Surface diffusion of oxygen atoms individually observed by STM

Surface Science 169 (1986) L295-L300 North-Holland, Amsterdam

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SURFACE SCIENCE LETTERS SURFACE DIFFUSION OF OXYGEN ATOMS INDIVIDUALLY O B S E R V E D BY S T M G. B I N N I G , H. F U C H S and E. S T O L L I B M Zurich Research Laboratory, 8803 R~schlikon, Switzerland

Received 12 September 1985; accepted for publication 5 December 1985

We report on the first dynamical studies with scanning tunneling microscopy (STM). Single excess oxygen atoms migrating on the oxygen-induced (2x 1) and c(2x2) reconstructions on Ni(110) and Ni(100), respectively, are detected space and time resolved. In principle, for varying temperature, the data reveal the local diffusion coefficient, the activation energy and the density of adatoms on terraces and at steps. For the Ni(ll0) (2×1), even the adsorption sites are identified.

U n d e r s t a n d i n g of the physical a n d chemical p r o p e r t i e s of solid surfaces d e p e n d s vitally on the e x p e r i m e n t a l access to their t o p o g r a p h i c a n d electronic structure and, similarly i m p o r t a n t , to the e l e m e n t a r y processes c o n s t i t u t i n g surfaces d y n a m i c s . As b o t h structure a n d d y n a m i c s d e p e n d on each other, an e x p e r i m e n t a l a p p r o a c h s i m u l t a n e o u s l y p r o v i d i n g i n f o r m a t i o n on b o t h in realspace is most desirable. F i e l d ion m i c r o s c o p y ( F I M ) is a well-established m e t h o d for s t u d y i n g m i g r a t i o n of atoms, molecules or clusters on structurally well-characterized areas [1]. However, F I M is restricted to structures on tips, a n d to tip materials a n d a d s o r b e d species not being f i e l d - d e s o r b e d d u r i n g imaging. T r a n s m i s s i o n electron m i c r o s c o p y ( T E M ) being a p p l i c a b l e to m o r e e x t e n d e d surfaces [2], has less resolution for surface structures a n d usually requires strong electron scatterers, i.e., elements of high a t o m i c n u m b e r s for their visualization a n d their diffusion process on the surface. In this report, we present the first d y n a m i c a l studies by S T M [3]. T h e y d e m o n s t r a t e that S T M is c a p a b l e of detecting m i g r a t i n g a d a t o m s locally while, at the same time, the structure of the s u b s t r a t e with all the defects like steps a n d grain b o u n d a r i e s can be resolved. As an e x a m p l e for w e a k l y - b o u n d p a r t i c l e s , we investigated oxygen on two different Ni surfaces. The l o c a t i o n of the m i n i m u m in the interaction p o t e n t i a l of the oxygen a d a t o m s was o b s e r v e d direct in one case, the statistical b e h a v i o r was d e d u c e d a n d the diffusion coefficient e s t i m a t e d direct a n d locally. These first results should be viewed m o r e as a d e m o n s t r a t i o n of new e x p e r i m e n t a l possibilities o p e n e d up by S T M rather than a c o m p l e t e c h a r a c t e r i z a t i o n of a special d y n a m i c a l p r o b l e m . 0 0 3 9 - 6 0 2 8 / 8 6 / $ 0 3 . 5 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division)

G. Binnig et al. / Surface diffusion of oxygen on nickel

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The N i ( l l 0 ) and Ni(100) samples were prepared under UHV conditions (10 -s Pa) by successive He bombardment at 500 eV, and annealing at 600 ° until a sharp LEED image was obtained, and the Auger spectra did not detect measurable impurities on the samples. Then, large flat terraces separated by steps were observed by STM. On oxygen exposure of the N i ( l l 0 ) surface in excess of 1 L (1 langmuir ( L ) = 10 4 Pa s), the LEED image changed to the characteristic 2 × 1 pattern [4]. Exposing the Ni(100) surface to 25 L and annealing the sample at 300°C, the characteristic c(2 × 2) reconstruction appeared. The STM measurements were performed at 10 s Pa. In addition to the surface structure, the STM traces reveal (in contrast to the result on clean surfaces tiny spikes, caused by transient current increases of upto nearly two i

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Fig. 1. Events observed in the tunneling current (a) and (b) on the oscilloscope and the STM traces (c) at 573 K (~) and 353 K (,8) for the c(2 ×2) on Ni(100). For the lower temperature (353 K), the current pulses get wider (a) and denser (b). (The labels a - 6 in (b) mark succedent traces on the oscilloscope. The current zero is given by the base line of each trace.) The asymmetry in the pulses (a) is caused by the variation of the time constant of the electronics with gap resistance (current). Inset (c) shows the original data (1), the data minus the high-frequency components (II) and the high-frequency components alone (IIl). In steps, the events frequently occur at 572 K (c.a) and nearly disappear at 353 K (c.,8). They almost disappear even somewhat apart from the step at 353 K, pointing to a repulsion of the excess oxygens eventually caused by frozen excess oxygen atoms located at steps (see text).

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orders of magnitude (see fig. 1). We attribute these events to single diffusing oxygen atoms crossing the tunneling region. Other possible explanations for current discontinuities do not appear to apply in this case because of the following experimental findings: Above certain current or field values depending on the material or the structure of the sample or tip, current pulses might originate from instabilities in the tip or sample surface. In contrast to results obtained under such conditions, in the measurements reported here, after a spike the tip moves exactly (_+0.02 ,~) to the same vertical position. In other words, subtracting the spikes the STM traces reveal a picture without any discontinuities. The temperature of the Ni(100) sample was changed between 360 and 580 K resulting in dramatic changes of the density of the current pulses and their widths (see fig. 1). From measurement of the strong thermopower occurring at the tunneling gap by annealing the sample, we found as expected that the temperature of the tip stays close to room temperature, i.e., at a nearly constant value. This and also the strong local variations of the experimental results at steps or terraces point to the fact that the processes occur at least predominately on the sample and not at the apex of the tip. In general, dynamical kinds of STM investigations have two aspects: to resolve the migration of the atoms in space, i.e., the location of their adsorption sites relative to the structure of the substrate, and to measure the time for the adatom to pass the tunneling area. The first kind of measurement requires atomic resolution. The conditions for resolving frozen adsorbates or thermally-excited ones hopping from one adsorption site to the neighboring ones are essentially the same. The hopping itself, much too fast at room temperature (ps range), is not observed. The adsorbates are only detected when they remain sufficiently long (1 /~s) in a circular region on the sample vis-a-vis the tip in which they can influence the tunnel current. The resulting intensities of the current pulses are not only determined by the lateral tip-adsorbate spacing but also by the time the adsorbate stays at that site. The local maxima of a spatially resolved distribution of mean peak heights with respect to substrate would give the adsorption sites, as the tunnel current peaks when the adatom is centrally located in the tunneling region. For the (100) surface, our results are not sufficiently voluminous to justify obtaining such a distribution. Their local maxima with respect to substrate would give the adsorption sites. For the (110) surface, in contrast to all our measurements on the (100) 'surface, the spatial distribution of current spikes shows large spikes predominantly at equivalent sites. The lateral resolution is poor (--3.5 A), and the interpretation of this picture is only possibleobecause of the earlier measurements [4] where high ( ~ 2 ~,) and low (-- 3.3 A) resolution could be compared to each other. In fig. 2, the largest spikes occur at equivalent sites at the edge of domain walls and within the domains at the center between four maxima.

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G. Binnig et al. / Surface diffusion of oxygen on nickel

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Fig. 2. STM traces (a) and related top view of the ( 2 x l ) on Ni(110) (b). The spikes occur predominantly between the maxima of the (2 × 1) and at the edge of their domain walls. The crosses of different sizes mark the positions of the spikes and indicate their heights. Arrows locate the domain walls. The lines in the top view are guides for the eye and cross at positions where we believe the oxygens of the 2 x 1 are located [4].

T h e l a t t e r result agrees v e r y well w i t h o u r earlier i n t e r p r e t a t i o n [4] to a t t r i b u t e the m a x i m a to the o x y g e n a t o m s of the 2 x 1, a n d w i t h the e x p e c t a t i o n that the excess o x y g e n s are s p a c e d as far a p a r t f r o m t h e m as possible. In this sense, the l o c a t i o n of the a d s o r p t i o n sites at the d o m a i n walls is surprising, a n d will o n l y b e u n d e r s t o o d b y f i n d i n g the e x p l a n a t i o n for the w i d e d o m a i n walls. T h e d i f f e r e n c e in r e s o l v i n g the a d s o r p t i o n sites for the t w o N i faces m a y h a v e its o r i g i n in their d i f f e r e n t s u r f a c e c o r r u g a t i o n s . T i m e - r e s o l v e d m e a s u r e m e n t s w e r e o n l y p e r f o r m e d o n Ni(100). A statistical

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analysis of the time intervals between succeeding pulses in fig. l b or of the spikes in the S T M traces d i d not show a n y n o t i c e a b l e d e v i a t i o n from a Poisson d i s t r i b u t i o n . H a v i n g no real statistical analysis of the pulse shapes available, we only show in fig. l a short t e m p o r a l sequences on the oscilloscope. A t T = 80°C, the events are at least ten times d e n s e r than at 300°C a n d wider so their actual shapes are not too much d i s t o r t e d b y the slow electronics. A s s u m i n g r a n d o m walk, the average n u m b e r of j u m p s to cross the center of the tunneling region ( n ) can be estimated. It is d e s i r a b l e to have a t o m i c r e s o l u t i o n to o b t a i n a well-defined n u m b e r with n = 1. However, u n d e r these conditions, our electronics were not fast enough to follow the current pulses, a n d we o p e r a t e d with a lateral resolution of a b o u t 12 ,~, d e d u c e d from the b r o a d e n i n g of the S T M traces at m o n o - s t e p s . This leads us to roughly ( n ) = 25. W i t h a m e a s u r e d m e a n time to cross the center of the tunnel region of ( t i n ) = 1.2 × l 0 -3 S (see fig. 1), we o b t a i n a j u m p frequency of u = ( 2 5 / 1 . 2 ) × 10 3 s = 2 x l 0 4 s - 1 a n d a diffusion coefficient of D = a 2 u / 4 = 6 x 1012 c m 2 / s . T a k i n g into a c c o u n t u = ~n)/{tm) = % exp(E/kT),

(1)

we derive an activation energy of E = 0.57 eV for T = 353 K, and an a s s u m e d a t t e m p t frequency of % = 1012 s 1 ( E = 0 . 6 4 eV for % 1013 s - l ) . By m e a s u r ing carefully ( t m ) a d d i t i o n a l l y at a lower or s o m e w h a t higher temperature, u0 can also be o b t a i n e d with E r e m a i n i n g c o n s t a n t in most cases. U n f o r t u n a t e l y , we stored d a t a only at 353 a n d 573 K. C a l c u l a t i n g ( t m ) a t 573 K with (1) a n d E = 0.57 eV, we end up with ( t m ) = 2.4 × 10 - 6 S, which is too fast to be seen in the traces a n d only s o m e w h a t a b o v e the high-frequency limit of the electronics to be resolved as a c u r r e n t pulse. To u n d e r s t a n d that nevertheless we observe p r o n o u n c e d spikes at 573 K, we m a d e some M o n t e - C a r l o calculations for low coverage. T h e y d e m o n s t r a t e d that the p r o b a b i l i t y for the same m i g r a t i n g a t o m to cross the tunneling area even ten or m o r e times in a row is quite high. T h e occurrence of such an event is roughly 500 times m o r e likely at 573 K c o m p a r e d to 353 K because of the difference in u. W e believe that the spikes in the traces a n d the larger current pulses at 573 K are only such m u l t i p l e - e v e n t s detected as a single p e a k b y the slow electronics. This in turn e x p l a i n s the low d e n s i t y of spikes at 573 K although u(573 K) is higher than u(353 K). A l s o in this context, our findings at steps can be u n d e r s t o o d in terms of the b i n d i n g energy being higher there than on terraces: at 573 K, the spikes at steps look similar to the ones on terraces at 353 K, suggesting lPstep s (573 K) = Pt. . . . . . (353 K) a n d therefore Estep ~ 0.9 eV (for v0 = 1012 S 1 ) . W i t h this value, Vstep (353 K) = 1.5 S - 1 i.e., the excess oxygen a t o m s are p r a c t i c a l l y frozen at steps in a g r e e m e n t with the s m o o t h S T M traces at steps for 353 K. T h e s o m e w h a t higher d e n s i t y of events at steps for T = 573 K than on terraces for T = 353 K, we a t t r i b u t e to a higher density of a d a t o m s Pn. This d e n s i t y can be e s t i m a t e d b y the m e a n time a particle spends in the tunnel region p e r time

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u n i t d i v i d e d by the t u n n e l a r e a of a b o u t 100 ,~2. F r o m o u r data, at 353 K we o b t a i n p,, = 1 0 - 4 ~,--2. F o r all these c o n s i d e r a t i o n s , the p o s s i b i l i t y that on this c o m p l e x s y s t e m excess o x y g e n s i n t e r c h a n g e sites with the o x y g e n s of the f r o z e n p h a s e has n o t b e e n t a k e n into a c c o u n t . F i n a l l y , we s h o u l d recall the i n f l u e n c e of the tip o n the results. F u t u r e i n v e s t i g a t i o n s will clarify h o w s t r o n g l y the s u r f a c e p o t e n t i a l is m o d i f i e d by the p r e s e n c e of the tip by v a r y i n g field, t u n n e l c u r r e n t a n d state of the tip. W e are h a p p y to t h a n k H . W . F i n k a n d R. G o m e r discussions.

for v e r y s t i m u l a t i n g

References [1] See, for example, D.W. Bassett, in: Surface Mobilities on Solid Materials, Ed. V.T. Binh (Plenum, New York, 1983) pp. 63 and 83; G. Ehrlich, in: Proc. 9th Intern. Vacuum Congr. 5th Intern. Conf. on Solid Surfaces, Invited Speakers Volume, Ed. J.L. Segovia (ASEVA, Madrid, 1983) p. 3. [2] See, for example, S. Iijima and T. Ichihashi, Japan. J. Phys. 24 (1985) L125; Nature 315 (1985) 628; U. Utlaut, Phys. Rev. B22 (1980) 4650; K. Takayanagi, UItramicroscopy 8 (1982) 145. [3] G. Binnig and H. Rohrer. in: Trends in Physics 1984, Vol. 1, Eds. J. Janta el al. (Eur. Phys. Soc., Geneva, 1984) pp. 38-46. [4] A.M. Baro, G. Binnig, H. Rohrer, Ch. Gerber, E. Stoll, A. Baratoff and F. Salvan, Phys. Rev. Letters 52 (1984) 1304.