Applied Surface Science 37 (1989) 369-380 369 North-Holland, Amsterdam

T O P O L O G I C A L S T U D Y O F T H E M U L T I - A L K A L | P H O T O C A T H O D E D E S T A B I L I Z A T I O N D U E T O MOLECUII ,AR O X Y G E N

,l. P A N C i R and I. H A S L I N G E R O V A

J. Heyrovsky~ Institute of Physical Chemistry and Electrodwmisto', C:echoslol'ak Academy of Sciences, DokT.~kot,a 3, 182 23 Prague, 8, Czedwslot,akia

Received I March 1989; accepted for publication 5 May 1989

A multi-alkali antimonide destabilization brought about by an interaction with such a common agent as molecular oxygen is studied theoretically. Adsorption energies and molecular diagrams of all phases which can temporarily exist in the Sb(NaKCs) multi-alkali photocathode systems were studied by the recently developed quantum chemical topological method. Systems under study were modelled as semi-infinite (SbXYZ). crystals, X, Y. and Z being Na. K. or Cs. Calculations were performed with cubic and hexagonal crystal lattices. Molecular oxygen can substantially r,.'duce the stability of photocathodes in which Sb and Cs atoms are present on their surface. The presence of even small amounts of oxygen in the surrounding medium should be avoided and/or surface Sb atoms should be adequately protected from the oxygen attack during the manufacturing of such photocathodes.

| . In~m[nctinn

The subjec t o f this s tudy is a theoret ical inves t iga t ion o f alkali a n t i m o n i d e des tabi l iza t ion due to such a c o m m o n impur i ty as molecu la r oxygen. T h e sy s t ems unde r s t udy f rom c o m m o n l y used mul t i -a lkal i p h o t o e a t h o d e s which exhibi t the h ighes t integral sensi t ivi ty o f all p h o t o c a t h o d e s wi th a posi t ive e lect ron aff in i ty which are p roduced nowadays . Alkal i a n t i m o n i d e s have been used commerc ia l ly as pho toemiss ive p h o t o n de tec tors for m a n y years [1,2]. A m o n g o the r propert ies , the s tabi l i ty o f Sb(Na , K, Cs) s y s t e m s is very im- p o r t a n t for their commerc ia l uti l ization.

Surface layers were mode led by the semi- inf in i te (SbXYZ), , crysta ls , X, Y, and Z be ing Na , K. or Cs. Ca lcu la t ions were p e r f o r m e d on cub ic a n d

hexagona l crystal lattices. The ca lcula t ions were p e r f o r m e d by a q u a n t u m - chemical topological m e t h o d which has been deve loped recent ly by one o f u s

[3].

2. Theore t ica l me¢lu~l and n m ~ s y s t e m s

In the p resen t s t u dy the s ame p rocedure ha s been used as in refs. [3-5] a n d we refer to these pape r s for details .

0 1 6 9 - 4 3 3 2 / 8 9 / $ 3 3 . 5 0 © Elsevier Science Publ i shers B.V. ( N o r t h - H o l l a n d Physics Pub l i sh ing Divis ion)

370 J. ~sr~r[?. [. H~z~{m~er{~cd / ),(~dtt.alkali photocathode destabilization

Tab~¢ \'a!~e~, ~£ C~uio;~;b in~e~-als c~ in MJ/m~l which v~ere used in this study

A~on~ Q C~p '~d

Sb - 1245 - 0.669 0.0 Na - 0.402 - 0.199 K -0.287 -0.i60 Cs -0.272 0.098 © - 0.635 - 0.564

The parametrization procedure was the same as in paper [5]. Resonance integrals were calibrated on heats of format ion of small molecules of similar valence types. In addition, orbital electronegativities (table 1), used as a basis for calculations of Cou lomb integrals, were empirically scaled to r e p r o d u c e correctly charge distr ibutions in small systems [3]. The AO basis set consisted

of 372 orbitaIs. Similarly as in ref. [5] we used two model s tructures - cubic and hexagonal

lattices. Models of these lattices are presented in figs, 1 and 2 in which elementary cells are outlined. The basic clusters to be calculated c o n s i s t e d o f 6 crystallograpl-dc planes. Odd planes of the cubic lattice included 6 Sb a toms and 12 alkali a toms whereas even planes only 6 alkali atoms. Odd planes o f

C

Fig. 1. Elementary cell and basic cluster of the cubic lattice. Atoms in planes parallel to the surface are linked by solid lines, atom~ in successive planes are linked by dotted lines.

J. PancfT', 1. Haslingerov~J /Multi-alkali photocathod "ffestabilization 371

Fig. 2. Elementary cell and basic cluster of the hexagonal lattice. For further details see fig. I.

the hexagonal lattice consisted of 6 Sb atoms and 9 alkali atoms whereas 3 other alkali atoms were lying on even planes. The proper symmetry of the infinite crystal in x- and y-dimensions was preserved by introducing periodic boundary conditions [5]. In addition, the uppermost of lowermost planes were fully saturated by molecular oxygen which was linked either to Sb atoms (the uppermost plane) or to alkali metals (the lowermost plane).

3. Results and discussions

In our calculations all possible combinat ions which can be formally derived by the replacement of Na, K, or Cs for X, Y, or Z in both lattice types (SbXYZ),, were considered. In this particular case the calculations were

372 27 ~:~4~ ~(~Y. : ::ca~ke!ger~<-~i / 3/u/r,.a/kail phorocarhode destalTilization

Ta~ic 2 .,kd:,~rT:~ion ene:g;.cs o~" s3s~ems under study (kJ/mol)

N) sle,nas Capture on Sb atoms Capture on alkali atoms the third atom points to 0 2

Hex~gor~a~ Cubic Hexagonal Cubic s~ructura structure structure structure

NaNaNa - 460 -441 30 28 NaNak -515 -453 14 121 NaNaCs - 445 - 418 - 11 112 KKK - 577 - 539 7t 103 KKNa - 518 - 513 67 58 KKCs - 607 - 551 38 92 CsCsCs -643 -613 55 30 CsCsK -612 -598 68 54 CsCsNa - 553 -569 30 - 6 0 CsNaK - 673 - 52 39 111

p e r f o r m e d for 10 s t r u c t u r e s in b o t h h e x a g o n a l a n d c u b i c la t t ices , e a c h fo r b o t h

s u r f a c e t ypes o f 0 2 c o v e r a g e s .

A d s o r p t i o n e n e r g i e s a n d m o l e c u l a r d i a g r a m s o f all t hese s y s t e m s a r e

p r e s e n t e d in t ab le 2 a n d in figs. 3 a n d 4. In these f i gu res o n l y o n e " c h a i n o f

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-~.e~ " ~ s oa,~ 0 o ~ 4 . t 7 5 ~

o < . . . . ~l . . . . f . . . . . . .

o=s ) o 71 ~ ~ss ,0.75

~50 ~ ® o ~ _ t . 7 s ~ ) ) ~ ) ~ T a o~ Q~5 -t3t: ~ 0 " 7 3 - Z 3 : ~ 0~$-1'bl

l .... )-o.~, )-o~,

Fig. 3a. Molecular diagrams of cubic lattices of Na3Sb. K3Sb. Cs~Sb, Na2KSb, and Na~CsSb systems with molecular oxygen absorbed on the alkali metal sides. Squares. closed circle.

half-filled circle, small open circle, large open circle stand for Sb. Na. K. Cs. and O. respectively.

0.~

O.55

o.7l

0.71

% r ~ o.0 #..~0." ®o~4_1.7 s . 3 ~ ~o.~

~o~O o.57

)-~2o -lisa

J. Panci~, I. Haslingerovd / Multi-alkali photocathode destahdi:ation

oTs o.t~ o~ o~t,

-o.~

b -on '

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,+ + cm (

) ..... (

)o.~

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o~O,,a .... m+o~o-,,,|

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o ~ o o6 o ~° o~a

~032 bo,~ . ? o.~5

)OZ6 iO.**O O~2

o. ~ 0@7 o.¢,j+ 08++ o Qmm

ooo, .... .... 20°

m + o.++ C +o ++ +.m

) . . . . ( ) . . . . (

Fig. 3b. Molecular diagrams of cubic lattices of K2NaSb. K:CsSb. Cs:NaSb. Cs2KSb. and CsNaKSb systems. For further explanation see fig. 3a.

Fig. 3c. Molecular diagrams of hexagonal lattices on Na~Sb. K~Sb. Cs~Sb. Na.KSb. and Na zCsSb systems. For further explanation see fig. 3a.

d

5 ~ "/¢e:>ae~.=.~:~/d / .~.Itdrt-J//~ah [Jhomcadlode de.~mbiliT.ano~l

Fig. 3d. Molecular diagrams of hexagonal lattices of K2NaSb, K : C s S b , C2NaSb, Cs2KSb, and CsNaKSb systems. For further explanation see fig. 3a.

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-ols -o3~

o )0s0

-l~z~ o ¢'° o4'9 c~

~ 0.~9

041 Or~

) . . . . © - , .~

) . . . . () .....

-," 0~9 0 ~ 0~3

0 ~ 0~4

~ ; : 7 . .... ~ ° 84-'t ~

0.71 ) 0 8 4

Q r~

,0.'~2 ~OeS

0/4~ ,'~ 0.43

~0.77 ?0~9

o~, ~ o.4~ o ~ / ~ o,4e'

~oTs ~ g

0.87 0.'~4

Fig. 4a. Molecular d iagrams similar to those in fig. 3a where molecu, r oxygen is adsorbed on tile Sb metal sides.

J. Pantie, L ttaslingerov~J / Mvlti-alkali photocathode destabdization 375

I ~ ..... 9 . . . . 9 .... 9 . . . . . . .

oro o o o 6 s ~ o a a ~ oe4 o~>.~

-o.~o ~fo1~ ~o7~ ~IoIs ~o?s ~vfolo C)OSa I oE~9~_~os5 Jo~.- 0o2~

.... ~1 o*~ ~°'° ~ .... o,, oo~

)o.,*1 )o 0~, i #.3.. Do 6t~ ~

Fig. 4b. Molc,'.ular d,agrams similar to those irx fig. 3b where molecular oxygen is adsorbed on the Sb ,uetal sides.

170

0~.6 070 0~4 08,8 C~8

Fig. 4c. Molecular diagrams similar to those in fig. 3c where molecular oxygen is a d ~ r b c d on the Sb melal sides.

376 ) ~%:,~;b r. L 5~,zsf~;~!,Nro~.,J / Mul~t-dtkuh photoca[hode dcstahih2atiml

:.. ,s 7 ~ 0 2,.; ~,t 3 033 "? '? '?

~/6- ~ ~ ~ -0 ,'-

~_~ ~ t .~

d o.~?oa6 o55~J'o~087 06; o930'o~? o.93

0.71 Q0,55~.112 ~ -2~,0 o 495 014A1\0,,

09'2(~e(~ 0092 0.6Y~([) 00.86 067 0.6?

Fig. 4d. Molecular diagrams similar to those in fig. 3d where molecular oxygen is adsorbed on the Sb metal sides.

atoms" is presented which points from the uppermost plane to the lowermost plane. These elementa~ structural units repeat in directions of x- and y-axes through the system.

Specific adsorption energies AE are given in table 2. (The more negative the value of AE the more feasible it is for an attack to occur.) Because of the sembempirical approximations used these values are not exact. However, we have assured ourselves in other fields of surface chemistry [4,6-11] that in a set of structurally similar lattices these values correspond to relative stabilities which are in good accord with the experiments. In this case, the binding of O 2 is somewhat stronger in hexagonal lattices than in cubic lattices, the attack of Sb being extremely feasible. This attack will be hindered by the increase of the concentration of Na atoms. Tbc reason for the activity of Sb can be seen in the molecular diagrams of fig3. 3 and 4. It can be seen that the 0 2 attack on alkali atoms resembles a formation of alkali superoxides with a very weak alkali-O bond as is seen from corresponding bond orders. However, the attack on Sb results in the o x i d e - l i k e formation with a strong S b - O bond. This effect, together with the existence of free valence electrons in surface Sb atoms, is probably the explanation for this behavior. Moreover, the strong charge flux from Sb to the terminal oxygen gives rise to the strongly reactive S b - O - O - species with the possibility to release O - ions.

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J. Panci~, I. Ha.~lingerovtl / Multi-alkali photocathode destabilization

Table 3 Energies of HOMO's and LUMO's (kJ/mol)

379

Systems Capture on Sb atoms Capture on alkali atoms the third atom point to 02

Hexagonal Cubic Hexagonal Cubic structure structure structure structure

Ell El. EH EL Ell EL EH EL.

NaNaNa -648 -588 -762 -760 -710 -709 -794 -794 KKK - 635 - 543 - 750 - 750 -690 - 685 - 792 - 792 CsCsCs -628 -616 -743 -742 - 670 -665 -786 -786 NaNaK - 644 - 583 - 761 -759 - 705 - 687 - 793 - 793 NaNaCs -641 - 580 - 761 - 759 - 703 - 670 - 793 - 790 KKNa -639 - 554 - 751 -750 - 704 - 703 - 702 - 7q2 CsCsNa - 632 - 552 - 744 - 744 - 689 - 688 - 788 - 787 KKCs -633 -538 -750 -749 -689 -668 -789 -788 CsCsK - 630 -522 -743 -743 - 681 -678 - 787 -787 CsNaK -636 - 5 ~ -756 -754 -697 -669 -792 -790

T o c o m p a r e the b o n d s t r e n g t h s a f t e r the o x y g e n a t t a c h w e c a n c o m p a r e

c o r r e s p o n d i n g b o n d o r d e r s o f t he se s t r u c t u r e s w i t h t h o s e o f p a r e n t s y s t e m s

(f ig. 5) w h i c h a re t a k e n f r o m ref. [5]. I n t h i s f i gu re t h e u p p e r d i a g r a m s

c o r r e s p o n d s to t h e r e f e r e n c e s t r u c t u r e s o f s y s t e m s w h i c h a r e i n f i n i t e in a l l

t h r e e d i m e n s i o n s . I t is s e e n t h a t t h e a t t a c k o f a l k a l i a t o m s b r i n g s n o s i g n i f i c a n t

c h a n g e s in the w h o l e s t r uc tu r e . H o w e v e r , s u b s t a n t i a l r e d u c t i o n o f b o n d o r d e r s

a n d t h u s b o n d s t r e n g t h s c a n b e f o u n d in the s u r f a c e l a y e r o f a l k a l i - S b b o n d s .

T h e p a r t i c u l a r l y s t r o n g b o n d s t r e n g t h r e d u c t i o n a p p e a r s vAth S b - C s b o n d s . I t

c a n t h u s b e c o n c l u d e d t h a t the a b s o r p t i o n o f 0 2 o n S b a t o m s c a n l e a d t c t h e

r e l e a s e o f s u r f a c e C s a t o m s a n d to the b r e a k d o w n o f t h e c r y s t a l s u r f a c e .

E n e r g i e s o f h i g h e s t o c c u p i e d m o l e c u l a r o r b i t a l s ( H O M O ' s ) a n d l o w e s t

u n o c c u p i e d m o l e c u l a r o r b i t a l s ( L U M O ' s ) a r e p r e s e n t e d in t a b l e 3. S i m i l a r l y a s

w i t h b o n d i n g e n e r g i e s t h e s e v a l u e s s h o u l d b e r e g a r d e d a s r e l a t ive . By i n s p e c t -

i o n o f t a b l e 3 c a n b e s e e n t h a t t h e a r i t h m e t i c m e a n o f H O M O ' s a n d L U M O ' s ,

w h i c h c a n b e r e l a t e d to t h e e l e c t r o n i c w o r k f u n c t i o n b e a r s l o w e r v a l u e s w i t h

t h e i n c r e a s i n g c o n c e n t r a t i o n o f C s a t o m s . H o w e v e r , b y c o m p a r i n g t a b l e s 2 a n d

3, i t s h o u l d b e c l e a r t h a t t h e d e c r e a s e o f t h e w o r k f u n c t i o n is a s s o c i a t e d w i t h

t h e d e c r e a s e of the s t a b i l i t y .

I t c a n b e c o n c l u d e d t h a t the p r e s e n c e o f t h e m o l e c u l a r o x y g e n c a n c o n s i d e r -

a b l y a f fec t the s t a b i l i t y o f the m u l t i - a l k a l i p h o t o c a t h o d e s , p a r t i c u l a r l y , i f Sb

a t o m s a r e p r e s e n t o n t h e su r face . P o s s i b l e d a m a g e is t h e r e l e a s e o f f ree C s

%,0 J. P~'ae.~?k L ~-~.:q'~agel,o< d fi Mulli-a/kali ph~)tocathodo destabih,zc~ri:m

atoms wi~ich is followed by surface reconstr~action. The possible preservat ion is ~o make the preparat ion in a strongly rednctive environment a n d / o r to pre~ent the existence of Sb particles on the surface. Finally, we should remark tha,~ !he s~abitity of muki-atkaIi photocathodes t o v a r d s other possible c u , - laminating agents such as nitrogen and carbon-con:a ining species is a topic ~i~ici~ will be further studied in this laboratory.

Ackaoutea~gen~en~

This paper required a rather high tevd of skiti in figure-drawing, The authors are greatly indebted to Mtss Dita Jon'a[ov~ for her valuable technical help.

Re~erences

[1] A.H. Sommer, Photoemissive Materials (Wiley, New York, 1981). [2] L. Gatan and C.W. Bates, 3. Phys. D (Appl. Phys.) 14 (1981) 293. [3] J. PanciL Collection Czech. Chem. Commun. 45 (1980) 2452, 2463. [4] J. PanciL 1. Haslingerov5 and P. Nachtigall, Surface Sci. 181 (1987) 4t3. [5] J. PandL I. HaslingerovS. and P. Nachtigall, Appl. Surface Sci. 25 (1986) 167. [6] J. PanciL L Haslingerov/t and P. Nachtigall, Chem. Phys. 119 (1988) 289. [7] J. FanciL J. HorS.k a~d Z. Star 9, Phys. Status Solidi (a) 103 (1987) 517. [8] J. Pancff. I. Haslingerov5. and P. Nachtigall, Collection Czech. Chem. Commun. 53 (1988)

2064. [9] J. PanciF and I. HaslingerovS.. Collection Czech. Chem. Commun., in press.

[10] J. PanciF and I. Haslingerov& Czech. J. Phys., in press. [11] J. PanciF and I. Haslingerov'a, Appl. Surface Sci., to be published..