Cesium and oxygen adsorption on NiO(100)

312

Surface

Cesium and oxygen adsorption S. Kennou, M. Kamaratos Department

Received

1991; accepted

on NiO(lOO)

and C.A. Papageorgopoulos

of Physrcs, University of loannina,

11 February

Science 256 (1991) 312-316 North-Holland

P.O. Box 1186, GR-451 10 loannina,

for publication

Greece

2 May 1991

The interaction of Cs and 0, on NiO(100) has been studied by AES, TDS and WF measurements at room temperature. A correlation of the experimental results shows that Cs is initially adsorbed as ionized adatoms which interact strongly with the oxygen of the outmost layer of NiO. The initial dipole moment of Cs on NiO is 8 D, while the sticking coefficient found to be - 0.7. With increasing coverage Cs remains disordered and forms islands on the surface. Oxygen adsorption on Cs-saturated NiO takes place on top of Cs. The deposited oxygen interacts with Cs and increases the binding energy of the latter.

of Cs and 0, interaction on NiO(100) and their comparison with metals and semiconductors.

1. Introduction A lot of studies has been reported in the last years on the interaction of alkali and oxygen on metals and semiconductors [l-5]. Alkali on metals tend to form uniform metallic overlayers, whereas the behavior of alkali on semiconductors depends on the nature of the substrate. It has been found that, upon adsorption of 0, on Cs saturated metallic and semiconducting surfaces, oxygen is initially bound to the alkali-substrate interface with simultaneous increase of the Cs binding energy. While, with increasing exposure, oxygen is adsorbed on the top of Cs layer. Recent studies of Cs on some layer compounds and oxidized Ni(100) ad W(110) surfaces suggest that Cs does not form uniform layers [4,5]. In a very recent publication of K adsorption on NiO(100) at room temperature it has been found that K forms 3D clusters [6]. The studies of the interaction of alkali with metal oxides are limited. The clarification of this interaction requires more research effort. In this work we report the results 0039-6028/91/$03.50

0 1991 - Elsevier Science Publishers

2. Experimental The experiments were performed in a UHV system equipped with AES, LEED, TDS, WF, and Cs flux facilities, described elsewhere [7]. A clean NiO(100) surface was prepared by annealing the sample up to - 1200 K. The sample heating is also described in ref. [7]. The clean NiO(100) exhibit an Auger peak height ratio of the 0 (KL,L,) transition at 520 eV to Ni(L,M,,,M,,,) at 850 eV of - 1.8. This ratio was reproducible after each experimental run and heating. Cesium was deposited from a SAES-Getter source to various coverages up to saturation. While the background pressure was - 1 X lop lo Torr. The Cs flux was kept constant at - 3 X 10” atoms cm-* s-‘. The cesiated surfaces were exposed to increasing 0, doses of -5x10-’ Torr up to a maximum exposure of 10 L, which was always enough to reach saturation.

B.V. All rights reserved

S. Kennou et

al. / Cs and 0, ahorption

313

on NiO(lO0)

eV. Further increase of the exposure of did not cause any WF change. When Cs-saturated NiO surface is exposed increase of the WF by - 1.7 eV is also 7 ?!

2

I

o* 5L c---------•

,x- -:: I

3.2. Auger electron spectroscopy

f

l

I I I I

-2 -

/ IX

/

:

\ -3 -

0, (10 L) a nearly to 0, an observed.

x_._._.

0 Cs

-O---A

111 0

I

I

500

1000

DEPOSITION

TIME

15oc (s )

Fig. I. Work-function change versus Cs deposition time on NiO (continuous line) and oxygen exposure on the Cs-covered surface (dashed lines).

3. Results 3. I. Work-function

measurements

Fig. 1 shows the work-function (WF) change, A+, versus Cs deposition time (solid lines) and upon oxygen exposure on cesiated surfaces with different Cs coverages (dashed line). The behavior of the WF curve during Cs deposition on clean NiO is quite similar to that of K/NiO, that is, initially there is a fast decrease and then it reaches a constant value without passing through any minimum. The total decrease of the WF value after Cs adsorption on NiO is - 3.1 f 0.2 eV. In order to eliminate surface charging effects, which causes a relatively large uncertainty (+ 0.2 eV) in the final WF value, we heated the crystal to 200” C after each alkali dose and then the WF measurements were taken as the crystal was cooling down to - 100°C. We should note here that the annealing at 200°C does not cause any Cs desorption from the surface. When the NiO(100) is covered with Cs for 100 s and the system is exposed to 0, up to 5 L the WF increases by 0.5

Fig. 2 shows the variation of the Auger peakto-peak heights of Cs(563 eV), Ni(850 eV), Ni(61 ev) and O(520 eV) as a function of Cs deposition time on NiO at room temperature. Initially, the Cs peak increases almost linearly up to - 300 s, it levels off at 600 s and then continuous to increase slowly to saturation. During deposition, the decrease of the substrate peaks follows the growth of the Cs peak. Near saturation coverage, the peaks of Ni(850 eV) and O(520 eV) are attenuated by - 60%. The attenuation of the Ni(61 eV) is - 90%. Upon oxygen exposure on cesiated NiO with different Cs coverages (not shown here) the increase of the O(520 eV) at saturation is relatively small - 10%. Fig. 3 shows the energy variation of the O(520 eV) peak during Cs adsorption. Initially the oxygen peak is shifted to lower energies up to - 150 s of Cs deposition time and then it returns slowly to its initial position at saturation. This behavior is observed only for the oxygen peak.

l-Cs(563eV)Xtj

2 _ pb.&*A-A-A*&_A_A_A La_ A-~-b-&_r -.-.-. / I I 0 500 1000 Cs DEPOSITION TIME

-f 1500 ( s )

Fig. 2. Auger peak-to-peak height of Cs(563 eV), Ni(850 ev), Ni(61 eV) and O(520 eV) versus deposition time on NiO(lOO).

S. Kennou et al. / Cs and 0, nhorption on NiO(iO0)

314

(b)

cs (15OOsj

1 Fig. 3. Energy

I

I

I

1000 500 Cs DEPOSITION TIME (sf

0

shift of the O(520 eV) Auger deposition time.

1500

peak versus

Cs

3.3. Thermal ~e~or~tio~ spectroscopy In fig. 4 a series of thermal desorption spectra of Cs on NiO(100) is shown. The heating rate between 500 and 1200 K was - 15 K/s. As it is seen in this figure, Cs desorbs between 650 and 2150 K in three desorption states. The first one, which appears at low Cs coverages, corresponds to the main peak near 970 IL As Cs exposure in-

a00

1000

1200

T (K) Fig. 5. Thermal

desorption adsorption

curves of (a) O,, and (b) Cs after 0, on Cs-covered NiO.

creases above 150 s two new peaks appear at - 800 K and 900 K, respectively. Fig. 5 shows a series of thermal desorption spectra of (a) 0, and (b) Cs after oxygen adsorption on Cs-covered NiO(100) surfaces. As is seen in this figure, with small oxygen coverage a broad desorption peak of oxygen appears at - 1250 K. As oxygen exposure increases, two new peaks appear at - 750 and - 1000 K, respectively. A comparison of fig. 5 with fig. 4 suggest that, in the presence of oxygen, the high-energy TD peak of Cs is shifted to higher temperature by more than 150 K. It should be mentioned that the mass spectrometer never showed any desorption of Cs oxides such as Cs,O, CsO.

4. Discussion

1

Fig. 4. Thermal

700

900 T (K)

desorption

1100

curves of Cs on NiO.

In order to estimate the sticking coefficient and the absolute coverage of Cs on NiO(100) we compared the area under the thermal desorption spectra of Cs versus Cs deposition time on NiO and

S. Kennou et al. / Cs and 0, adsorption on NiO(100)

h

l

NiO(100)

x

Si (100) I,

60 /*

9 .A*

0

1000 TIME

500 DEPOSITION

Cs

1500 (s)

Fig. 6. Total area under the thermal desorption curve versus Cs deposition time.

on a Si(100) surface taken in the same chamber under exactly the same experimental conditions (fig. 6). From this comparison we found that the sticking coefficient of Cs on NiO is constant up to 600 s of Cs deposition time and close to the initial sticking coefficient of Cs on Si(lO0). It has been found that the initial sticking coefficient of Cs on Si(lO0) is about 0.7 [S]. Above this Cs coverage the sticking coefficient decreases suddenly to about one half of its initial value. This change is similar to that of the slope of the Cs Auger peak height curve (fig. 2). This value of the initial sticking coefficient (- 0.7) is consistent with measurements referred to Cs deposition on clean and oxidized Ni(lOO). In fig. 7 the variation of the

l

y 0

oxidized

NitlOO)

I

I

I

10

20

30

CS DEPOSITION

TIME

(min)

Fig. 7. Auger peak-to-peak height of Cs(565 eV) versus Cs deposition time on clean and oxidized Ni(100).

315

Cs(563 eV) Auger peak height versus Cs deposition time on clean and on oxidized Ni(100)is shown. The slope of the Auger curve on the oxidized surface is about 30% smaller than that on clean Ni, indicating a decrease of the sticking coefficient upon Ni oxidation. It is well-known that the sticking coefficient of Cs on clean Ni(100) is one, this leads to the conclusion that the initial sticking coefficient of Cs on oxidized Ni(lOO) is 0.7 which is the same as that estimated from the comparison of Cs on NiO and Si. The present experimental results suggest that the behavior of Cs on NiO is similar to that of K on NiO [6], i.e., initially, Cs is adsorbed as strongly ionized adatoms with increasing coverage it forms 2D clusters on the surface. The strong ionic character of the Cs adatoms is evidenced from the drastic linear decrease of the WF curve and the high temperature desorption state. The obtained dipole moment from the initial slope of the WF curve is - 8 D in agreement with that of Cs on metals and semiconductors (7-10 D) [7,9]. The chemical shift of the O(520 eV) Auger peak at early stages of Cs adsorption (fig. 3) indicates a strong interaction of Cs with the oxygen of the outmost layer of NiO. At low coverage, the desorption activation energy of Cs on NiO, calculated from the high-T peak of the TD spectra, is found to be E, - 2.6 eV as compared to 2.7 eV for Cs/GaAs(llO) [9] and Cs/Ge(lll) [lo]. At - 300 s of Cs deposition the Auger curve deviate from linearity while the Cs TDS area versus Cs deposition time curve continues to be linear up to 600 s. This indicates that the change of the slope of the Auger curve at 300 s (fig. 2) is not due to a decrease in sticking coefficient but to the tendency of clustering. Also two more peaks start to appear in TD spectra (fig. 4). The variation of the Auger and WF curves with Cs deposition is similar to those reported for 2D and 3D cluster formation [7,11]. However, the substantial WF lowering which is about 3.2 eV (fig. 1) Indicates that most of the NiO surface is covered with Cs. Therefore, instead of clusters we may have large islands. The WF curve of Cs on NiO (fig. 1) is almost the same as that observed for Cs adsorption on oxidized Ni(lOO) [4]. Cesium does not form any uniform metallic adlayer on oxidized

316

S. Kennou et al. / Cs and 0, aakorption on NiO(IO0)

Ni(lOO). The behavior of Cs on NiO should be similar. As is mentioned in section 3.1, adsorption of oxygen on Cs-saturated NiO(100) causes an increase in the WF of Cs. This is in contrast to most cases of oxygen exposure on alkali-covered metallic and semiconducting surfaces where an initial lowering of the WF has been observed [4]. This increase of the WF upon 0, exposure means that oxygen atoms are adsorbed on top of Cs. Most likely, the strong binding of Cs to oxygen on the top layer of the NiO substrate prevents the diffusion of oxygen under the Cs overlayer. As is mentioned in section 3.3, in the presence of oxygen, the high-energy peak of Cs is shifted to higher temperature. This suggests that the present of oxygen causes an increase of the binding energy of Cs. Moreover, the high energy TD peaks of Cs and 0, appear at the same temperatures. This may indicate the formation of some Cs-0 complex. Probably, this complex dissociates during desorption, because the mass spectrometer did not

show desorption Cs,O and CsO.

of any Cs-0

compound

such as

References Dl R.E. Weber and W.T. Peria, Surf. Sci. 14 (1969) 13. VI C.A. Papageorgopoulos and J.M. Chen, Surf. Sci. 39 (1973) 313; 52 (1975) 40. I31 G. Rangelov and L. Sumev, Surf. Sci. 185 (1987) 457. [41 CA. Papageorgopoulos and J.M. Chen, Surf. Sci. 52 (1975) 53. I51 J.-L. Desplat and C. Papageorgopoulos, Surf. Sci. 92 (1980) 97. 161 S. Kennou, M. Kaxnaratos and C. Papageorgopoulos, Vacuum 22 (1990) 41. [71 S. Kennou, S. Ladas and C. Papageorgopoulos, Surf. Sci. 152/153 (1985) 1213. PI S. Kennou, M. Kamaratos, S. Ladas and C. Papageorgopoulos, Surf. Sci. 216 (1989) 462. 191 J. Derrien and F. Arnau d’Avitaya, Surf. Sci. 65 (1977) 68. IlO1 L. Sumev and M. Tikhov, Surf. Sci. 85 (1979) 413. illI M. Kamaratos and CA. Papageorgopoulos, Surf. Sci. 160 (1985) 451.