of the Sm NVV transition at this point strongly implies the onset of second layer formation. The 293 K Sm Auger data and the 150 K XPS data indicate termination of a second important stage at 16 min, as film growth switched from layer-by-layer mode to simultaneous multilayer mode. Similar Stranski-Krastanov behaviour has been observed for Sm/Cu(lll) [6,16], Eu/Pd(lll) [171 and Yb/ Ni(100) [18]. Appearance of the (6 x J?;)R30” LEED pattern correlated with near completion of the first monolayer (8 min), which we associate with a fractional surface coverage of l/3 with respect to the Ni(lll) substrate, i.e. the adatom density in the first Sm overlayer (1 ML) is 6.2 x 10” m-*. The LEED results also show that further Sm adsorption occurred in registry with
this contact layer, yielding a bilayer of overall fi symmetry. This bilayer was completed at - 16 min deposition and corresponds to a relatively - 33% expanded with respect to open structure, bulk Sm metal. The relatively low attenuation of the Ni 60 eV signal at 1 ML is understandable in terms of the low density \/;5 monolayer. Correspondingly, the absence of a clear break in the Sm NON Auger intensity at 1 ML may reflect the fact that Sm atoms in the contact layer are not fully shadowed by second layer Sm atoms. (The contribution from NiMVV intensity at 100 eV would also have tended to obscure the Sm break point at 1 ML.) The possibility that Ni/Sm intermixing occurs at 293 K may be excluded on the basis of the essentially identical uptake behaviour observed at low temperatures (e.g. 220 K, Fig. 8B). Furthermore, in annealing experiments, no change in AR occurred until reaching a temperature of 400 K. Low density rare earth overlayer structures similar to that postulated here have also been reported for Nd/Cu(lll) [4], Nd/ Cu(lO0) [19,201, Sm/Cu(lll) [6,16], Sm/Cu(llO) [16], Sm/Cu(lOO) [16] and Eu/Pd(lll) [17]. The Sm/ Cu(ll1) system is ostensibly most closely related to the present case, but there are significant structural differences [6]. On Cu(l 11) a low density contact layer corresponding to 0 = 0.36 -t 0.03 is also formed (cf. 0 = 0.33 on Ni(ll1)) but with a quite different structure: a ((2,-i) mesh incorporating additional adatoms. This is followed by a fi second layer, as in the present case. On Ni(ll1) beyond 2 ML the Auger data suggest three-dimensional growth, at which point LEED observations indicate progressive collapse of the metastable bilayer with eventual extinction of the fi beams leaving only a diffuse (1 X 1) pattern. In this respect, the behaviour of Sm/ Cm11 1) is very similar [61. The work function decrease with increasing Sm coverage is characteristic of an electropositive adsorbate on a high work function substrate: note that A4 did not level off until bilayer completion. A value for the low coverage surface dipole of 0.6 D per adatom can be calculated by means of the Helmholtz equation [21], implying charge transfer of 0.05 electrons per Sm atom - it may be that repulsive interactions between the electron defi-
G.M. Roe et al. /Surface
cient Sm atoms contribute to stabilisation of the low density bilayer. The evolution of the 100 eV SmNON, 127 eV SmNNV and 129 eV SmNVV relative intensities with increasing Sm coverage can be rationalised in terms of progressively decreasing charge transfer from Sm valence levels. Thus at low coverages, a significant degree of charge transfer should not affect the NON transition but would decrease the probability of NNV transitions; NVV transitions would be even more attenuated. The observed behaviour of all three transitions is entirely in line with this argument. The rise in crystal current to a maximum at - 8 min (- 1 ML) might be anticipated as decreasing 4 enhances the escape of secondary electrons. The subsequent decrease may be attributed to effects determined by surface structure [121; changes in the density of states above E, [22] would also be expected to contribute to this behaviour. For the above reasons, and because of different experimental geometries (40” versus 75” electron incidence angles) it is not unexpected that the present crystal current data are not identical to those reported for Sm/ Cu(ll1) [6]. Sm 3d,,, spectra show that below 2 ML the Sm overlayer appeared to be entirely trivalent (see Fig. 91, the divalent component appearing only at > 2 ML. (The Sm(II1) metallic state is not to be confused with partially ionic Sm, resulting, for example from oxidation; the latter has a binding energy - 1 eV higher than Sm(III).) Similar trivalency exhibited by isolated Sm atoms on Pd(100) [23] has been reported, whereas on Cu [5,24] and MO [25], the Sm adatoms were initially divalent. At high coverages the effective valency CC; u = 2 + l,/(Z, + 1,)) tends to a value of - 2.73, close to the limiting values found for thick Sm films on Pd(100) [23] and Cu(100) [24,26]. Our angle-resolved data clearly indicate that the divalent Sm was surface localised, suggesting that the mixed valency observed at 2 2 ML was heterogeneous rather than homogeneous. Faldt et al. [23] suggested that formation of trivalent Sm was favoured on substrates with a high density of states at the Fermi edge (N(E,)). It was proposed that AXE,) was significant because it determines the width of the adsorbate resonance and may therefore control the degree
Science 301 (1994)
39-51
47
of charge transfer from the adsorbate to substrate: increased charge transfer was thought to favour the trivalent configuration. Our results do not accord with the above view when considered alongside earlier findings [5,6] for the closely related system Cu(lll)/ Sm. Application of the Helmholtz equation to the low coverage data yields values of 0.1 e-/ Sm and 0.05 e-/Sm for Cu(ll1) [5] and NXlll), respectively, but at low coverages Sm is diualent on Cu(ll1) [6]. Even allowing for the approximate nature of the Helmholtz treatment, the difference between the two cases seems large enough to contradict the notion that increased charge transfer favours the trivalent state of Sm. An alternative explanation may be sought by analogy with the behaviour of pure bulk Sm. Johansson [27] has convincingly argued that in the case of certain rare earths, the trivalent configuration is stabilised for atoms in the bulk because the associated gain in cohesive energy outweighs the relatively modest f + d promotion energy required for formation of the trivalent state. At the surface, loss of coordination tips the balance the other way and the divalent state is stabilised. Extending this argument to the case of Sm chemisorption, one might therefore expect increased bonding interaction between adatoms and substrate to stabilise the trivalent state. In the case of NKlll) the bonding overlap with Sm is considerably greater than for Cu( 11 I) because the 3d band is involved in the former case. A stronger chemisorption bond is therefore formed on Ni(ll1) compared to Cu(lll), stabilising the trivalent configuration in the former case. That is, the key factor which determines Sm valency is the strength of the bonding interaction rather than the degree of charge transfer which accompanies it. This is determined by the extent of hybridization between the adsorbate and substrate valence levels, thus explaining why the trivalent Sm state is generally favoured on substrates with a high density of states at the Fermi edge. The preceding simple argument appears plausible with respect to the Cu/Ni comparison where the difference in N(E,) is very large; it is also consistent with other reported work on group IB and group VII metals. However, it is too crude to rationalise the behaviour of Sm on the (110) faces of Cr and
G.M. Roe et (11./Surfuce Sciencr 301 (1094) 39-51
48
ratios of these parameters to the length of the Ni(l11) unit cell vector. The match between the various bulk structures and surface periodicities determined by LEED are obvious, suggesting that thermal treatment of Sm films results in the formation of surface alloys, sometimes accompanied by Sm overlayers. The (2v% X 2v%)R30” structure is most likely due to a surface alloy with the Ni,,Sm, hexagonal structure oriented with the (001) plane parallel to the Ni(ll1) surface. This is the most important surface phase in that it is the final structure formed at 2 X00 K, regardless of initial conditions. It is also the first to appear (in this case without accompanying (1.5 X 1.5) beams). With regard to the (2 X 2) phase, it is important to note that the structure of Ni,,Sm, is closely related to that of Ni,Sm: both alloys consist of mixed Ni/ Sm layers interleaved with pure Ni layers, and the (2 X 2) surface alloy is explicable in terms of the Ni,Sm structure. However, this (2 X 2) phase may also be rationalised in terms of (111) planes of the alloy Ni,Sm or (001) planes of Ni,Sm - both of which are structurally quite different from Ni ,,Sm ?. Thus crystallographic relationships favour the first interpretation ((2 x 2) = Ni,Sm) while the AR data (Fig. 4) indicate a stoichiome-
MO (trivalent and divalent, respectively.) Here, the N(E,) values are very similar and additional complicating electronic effects arise in the case of Cr, as discussed by Dubot et al. [281. 4.2. High temperature behaviour Annealing Sm overlayers gave rise to a range of ordered surface phases, the appearance of any given phase being determined by the initial Sm loading and degree of subsequent thermal treatment. The accompanying decrease in AR which upon annealing may have resulted from: (i) aggregation of Sm containing species into crystallites; (ii) interdiffusion of Sm and Ni (iii) desorption of Sm. The third possibility may be ruled out on the basis of TDS control experiments which showed no detectable desorption of Sm at temperatures up to * 1000 K. The drop in AR may therefore be attributed to interdiffusion and/ or aggregation, the following analysis indicating that the first of these effects was of greatest importance at lower coverages, the second becoming significant at higher coverages. The unit cell dimensions of crystalline Ni,,Sm?, Ni,Sm, Ni,Sm, Ni,Sm, NiSm, Ni and Sm are given in Table 1 which also shows the
Sm/ Ni intermetallic Bulk phase
Ni,,Sm,
phases
and correspondence
with observed
Structural
Unit cell
description
dimensions
Hexagonal
a = X.65
surface
structures a,/~,,
L&J
Niflll)
Refs.
+ periodicity 3.47 + (26
x 26,
[2Yl
c=8 Ni,Sm Ni,Sm
Ni,Sm NiSm (Y Sm /3 Sm ( > 734°C) rSm(>4GPa)
Hexagonal CaCu, type Hexagonal Ni,Pu Cubic MgCuz type
a = 4.03 c = 3.79 a = 5.00 c = 24.59 a = 1.22 a,,(111)=5.11
Orthorhombic CrB type Hexagonal
1.08 * (2 x 2)
[30,31]
2.01 + (2 x 2)
[321
2.05 + (2 x 2)
[30.3 1,331
c = 4.26
1.71 --f tJS
[341
a = c= (I = c= a = c=
1.46 + 1.5 x I.5
3.629 26.202 3.663 5.8488 3.618 11.660
x Ji,
1.47 --) 1.5 x 1.5 1.45 + 1.5 x 1.5
[I51
G.M. Roe et al. /Surface
Fig. 11. Proposed surface structures. (A) (2X 2) surface based on Ni,Sm layer from Ni,Sm bulk structure. Large hatched circles: Sm, small open circles: Ni (substrate), filled circles: Ni (alloy film). (B) A 6 surface alloy based on the NiSm structure.
alloy cross small layer
try of Ni : Sm = 2 : 1 at the point where the (2 X 2) LEED pattern is at its most intense. Therefore the most probable interpretation is that the (2 X 2) phase consists of (OOl)-oriented Ni,Sm layers (Fig. 11A) derived from the Ni,Sm bulk structure. Analogous surface alloy structures have been invoked to explain the LEED results obtained for Sm/Cu(lll) [6], Nd/ Cu(ll1) [4] and Nd/ Cu(100) [19]. Finally, the c dimension of bulk NiSm closely matches the fia, vector of Ni(ll1) and a possible structure for the A-(J?; X fi)R30 phase based on NiSm is shown in Fig. 11B. The basal (0001) planes of (Y, p and y Sm all consist of hexagonally packed Sm atoms with lattice spacing is - 3.6 A, (i.e. - 1.5~ Ni(lll)). Thus the (1.5 X 1.5) and (1.5 X 1.5>R30° struc-
Science 301 (1994) 39-51
49
tures observed in association with the (2fi x 2fi)R30”, (2 X 2) and A-(& X &)R30“ alloy structures are probably due to pure Sm layers. The hcp Sm layer could overlay the surface alloys with which it coexists, or the two structures could have occurred in separate domains. In the 2fi case, the observation of diffraction beams corresponding to an apparent (6x6) periodicity in the direct lattice could arise from double diffraction between the 26 alloy and the (1.5 X 1.5) Sm overlayer. However, the same diffraction pattern would also result from coexisting domains of 26 surface alloy and (1.5 x 1.5) Sm overlayer, both in contact with Nitlll). We reject this latter possibility on the following grounds: (i> upon annealing a Sm overlayer, it is not obvious why some of it should form an alloy by interdiffusion of Sm and Ni while some does not; (ii) (1.5 X 1.5) Sm overlayers are not formed by direct deposition of Sm at 293 K. It therefore seems most likely that the Sm and alloy phases lie one on top of the other. In particular, based on angle-resolved photoemission data obtained for the analogous Cu(lll)/Sm system 151 we favour the view that the 26 surface alloy is capped with a Sm overlayer. More generally, we may suggest that both the type (v> and (vi> structures (A-(& X fi>R30° + (2 X 2) + (1.5 X 1.5>R300 + (1.5 X 1.5) and (20 x 2~%>R300 + (1.5 x 1.5)) may be associated with surface alloys capped by hcp Sm. The appearance of the Sm overlayers at temperatures above which Ni/Sm alloy phases had already formed presumably reflects strong surface segregation of the alloy component with lower surface energy: XPS results indicate that the Sm capping consisted of the trivalent form. The “phase diagram” (Fig. 6) implies that during Sm uptake at 800 K the Ni,,Sm, structure coexisted with (1.5 X 1.5) hcp Sm, and as already noted, the data indicate that the Ni,,Sm, surface alloy coexisted with a (capping) layer of hcp Sm. Given that the “coverage” corresponding to a single Ni,Sm, bimetallic layer of the alloy structure is 0.17, a satisfactory quantitative explanation of the uptake data is obtained by taking Ni,,Sm, as the predominant component: the break at 16 min (Fig. 1B) can be attributed to the formation of a surface alloy containing four com-
so
GM
Roe
et al. /Surface
plete layers of NisSm,. Island formation at Sm loadings > 2 ML presumably involved aggregation of the Ni,,Sm,, and both the high temperature A4 and crystal current results are consistent with this description. The maximum in crystal current at 1 ML initial Sm loading could thus be associated with the completion of about one unit cell depth of Ni,,Sm,; the levelling of A+ at > 1 ML may be similarly attributed. Finally, the weak feature at 1086.6 eV BE, which was observed only at very low coverages, remains a puzzle. It may be significant that a similar high binding energy feature was observed (but not discussed) in the case of Pd(lOO)/Sm 1231, whereas it did not occur for Cu(lll)/ Sm [51. This suggests that the presence of an unfilled substrate d-band is of significance for the formation of this minority species, but we cannot advance a plausible suggestion as to its identity: certainly, it has nothing to do with adventitious oxidation of Sm.
5. Conclusions
(1) At 293 K growth of Sm on Ni(ll1) proceeds in the Stranski-Krastanov mode: the contact layer consists of a low density fi Sm bilayer. Sm atoms in this bilayer are trivalent, surfacelocalised divalent Sm appearing in thicker films, therefore the mixed valency appears to be heterogeneous in character. (2) Depending on the initial Sm loading and subsequent temperature of heating, thermal treatment leads to formation of ordered Ni/Sm surface alloys whose structures are understandable in terms of known bulk phases. The most stable structure with 2fi periodicity is based on Ni,,Sm, and appears to be capped by a layer of Sm metal. (3) The different valence behaviour exhibited by Sm on NXlll) and Cu(lll) is understandable in terms of differing chemisorption bond strengths: stronger bonding stabilises the trivalent configuration relative to the divalent by offsetting the 4f --f 5d promotion energy.
Science
301
(1994)
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Acknowledgements G.M.R. acknowledges the award of a Barclays Scholarship by the Cambridge Commonwealth Trust. C.M.C.deC. thanks Coordenacao de Aperfeicoamento de Pessoal de Nivel Superior for the award of a Visiting Fellowship. We thank Mintcho Tikhov for helpful discussions.
References [l] P.A. Dowben, D. La Graffe and M. Onellion, J. Phys. (Condens. Matter) 1 (1989) 6571. [2] C. Schild, A. Wokaun and A. Baiker, Surf. Sci. 270 (1992) 520. [3] W.E. Wallace, J. France and A. Shamsi, in: Rare Earths in Modern Science and Technology, Vol. 3 (Plenum, New York, 1982). [4] R.M. Nix, R.W. Judd and R.M. Lambert, Surf Sci. 205 (1988) 59. [5] D.M. Jaffey, PhD Thesis, Cambridge University, 1987. [6] D.M. Jaffey, A.J. Gellman and R.M. Lambert, Surf. Sci. 214 (1989) 407. [7] E.A. Shaw, R.M. Ormerod and R.M. Lambert, Surf. Sci. 275 (1992) 157. [8] C. Laubschat, G. Kaindl, W.-D. Schneider, B. Reihl and N. Mirtensson, Phys. Rev. B 33 (1986) 6675. [9] Valence Instabilities, Eds. P. Wachter and H. Boppart (North-Holland, Amsterdam, 1982). [lo] G.E. Rhead, M.-G. Barth& and C. Argile, Thin Solid Films 82 (1981) 201. [II] C. Argile, M.-G. Barth&Labrousse and G.E. Rhead, Surf. Sci. 138 (1984) 181. [12] G.K. Wertheim and G. Crecelius, Phys. Rev. Lett. 40 (1978) 813. [13] M. Campagna, G.K. Wertheim and Y. Baer, in: Photo emission in Solids 11, Eds. L. Ley and M. Cardona (Springer, Berlin, 1979). [14] D.A. Shirley, Phys. Rev. B 5 (1972) 4709. [15] D.R. Lide, Ed, CRC Handbook of Chemistry and Physics (CRC Press, Boca Raton, 1990-1991X [16] B. Jorgensen, M. Christiansen and J. Onsgaard, Surf. Sci. 251/252 (1991) 519. 1171 F. Bertran. T. Gourieux, G. Krill, M. Alnot, J.J. Ehrhardt and W. Felsch, Surf. Sci. Lett. 245 (1991) L163. [18] J.N. Andersen, J. Onsgaard, A. Nilsson, B. Eriksson and N. Mirtensson, Surf. Sci. 202 (1988) 183. [19] R.M. Nix and R.M. Lambert, Surf. Sci. 186 (1987) 163. [20] R.M. Nix, R.W. Judd and R.M. Lambert. Surf. Sci. Lett. 215 (1989) L316. [Zl] M.W. Roberts and C.S. McKee, Chemistry of the MetalGas Interface (Clarendon, Oxford, 1978). [22] S.D. Parker and P.J. Dobson, Surf. Sci. 171 (1986) 267.
GM [23] [24] [2.5]
[26] [27] [28]
Roe
et al. /Surface
A. Fildt, D.K. Kristensson and H.P. Myers, Phys. Rev. B 37 (1988) 2682. A. Fgldt and H.P. Myers, Phys. Rev. B 30 (1984) 5481. A. Stenborg, 0. BjGrneholm, A. Nilsson, N. Mirtensson, J.N. Andersen and C. Wigren, Phys. Rev. B 40 (1989) 5916. A. F%ldt and H.P. Myers, J. Magn. Magn. Mat. 47/48 (1985) 225. B. Johansson, Phys. Rev. B 19 (1979) 6615. P. Dubot, E. Alleno, M.-G. Barth&s-Labrousse, C. Binns, C. Nicklin, C. Norris and D. Ravot, Surf. Sci. 282 (1993)
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(1994)
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51
[29] W.E. Wallace, Prog. Solid State Chem. 16 (1985) 127. [30] K. Nassau, L.V. Cherry and W.E. Wallace, J. Phys. Chem. Solids 16 (1960) 123. [31] W.B. Pearson, The Crystal Chemistry and Physics of Metals and Alloys (Wiley, New York, 1972). [32] H.M. Ondik and A.D. Mighell, Eds., Crystal Data, Determinative Tables, 3rd ed., Vol. 4, NSRDS and JCPDS (1978). [33] J.D.H. Donnay and G. Donnay, Eds., Crystal Data, Determination Tables, ACA Monograph, No. 5 (1963). [34] A.E. Dwight, R.A. Conner and J.W. Downey, Acta Cryst. 18 (1965) 837.
surface
science
....>...;:; :‘.“.“‘.“.~.‘.‘.‘.‘. ...::.>... ., .,.,. . ,,, ~:.:,.::,.:::::‘:::i::: ,...,........ .....,....: ,.) :.~ ,:::.:., ~::,, ., :::ji:;::.:::.:., :‘:.:“::::..:::::::.;.:,.,:.: ~,,.....: .“‘.‘.:.‘:.:::i:i~~li:~::.~::~:::~:~:~:~::~~~ ..:.‘.” Surface Science 301 (1994) 39-51
ELSEVIER
Structure and properties of samarium overlayers and Sm/Ni surface alloys on Ni( 111) Gerard
M. Roe ‘, Caio M.C. de Castilho
2, Richard
M. Lambert
*
Department of Chemistv, Unicersity of Cambridge, Lensfield Road, Cambridge CB2 lEU! UK (Received
4 July 1993; accepted
for publication
17 August
1993)
Abstract Development of the Ni(1 ll)/ Sm interface as a function of rare earth loading and substrate temperature has been studied by LEED, XPS, UPS, Aq5 and auxiliary measurements. At ambient temperature growth proceeds by the Stranski-Krastanov mechanism involving initial formation of a metastable low density, fi bilayer of Sm adatoms. Sm atoms in the bilayer are trivalent, divalent character appearing for thicker films; angle-resolved XP data suggest that the divalent Sm is surface-localised and therefore that this mixed valency is heterogeneous rather than homogeneous in nature. Thermal treatment results in the formation of a series of ordered surface alloys whose structures may be rationalised in terms of known Ni/Sm bulk alloys. The most important (most stable) of these surface alloys is based on the Ni,,Sm, structure and it appears to be capped by an overlayer of trivalent Sm.
1. Introduction The physics and chemistry of rare earth/ transition metal intermetallic systems are of interest with respect to bulk properties such as magnetism and hydrogen storage. Surface properties of such systems are also of considerable interest in connection with the formation of magnetic overlayers [l], and, in the case of alloy surfaces, with regard to catalytic behaviour [21. Although the catalytic chemistry of rare earth/
* Corresponding author. address: Comalco Research Centre, Thomastown, Melbourne, Victoria 3074, Australia. ’ Present address: Instituto de Fisica, UFBa Campus Universitsrio da Federacao, 40 210 Salvador, Bahia, Brazil.
’ Present
0039-6028/94/$07.00 0 1994 Elsevier XSDI 0039-6028(93)E0497-I
Science
transition metal alloys has received a great deal of investigation [3] there have been relatively few studies of the surface structure, composition and reactive properties of such systems under well defined conditions. Our earlier UHV/ single crystal work on rare earth/Cu systems indicates that such observations can provide important clues to the origin of the extraordinarily effective catalysts which can be produced from these materials 14-71. In the case of Sm, an additional interesting feature arises, namely the phenomenon of variable valency. Metallic Sm is trivalent in the bulk (Xe 4f5(5d6sj3), the gaseous atom is divalent (Xe4f6(5d6sj2), and the surface of trivalent metallic Sm exhibits divalent character. Intermetallic compounds of Sm often also exhibit divalent Sm in the surface layer, whatever the bulk electronic properties [8,9]. The present paper
B.V. All rights reserved
40
G.M. Roe rt ul. /Surface
deals with the Ni(ll l)/Sm system with particular reference to the nature of overlayer structures, the conditions necessary for their conversion to surface alloys, the structure and composition of such alloys, and the associated electronic properties. At ambient temperatures a metastable, low density bilayer of trivalent Sm forms on the Nit 111) surface; thicker layers contain surfacelocalised divalent Sm. Thermal treatment leads to the formation of a series of well ordered surface alloys, the most important of which (Ni,,Sm ,) appears to be capped by a layer of Sm atoms. It is of interest to compare the results reported here with earlier work on the closely related Cu(lll)/ Sm system [.5,6].
2. Experimental Experiments were carried out in two separate UHV chambers. Chamber A was a modified VG ADES 400 angle dispersive electron spectrometer system, incorporating LEED, AES, UPS and XPS facilities. Chamber B was used for LEED, AES and TDS experiments, and both chambers operated at a base pressure of - 1 X 1OW”’ Torr. The Ni(lll> single crystal wafer was cut from a 99.999% pure ingot (Metal Crystals Ltd) after orientation to within 0.5” of the (111) plane following mechanical polishing, a chromel/ alumel
1.2
thermocouple was spot welded to the back face of the specimen which was mounted on a liquid nitrogen cooled manipulator. Cleaning was achieved by Ar+ bombardment at room temperature, surface order being restored by annealing at - 900 K for 10 min. In both chambers, Sm was evaporated from collimated, shuttered sources which were heated by electron bombardment; during Sm deposition the increase in system pressure never exceeded 1 x lo-” Torr.
3. Results 3.1. Auger spectroscopy At deposition times ,< 8 min (Sm coverage < 1 ML, see below) the Sm transitions at 100 eV (N,,O,,N,,) and 129 eV (N,,N,,V) were observable, but the 138 eV (N,,VV) emission was below the detection limit; the latter only became apparent at - 1 ML coverage. For Sm coverages between 1 and - 2 ML, the 138 eV (NdsVV) transition became progressively stronger relative to the 100 eV (N,,O,,N,,) and 129 eV (N4sNh7V) transitions, while for coverages - 2 ML, the relative intensities of the three signals were constant. Typical low coverage and high coverage Sm spectra are shown in the inset to Fig. 1 which illustrates the variations in intensity of the SmNON
a
TJ
0.8
.s
.b 1.0 c/l
i
Kj 0.8
0.6
Q u
-0 a 0.6 .v, iT E 0.4
.$ 0.4 T!
E c&
b c
Science 301 (IYYJ) 39-51
0.2
0.2
deposition time/minutes 0.0
0
8
16
24
32
0.0
Fig. 1, Normalised Sm NON and Ni MVV intensities as a function of Sm deposition deviation +5%. Inset to (A) shows Auger spectra for 1.0 and 4.1 ML Sm films.
0
8
16
24
32
time at 293 K (A) and 800 K (B). Standard
GM Roe 81al. /Surface Science 301 (1994139-51
0
8
16
24
32
40
deposition time/minutes Fig. 2. Auger ratios (AR) for Sm uptake at 293 and 800 K.
eV) and NiMVV (60 eV) transitions as a function of Sm deposition time at 293 and 800 K. Signal intensities may also be expressed in terms of the ratio of Sm-to-Ni peak heights (Auger ratio: AR), 3 quantity which is independent of incident beam current. Variations in the AR as a function of Sm coverage and deposition time at 293 and 800 K are shown in Fig. 2. At 293 K the Sm intensi~ exhibits linear growth with a clear break at 16 min deposition time; beyond this point the graph is slightly curved. No clear breaks are evident in the corresponding Ni data. However, at 800 K, clear breaks occur at 16 min deposition time in both the Sm and the Ni data; the principal differences between the 293 and 800 K uptake curves emerge at deposition times > 16 min. The AR plot (Fig. 2) highIights (100
31
these differences, as reflected by the divergence of the 293 and 800 K data at 16 min deposition time. The Auger data for deposition at 293 and 800 K are characteristic of Stranski-~astanov growth: at 800 K, island formation occurs beyond 16 min deposition, whereas at 293 K simultaneous multilayer growth ensues at this point 1103. The LEED, workfunction (A+) and TDS results presented below are consistent with formation of 3 Sm bilayer structure at 293 K, completion of this bilayer occurring at 16 min deposition. (This also provides the calibration point which is used to obtain the quoted Sm coverages.) The sample current to ground measured during Auger spectroscopy experiments can provide useful information about the structural evolution of systems of this type [II], and Figs. 3A and 3B show the results obtained at 293 and 800 K. In both cases the sample current initially rose to a m~imum at the monohyer point (8 m); at 293 K, it then fell continuously, whereas at 800 K it fell and then levelled out after 16 min deposition. Note that this behaviour resembles but is not identical to that reported for Sm/Cu(lll); in that case a change in slope was observed at the monolayer point (see Section 4). Variations in the AR with temperature for a series of different initial Sm loadings are shown in Fig. 4; experiments were performed by depositing Sm at 293 K followed by annealing for 2 min at the temperatures specified. At Sm coverages
B
. A,
deposition time/minutes
deposition time/minutes
5.6 0
8
16
24
32
8
76
24
Fig. 3. Variation in crystal current with Sm deposition time at 293 K(A) and HO0K(B).
32
G.M.Roe etal./ Surface Science 301 (1994)39-53
42
G 2 ML no significant changes occurred, whereas for higher coverage films (2.2, 3.5, 4.3 ML) annealing caused an AR decrease to a limiting value between 1.6 and 1.8 at 800 K. Note that for 4.3 and 3.5 ML films a distinct plateau appears between 600 and 700 K. The ARs observed after thermally treating Sm films of a given coverage were similar, whether the overlayer was deposited at a particular elevated temperature, or deposited at 203 K and subsequently annealed to that temperature (compare Figs. 2 and 4).
293 K, indicating that the associated surface structures were not the same. The 6 structure resulting from annealing is henceforth designated as A-(& x v%)R30” in order to distinguish it from the 293 K v% structure. The 1.5 X 1.5 and (1.5 X l.5>R30° patterns occurred only in combination with other structures, and the A-t&? X fi>R30” and (2 X 2) patterns were often found to coexist. The various phases appearing, either separately or in coexistence with other structures, are summarised below, and representative diffraction patterns are shown in Fig. 5.
3.2. LEED (i)
The only overlayer LEED pattern observed during Sm deposition at 293 K corresponded to a tfi X fi)R30” surface structure (Fig. 5). This pattern appeared at a coverage of - 0.6 ML, reached its maximum intensity at - 1.5 ML, and then progressively faded. By - 3.5 ML the pattern had disappeared, although diffuse (1 X 1) beams were still risible; the latter were extinguished at Sm coverages - 4.5 ML. A variety of LEED patterns were seen upon subsequent thermal treatment of 293 K deposited films, corresponding to (26 X 2v%)R30°, tv% X fi)R30”, (2 X 21,(1.5 1.5) and (1.5 X 1.5)R30° periodicities. The intensity versus beam voltage characteristics of this second tfi X &)R30” phase were entirely different to those of the v% structure observed at
(Zfi~?fi)R30",
(iv)
A-(&
Xfi)R.?O"
+(2x2). (ii)
A-(6
x\/i)R30",
(v)
A-(fiXfi)R30 +(2x2)+(1.5x1.5) +
(iii)
(2X2),
(vi)
(26
( 1.5 x I .S)RW. x2fi)R30
+(1.5x1.5).
Occurrence of these surface structures was dependent on Sm pre-coverage and subsequent annealing temperature; results are summarised in Fig. 6, LEED observations being performed at each of the points indicated after heating to the relevant temperature for 2 min. It is apparent that the (26 x 2fi)R30” structure alone occurred only at coverages G 2 ML; above 2 ML, the A-(v% X v%)R30” or (2 X 2) phases resulted from heating at 500 K, the latter predominating at higher coverages (up to - 4.3 ML). For higher temperatures (600 and 700 K) ( 1.5 X 1.5)R30° and (1.5 X 1.5) patterns were associated with the ACJ? X &)R30” and (2 X 2) structures, while for temperatures 2 X00 K only the coexisting (2fi X 2v%)R30”) + (1.5 x 1.5) phases were produced, regardless of coverage. 3.3. Photoemission
a
0.5
0
c -
q
300
500
700
900
temperature/K Fig. 4. Dependence were
deposited
temperatures
of AR
on temperature.
at 293 K and
specified.
annealed
for
Sm overlayera 2 min
at the
3.3.1. As-deposited Sm orlerlayers: XPS Fig. 7 shows a series of representative Sm 3d,,: XP spectra as a function of Sm coverage following deposition at 293 K, quoted coverages are calibrated with respect to the Auger spectroscopy data, as noted above. The high coverage spectra are typical of those exhibited by bulk Sm, showing transitions due to both divalent Sm (SmtII), BE
GM
Roe et al. /Surface
= 1075.9 eV) and trivalent Sm (Sm(III), BE = 1083.6 eV): the lower BE associated with Sm(I1) could arise as a result of either initial state effects (higher inter-electron repulsion), final state effects (increased screening of core hole), or both. Both the divalent and trivalent emissions are broad due to their multiplet structure which arises as a result of 3d-4f coupling [12,13]. At low coverages, only the characteristic Sm(II1) emis-
Science 301 (1994) 39-51
43
sion was detectable: Sm(I1) emission did not uppear until a threshold corjerage Lsalue of two Sm monolayers had been exceeded. This behaviour of the Sm(I1) and Sm(II1) intensities is illustrated in Fig. 8A for deposition experiments carried out at 150 K; essentially similar results were obtained at 298 K. No coverage-dependent binding energy shifts were apparent. Measurements carried out as a function of photoelectron exit angle for 6.5
Fig. 5. LEED patterns. (A) (v% X fi)R30” structure, 1.8 ML Sm at 293 K, 105 eV. (B) 2(&f x fi)R30” structure, 1.4 ML Sm. 61h K anneal, 110 eV. (C) (2 x 2) structure, 4.4 ML Sm, 611 K anneal, 120 eV. (D) 2(& X &)R30” + (1.5 X 1.5) structure, 4.4 ML Sm. 796 K anneal. 100 eV.
G. M. Roe et ul. /
SwfuceScience 301 (1994) 39-51
1000
800 & L g 600 s
E s 400
u.0
1 .o
2.0
3.0
coverage
4.0
5.0
(ML)
Fig. 6. Temperature-initial Sm loading diagram tence regimes for the various structures.
showing
exis10717 10762 lOB07 IO852 10897
Binding Energy/eV ML Sm films showed that the Sm(II) emission is enhanced at grazing exit angles, indicating that the divalent Sm atoms are surface localised. Fig. 8B summarises the divalent, trivalent and (II + III) integrated intensities over a wide range of coverage. Here, as in all other cases, the data were treated by subtracting a Shirley-type background [14] and normalized with respect to the intensity. In passing, we clean surface Ni 2p,,, note that at very low coverages (5 0.07 ML) an unusual weak Sm-derived feature appeared at a BE of 1086.6 eV.
Fig. 7. Sm 3d 5,2 spectra as a function K. Divalent (II). trivalent (III) and indicated.
of Sm deposition at 293 1086.6 eV BE features
3.3.2. Annealed films: XPS The intensity variation of the Sm(II + III) emission for a series of different Sm loadings as a function of annealing temperature is shown in Fig. 9 which also presents spectra illustrating decay of the divalent state. For initial Sm covcrages _<2 ML the Sm and Ni signals were essentially invariant over the entire temperature range,
III
(ML)
coverage
0.08 0 Fig. X. Divalent,
trivalent
and total Sm 3d,,,,
intensities
as a function
’ 5
of Sm deposition
IO
15
at IS0 K(A)
20
25
30
and 293 K CR)
G.M. Roeet
al./
Surface Science 301 (1994) 39-51
45
whereas at - 5.8 ML the Sm intensity decreased for annealing temperatures > 400 K. For higher coverage films, the Ni signal increased markedly for temperatures 2 500 K. The Sm(I1 + III) intensities for 2.8 and 5.8 ML films tended rapidly to zero for temperatures in excess of 1000 K while at the same time the Ni signals returned to the clean surface value: this behaviour is entirely consistent with the Auger data reported above. Again, thermal treatment led to no detectable BE . shifts m the Sm3d,,, and Ni 2p,,, spectra.
0.1 ML t ,
0.00 200
400
f ‘. 600
-, ? 800
temperature
f 1000
.1200
(K)
Sm 3d5/2 XPS
B,
-3.0’
-I
0
1
2
3
4
5
Fig. 10. (A) He1 UP spectra of clean Ni (a) and after deposition of 1.8 ML Sm at 293 K (b) and 800 K Cc). (B) AC#Jas a function of Sm deposition at 293 and 800 K.
3.3.3. As-deposited
and annealed
films:
UPS, AI$,
TPD
1071.7 1076 2 1080 7 1085.2 IOIN 7
Binding energy/e\/ Fig. 9. (A) Total Sm3dS,, intensity as a function of annealing temperature for a series of different initial coverages. (B) Sm3dsj2 spectra for a 5.8 ML Sm film annealed at 293 K (a), 623 K (b), and 906 K (cl.
Normal emission He I spectra for 1.8 ML Sm films deposited at 298 and 800 K are shown in Fig. 10A; these spectra exemplify typical features observed in the UPS data. Sm deposition led to attenuation and broadening of the Nid-band emission accompanied by growth of an Sm-induced feature at BE = 5 eV which may be assigned to 4f emission from a trivalent initial state [131. As might be expected, Sm deposition led to
46
G.M. Roe et ul. /Surface Science 301 (1994139-51
large decreases in work function and Fig. 10B shows A+ as a function of Sm coverage for depositions covered out at 293 and 800 K. At 293 K, A$ reached a limiting value of -2.5 eV after deposition of - 2.5 ML Sm, which is in agreement with the work function difference between Ni(ll1) and Sm metal [15]. The initial work function decrease was more rapid at 800 K, yielding a limiting value of A4 = -2.0 eV at 1 ML Sm. Thermal treatment of the 4.6 ML film caused a work function increase of 0.5 eV, which corresponds to the difference in limiting values between films deposited at 293 and at 800 K. Finally it is important to note that at 293 K, deposition of 1 ML Sm led to complete suppression of the Hz TPD feature associated with clean Ni(l11). This provides strong confirmation of monolayer calibration point (8 min deposition).
4. Discussion 4.1. Room temperature (293 K) behaviour The crystal current and H, TPD data clearly indicate that an important stage in the structural evolution of the Sm/ Ni(ll1) interface was reached at 8 m deposition time. In particular, the complete quenching of H, desorption from Ni sites provides strong evidence that the contact layer is completed at 8 min deposition time. Furthermore, the appearance threshold of the Sm NVV transition at this point strongly implies the onset of second layer formation. The 293 K Sm Auger data and the 150 K XPS data indicate termination of a second important stage at 16 min, as film growth switched from layer-by-layer mode to simultaneous multilayer mode. Similar Stranski-Krastanov behaviour has been observed for Sm/Cu(lll) [6,16], Eu/Pd(lll) [171 and Yb/ Ni(100) [18]. Appearance of the (6 x J?;)R30” LEED pattern correlated with near completion of the first monolayer (8 min), which we associate with a fractional surface coverage of l/3 with respect to the Ni(lll) substrate, i.e. the adatom density in the first Sm overlayer (1 ML) is 6.2 x 10” m-*. The LEED results also show that further Sm adsorption occurred in registry with
this contact layer, yielding a bilayer of overall fi symmetry. This bilayer was completed at - 16 min deposition and corresponds to a relatively - 33% expanded with respect to open structure, bulk Sm metal. The relatively low attenuation of the Ni 60 eV signal at 1 ML is understandable in terms of the low density \/;5 monolayer. Correspondingly, the absence of a clear break in the Sm NON Auger intensity at 1 ML may reflect the fact that Sm atoms in the contact layer are not fully shadowed by second layer Sm atoms. (The contribution from NiMVV intensity at 100 eV would also have tended to obscure the Sm break point at 1 ML.) The possibility that Ni/Sm intermixing occurs at 293 K may be excluded on the basis of the essentially identical uptake behaviour observed at low temperatures (e.g. 220 K, Fig. 8B). Furthermore, in annealing experiments, no change in AR occurred until reaching a temperature of 400 K. Low density rare earth overlayer structures similar to that postulated here have also been reported for Nd/Cu(lll) [4], Nd/ Cu(lO0) [19,201, Sm/Cu(lll) [6,16], Sm/Cu(llO) [16], Sm/Cu(lOO) [16] and Eu/Pd(lll) [17]. The Sm/ Cu(ll1) system is ostensibly most closely related to the present case, but there are significant structural differences [6]. On Cu(l 11) a low density contact layer corresponding to 0 = 0.36 -t 0.03 is also formed (cf. 0 = 0.33 on Ni(ll1)) but with a quite different structure: a ((2,-i) mesh incorporating additional adatoms. This is followed by a fi second layer, as in the present case. On Ni(ll1) beyond 2 ML the Auger data suggest three-dimensional growth, at which point LEED observations indicate progressive collapse of the metastable bilayer with eventual extinction of the fi beams leaving only a diffuse (1 X 1) pattern. In this respect, the behaviour of Sm/ Cm11 1) is very similar [61. The work function decrease with increasing Sm coverage is characteristic of an electropositive adsorbate on a high work function substrate: note that A4 did not level off until bilayer completion. A value for the low coverage surface dipole of 0.6 D per adatom can be calculated by means of the Helmholtz equation [21], implying charge transfer of 0.05 electrons per Sm atom - it may be that repulsive interactions between the electron defi-
G.M. Roe et al. /Surface
cient Sm atoms contribute to stabilisation of the low density bilayer. The evolution of the 100 eV SmNON, 127 eV SmNNV and 129 eV SmNVV relative intensities with increasing Sm coverage can be rationalised in terms of progressively decreasing charge transfer from Sm valence levels. Thus at low coverages, a significant degree of charge transfer should not affect the NON transition but would decrease the probability of NNV transitions; NVV transitions would be even more attenuated. The observed behaviour of all three transitions is entirely in line with this argument. The rise in crystal current to a maximum at - 8 min (- 1 ML) might be anticipated as decreasing 4 enhances the escape of secondary electrons. The subsequent decrease may be attributed to effects determined by surface structure [121; changes in the density of states above E, [22] would also be expected to contribute to this behaviour. For the above reasons, and because of different experimental geometries (40” versus 75” electron incidence angles) it is not unexpected that the present crystal current data are not identical to those reported for Sm/ Cu(ll1) [6]. Sm 3d,,, spectra show that below 2 ML the Sm overlayer appeared to be entirely trivalent (see Fig. 91, the divalent component appearing only at > 2 ML. (The Sm(II1) metallic state is not to be confused with partially ionic Sm, resulting, for example from oxidation; the latter has a binding energy - 1 eV higher than Sm(III).) Similar trivalency exhibited by isolated Sm atoms on Pd(100) [23] has been reported, whereas on Cu [5,24] and MO [25], the Sm adatoms were initially divalent. At high coverages the effective valency CC; u = 2 + l,/(Z, + 1,)) tends to a value of - 2.73, close to the limiting values found for thick Sm films on Pd(100) [23] and Cu(100) [24,26]. Our angle-resolved data clearly indicate that the divalent Sm was surface localised, suggesting that the mixed valency observed at 2 2 ML was heterogeneous rather than homogeneous. Faldt et al. [23] suggested that formation of trivalent Sm was favoured on substrates with a high density of states at the Fermi edge (N(E,)). It was proposed that AXE,) was significant because it determines the width of the adsorbate resonance and may therefore control the degree
Science 301 (1994)
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of charge transfer from the adsorbate to substrate: increased charge transfer was thought to favour the trivalent configuration. Our results do not accord with the above view when considered alongside earlier findings [5,6] for the closely related system Cu(lll)/ Sm. Application of the Helmholtz equation to the low coverage data yields values of 0.1 e-/ Sm and 0.05 e-/Sm for Cu(ll1) [5] and NXlll), respectively, but at low coverages Sm is diualent on Cu(ll1) [6]. Even allowing for the approximate nature of the Helmholtz treatment, the difference between the two cases seems large enough to contradict the notion that increased charge transfer favours the trivalent state of Sm. An alternative explanation may be sought by analogy with the behaviour of pure bulk Sm. Johansson [27] has convincingly argued that in the case of certain rare earths, the trivalent configuration is stabilised for atoms in the bulk because the associated gain in cohesive energy outweighs the relatively modest f + d promotion energy required for formation of the trivalent state. At the surface, loss of coordination tips the balance the other way and the divalent state is stabilised. Extending this argument to the case of Sm chemisorption, one might therefore expect increased bonding interaction between adatoms and substrate to stabilise the trivalent state. In the case of NKlll) the bonding overlap with Sm is considerably greater than for Cu( 11 I) because the 3d band is involved in the former case. A stronger chemisorption bond is therefore formed on Ni(ll1) compared to Cu(lll), stabilising the trivalent configuration in the former case. That is, the key factor which determines Sm valency is the strength of the bonding interaction rather than the degree of charge transfer which accompanies it. This is determined by the extent of hybridization between the adsorbate and substrate valence levels, thus explaining why the trivalent Sm state is generally favoured on substrates with a high density of states at the Fermi edge. The preceding simple argument appears plausible with respect to the Cu/Ni comparison where the difference in N(E,) is very large; it is also consistent with other reported work on group IB and group VII metals. However, it is too crude to rationalise the behaviour of Sm on the (110) faces of Cr and
G.M. Roe et (11./Surfuce Sciencr 301 (1094) 39-51
48
ratios of these parameters to the length of the Ni(l11) unit cell vector. The match between the various bulk structures and surface periodicities determined by LEED are obvious, suggesting that thermal treatment of Sm films results in the formation of surface alloys, sometimes accompanied by Sm overlayers. The (2v% X 2v%)R30” structure is most likely due to a surface alloy with the Ni,,Sm, hexagonal structure oriented with the (001) plane parallel to the Ni(ll1) surface. This is the most important surface phase in that it is the final structure formed at 2 X00 K, regardless of initial conditions. It is also the first to appear (in this case without accompanying (1.5 X 1.5) beams). With regard to the (2 X 2) phase, it is important to note that the structure of Ni,,Sm, is closely related to that of Ni,Sm: both alloys consist of mixed Ni/ Sm layers interleaved with pure Ni layers, and the (2 X 2) surface alloy is explicable in terms of the Ni,Sm structure. However, this (2 X 2) phase may also be rationalised in terms of (111) planes of the alloy Ni,Sm or (001) planes of Ni,Sm - both of which are structurally quite different from Ni ,,Sm ?. Thus crystallographic relationships favour the first interpretation ((2 x 2) = Ni,Sm) while the AR data (Fig. 4) indicate a stoichiome-
MO (trivalent and divalent, respectively.) Here, the N(E,) values are very similar and additional complicating electronic effects arise in the case of Cr, as discussed by Dubot et al. [281. 4.2. High temperature behaviour Annealing Sm overlayers gave rise to a range of ordered surface phases, the appearance of any given phase being determined by the initial Sm loading and degree of subsequent thermal treatment. The accompanying decrease in AR which upon annealing may have resulted from: (i) aggregation of Sm containing species into crystallites; (ii) interdiffusion of Sm and Ni (iii) desorption of Sm. The third possibility may be ruled out on the basis of TDS control experiments which showed no detectable desorption of Sm at temperatures up to * 1000 K. The drop in AR may therefore be attributed to interdiffusion and/ or aggregation, the following analysis indicating that the first of these effects was of greatest importance at lower coverages, the second becoming significant at higher coverages. The unit cell dimensions of crystalline Ni,,Sm?, Ni,Sm, Ni,Sm, Ni,Sm, NiSm, Ni and Sm are given in Table 1 which also shows the
Sm/ Ni intermetallic Bulk phase
Ni,,Sm,
phases
and correspondence
with observed
Structural
Unit cell
description
dimensions
Hexagonal
a = X.65
surface
structures a,/~,,
L&J
Niflll)
Refs.
+ periodicity 3.47 + (26
x 26,
[2Yl
c=8 Ni,Sm Ni,Sm
Ni,Sm NiSm (Y Sm /3 Sm ( > 734°C) rSm(>4GPa)
Hexagonal CaCu, type Hexagonal Ni,Pu Cubic MgCuz type
a = 4.03 c = 3.79 a = 5.00 c = 24.59 a = 1.22 a,,(111)=5.11
Orthorhombic CrB type Hexagonal
1.08 * (2 x 2)
[30,31]
2.01 + (2 x 2)
[321
2.05 + (2 x 2)
[30.3 1,331
c = 4.26
1.71 --f tJS
[341
a = c= (I = c= a = c=
1.46 + 1.5 x I.5
3.629 26.202 3.663 5.8488 3.618 11.660
x Ji,
1.47 --) 1.5 x 1.5 1.45 + 1.5 x 1.5
[I51
G.M. Roe et al. /Surface
Fig. 11. Proposed surface structures. (A) (2X 2) surface based on Ni,Sm layer from Ni,Sm bulk structure. Large hatched circles: Sm, small open circles: Ni (substrate), filled circles: Ni (alloy film). (B) A 6 surface alloy based on the NiSm structure.
alloy cross small layer
try of Ni : Sm = 2 : 1 at the point where the (2 X 2) LEED pattern is at its most intense. Therefore the most probable interpretation is that the (2 X 2) phase consists of (OOl)-oriented Ni,Sm layers (Fig. 11A) derived from the Ni,Sm bulk structure. Analogous surface alloy structures have been invoked to explain the LEED results obtained for Sm/Cu(lll) [6], Nd/ Cu(ll1) [4] and Nd/ Cu(100) [19]. Finally, the c dimension of bulk NiSm closely matches the fia, vector of Ni(ll1) and a possible structure for the A-(J?; X fi)R30 phase based on NiSm is shown in Fig. 11B. The basal (0001) planes of (Y, p and y Sm all consist of hexagonally packed Sm atoms with lattice spacing is - 3.6 A, (i.e. - 1.5~ Ni(lll)). Thus the (1.5 X 1.5) and (1.5 X 1.5>R30° struc-
Science 301 (1994) 39-51
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tures observed in association with the (2fi x 2fi)R30”, (2 X 2) and A-(& X &)R30“ alloy structures are probably due to pure Sm layers. The hcp Sm layer could overlay the surface alloys with which it coexists, or the two structures could have occurred in separate domains. In the 2fi case, the observation of diffraction beams corresponding to an apparent (6x6) periodicity in the direct lattice could arise from double diffraction between the 26 alloy and the (1.5 X 1.5) Sm overlayer. However, the same diffraction pattern would also result from coexisting domains of 26 surface alloy and (1.5 x 1.5) Sm overlayer, both in contact with Nitlll). We reject this latter possibility on the following grounds: (i> upon annealing a Sm overlayer, it is not obvious why some of it should form an alloy by interdiffusion of Sm and Ni while some does not; (ii) (1.5 X 1.5) Sm overlayers are not formed by direct deposition of Sm at 293 K. It therefore seems most likely that the Sm and alloy phases lie one on top of the other. In particular, based on angle-resolved photoemission data obtained for the analogous Cu(lll)/Sm system 151 we favour the view that the 26 surface alloy is capped with a Sm overlayer. More generally, we may suggest that both the type (v> and (vi> structures (A-(& X fi>R30° + (2 X 2) + (1.5 X 1.5>R300 + (1.5 X 1.5) and (20 x 2~%>R300 + (1.5 x 1.5)) may be associated with surface alloys capped by hcp Sm. The appearance of the Sm overlayers at temperatures above which Ni/Sm alloy phases had already formed presumably reflects strong surface segregation of the alloy component with lower surface energy: XPS results indicate that the Sm capping consisted of the trivalent form. The “phase diagram” (Fig. 6) implies that during Sm uptake at 800 K the Ni,,Sm, structure coexisted with (1.5 X 1.5) hcp Sm, and as already noted, the data indicate that the Ni,,Sm, surface alloy coexisted with a (capping) layer of hcp Sm. Given that the “coverage” corresponding to a single Ni,Sm, bimetallic layer of the alloy structure is 0.17, a satisfactory quantitative explanation of the uptake data is obtained by taking Ni,,Sm, as the predominant component: the break at 16 min (Fig. 1B) can be attributed to the formation of a surface alloy containing four com-
so
GM
Roe
et al. /Surface
plete layers of NisSm,. Island formation at Sm loadings > 2 ML presumably involved aggregation of the Ni,,Sm,, and both the high temperature A4 and crystal current results are consistent with this description. The maximum in crystal current at 1 ML initial Sm loading could thus be associated with the completion of about one unit cell depth of Ni,,Sm,; the levelling of A+ at > 1 ML may be similarly attributed. Finally, the weak feature at 1086.6 eV BE, which was observed only at very low coverages, remains a puzzle. It may be significant that a similar high binding energy feature was observed (but not discussed) in the case of Pd(lOO)/Sm 1231, whereas it did not occur for Cu(lll)/ Sm [51. This suggests that the presence of an unfilled substrate d-band is of significance for the formation of this minority species, but we cannot advance a plausible suggestion as to its identity: certainly, it has nothing to do with adventitious oxidation of Sm.
5. Conclusions
(1) At 293 K growth of Sm on Ni(ll1) proceeds in the Stranski-Krastanov mode: the contact layer consists of a low density fi Sm bilayer. Sm atoms in this bilayer are trivalent, surfacelocalised divalent Sm appearing in thicker films, therefore the mixed valency appears to be heterogeneous in character. (2) Depending on the initial Sm loading and subsequent temperature of heating, thermal treatment leads to formation of ordered Ni/Sm surface alloys whose structures are understandable in terms of known bulk phases. The most stable structure with 2fi periodicity is based on Ni,,Sm, and appears to be capped by a layer of Sm metal. (3) The different valence behaviour exhibited by Sm on NXlll) and Cu(lll) is understandable in terms of differing chemisorption bond strengths: stronger bonding stabilises the trivalent configuration relative to the divalent by offsetting the 4f --f 5d promotion energy.
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Acknowledgements G.M.R. acknowledges the award of a Barclays Scholarship by the Cambridge Commonwealth Trust. C.M.C.deC. thanks Coordenacao de Aperfeicoamento de Pessoal de Nivel Superior for the award of a Visiting Fellowship. We thank Mintcho Tikhov for helpful discussions.
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