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BEHAVIOR OF SINGLE Ir ADATOMS AND SMALL Ir CLUSTERS ON Ir SURFACES
C. Chen, T. Tsong
To cite this version:
C. Chen, T. Tsong. BEHAVIOR OF SINGLE Ir ADATOMS AND SMALL Ir CLUSTERS ON Ir SUR- FACES. Journal de Physique Colloques, 1989, 50 (C8), pp.C8-273-C8-278. �10.1051/jphyscol:1989846�.
�jpa-00229944�
COLLOQUE DE PHYSIQUE
Colloque C8, suppl6ment au n o 11, Tome 50, novembre 1989
BEHAVIOR OF SINGLE Ir ADATOMS AND SMALL Ir CLUSTERS ON Ir SURFACES
C. CHEN and T.T. TSONG
Physics Department, The Pennsylvania State University, university Park, PA-16802', U.S.A.
Abstract
-
The behavior of single Ir adatoms and small Ir clusters on the Ir (111) and (001) surfaces such as the surface diffusion, the structure transformation of clusters, the dissolution of the top surface layers etc. as functions of the temperature has been studied with the field ion micro~cope.~ From these experiments, diffusion parameters and the dissociation energy of plane edge atoms etc. have been derived. We have also observed a 'strange behavior' in the temperature dependence of cluster structure transformation which disagrees with a simple consideration of the binding energy difference of the two structures.I. Introduction
It is well known that quantitative data for the behavior of single atoms and small atomic clusters on metal surfaces, such as surface diffusion, atomic interactions, and formation of 2-dimensional overlayers etc., can be obtained from field ion microscope experiments. Despite rapid progress in other atomic resolution microscopies, up to the present time, no quantitative studies of the behavior of single atoms have been reported by other microscopies. In the past, most of FIM single atom studies are performed on the bcc tungsten surfaces, even though a few studies of surface diffusions and cluster shape changes have also been reported for fcc metals.' From the theoretical point of view, fcc lattices are easier to treat than bcc lattices. We report here a study of a variety of behaviors of single Ir adatoms and small Ir atomic clusters on the Ir (111) and (001) surfaces.
11. Experimental
Experimental procedures for FIM experiments with single atoms have been well established from years of studies. Details can be found in the literature.' In this experiment, we again pay great attention to the vacuum condition and the degassing of the deposition sources. Two methods of heating have been used. One uses a Lexel Model-85 Cu Ion CW Laser. This laser unit has the maximum power of 0.5 watt with the spectrum centered around X
-
5145A.
The output power can be continuously adjusted. With our laser focusing setup, a power in the range of 0 to about 500 mW is needed to change the tip temperature all the way from 20 to -600 K.The advantage of laser heating is that an equilibrium temperature can be reached in ps range if the heated spot is very small,2 thus heating periods can be adjusted all the way from 0.1 s or less to over a minute, and can be adjusted by simply chopping the laser beam. The final temperature reached is calibrated according to the
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989846
temperature dependence of the evaporation field. e" disadvantage is that there are some problems with the long term stability of the laser and with an accurate calibration of the final temperature. It is well suited where the heating temperature is well over 200 K. For studying surface diffusion of Ir adatoms on the Ir (111) surface, diffusion can occur around 100 K. The accuracy of temperature calibration is then not quite sufficient. In Fig. 1 we show an Arrhenius plot obtained with laser heating for the diffusion of a W adatom on the W (110) surface.
The diffusion parameters obtained are: Ed = 0.91 f 0.03 eV and Do =
~.~x(~")xICT:!
cm2/s. These values are in good agreement with those obtained earlier by resistivity heating methods. We also use a current heating method with a pulsed dc power supply. With this power supply, the final temperature can be reached in less than 0.5 s without any overshoot. Details of these heating methods will be described later.
In the course of this investigation, we also find an interesting image spot shape of single Ir adatoms on the Ir (111) surface. While in the past all the single adatoms of various species, deposited on various surfaces, studied so far all exhibit a circular image spot, single Ir adatoms appear on the Ir (111) surface a triangular shape with their sides parallel to the <110> directions. We believe this is produced by a triangular distribution of the electronic density of states right above the Fermi level around the site of each adatom. In Fig. 2 a few of these images are shown.
11. Result and Discussions
a). Surface Diffusion. The surface diffusion parameters of single Ir adatoms on the Ir (111) and (001) planes are derived by depositing one Ir adatom on a plane using pulsed-current heating of the tip mounting loop. Each heating period is 10 s.
Fig. 1
Fig. 2
Each data'point consists of over 100 measurements. The Arrhenius plots obtained are shown in Fig. 3. The activation energies and diffusivities on the (111) and the (001) are, respectively, Ed
-
0.2220.03 eV and D, = 8.84~(8*')~10'~ cm2/s, andEd =
0.93s.04 eV and Do = 1.4x(10*')~10-~ cm2/s. Thus within the accuracy of the measurement, Do is on the order of kT/h as has been concluded earlier from comparing the FIM data then existing.' At the present time, any significant deviation of Do from cm2/s should be considered an artifact by the limited accuracy of the FIM measurement. The accuracy of FIM measurement of Do is still too limited to detect a possible difference in Do which may arise from the dynamics of surface diffusion.
This is mainly due to the narrowness of the temperature range in which an FIM measurement can be carried out.
Fig. 3
On the (111) surface the 'onset' diffusion temperature, i.e. the temperature where <(~r)'>%/t-0.5 A/s, changes from about 95
K
to 435 K for clusters of two to twelve atoms. While, in general, this temperature increases monotonically with the size of the cluster, 4-atom clusters are found to start to diffuse at a slightly lower temperature than 3-atom clusters, thus the activation energy of surface diffusion of 3-atom clusters seems to be higher than that of the 4-atom clusters.Currently we are trying to make a more careful measurement of the diffusion parameters of Ir clusters of different sizes.
b). Dissociation Energy. When a crystal is heated to high temperature, the size of a surface of lower free energy will increase from that of a field evaporation end form. The top surface layer of this surface will dissolve gradually and eventually will disappear completely. It is possible to measure the dissociation energy of plane edge atoms by plotting the inverse of the time it takes to have the entire top layer disappear completely against the inverse of the temperature. We have carried out this measurement for the Ir (001) surface. Fig.
4(a) are field ion micrographs showing the gradual dissolution of a top Ir (001) layer while an Arrhenius plot is shown in Fig. 4(b). Let us assume that at the temperatures of these measurements, diffusion of adatoms on the terrace is not a rate limiting factor in the dissolution process, then the rate of loss of atoms is given by
Fig.
Fig. 4b.
dN 2rr
- - - -
exp (- Eb/kT)dt a {exp (-E~/~T)
+
expc- E,/ICT)}
exp(- Edis/kT)where r is the radius of the layer, and a is the diameter of the atoms, Eb is the barrier height at the plane boundary which is assumed to be reflective. By integrating this equation from 0 to r and from Ro to 0, where R,, is the initial radius of the layer, one obtains
where AEb r Eb - Ed, or the extra barrier height of the reflective boundary. When
AE, >> kT, the expression can be rearranged to
Thus the Arrhenius plot should be linear. The plot of Fig. 4(b), however, shows a significant deviation from the linearity at low temperatures. This deviation indicates that the approximation is not vaiid at low temperatures when surface diffusion may in fact limit the rate of the dissolution process. In any case, the slope of the high temperature section of this plot gives EdiS+AE, of -1.4 eV. In general, AEb is much smaller than either Ed or Edis, and can be neglected from consideration.
It can be shown that the binding energy of kink site atoms should be identical to the cohesive energy of the metal, and the binding energy of an adsorbed atom on a plane should be the difference between the binding energy and the,dissociation energy of a kink site atom. Thus it is possible to determine the binding energy of adsorbed atom on a crystal plane by simply measuring the dissociation energy of kink site atoms of the same plane and substract it from the cohesive energy.
Unfortunately we are not certain at the moment that in the dissolution of a surface layer, atoms are dissociated from kink sites. On the plane edge, there are three different types of atoms. They are kink.atoms, ledge atoms and edge atoms as shown in Fig. 5. It may be possible to use a pulsed-heating technique to establish the sites where atoms are dissociated from a plane. We intend to do this experiment ir, the near future.
c). Structures of Small Clusters and Structure Transformation. In 1972, it was found that 6-atom W-clusters on the W (110) plane exhibited two structures, one highly symmetric structure which can be formed below 390 K. When it was heated to 390 K, it transformed into a less symmetric stable struct~re.~ Bassett also found that Ni, Pd, Ir and Pt adatoms on the W (110) tended to form one-dimensional (1-D) chains if the number of atoms was small. A two-dimensional (2-D) island was the stable structure only if the number of atoms is greater than a critical n ~ m b e r . ~ Schwoebel and Kellogg found that for Ir on the Ir (OOl), this number was 6 . 6
Since the number of atoms in a cluster is very small, no singularity behavior as in critical phenomena can be expected. There is unlikely to have a well-defined critical number nor a sharply defined transition temperature in the 1-D and 2-D cluster structure transformation. In other words, atomic size effects in the structure transformation can be studied by FIM experiments. One would expect that the probabilities of observing the 1-D structure and a 2-D structure should depend on the free energy difference of the two structures and the surface temperature according to the Boltzmann statistics.
where p, and pz are the probabilities of finding the cluster in the 1-D and 2-D structures, W, and W2 are the statistical weights or the possible numbers of different configurations of the two structures, and AE12 = El-E, is the difference in the magnitutdes of the free energies of the cluster in the 1-D and 2-D
Fig.
6Fig. 5 ~Edge 2 )
Ir 3-atom clusters on Ir(001)
AE = 3.89eV
i
structures, respectively. A set of preliminary data for Ir3 on the Ir (001) is shown in Fig. 6. We are most surprised to find that ln(pl/p2) exhibits two distinct linear sections, one with an activation energy of 0.347k0.013 eV and one can be described as having an activation energy as high as 3.8950.22 eV. We believe that this is due to a structural phase transition of a very small system of 3 atoms interacting with a large substrate of many particles, or due to a size effect of the phase transition. At the moment, we are trying to understand this behavior, and to make similar and other types of measurements for clusters of different sizes.
Details of this study will be presented later.
References *Supported by NSF
1 . See for examples G. Ehrlich and K. Stolt, Ann. Rev. Phys. Chem. 31, 603 (1980);
D. W. Bassett, in Surface Mobilities on Solid Materials, V. T. Binh edit (Plenum, New York, 1981); T. T. Tsong, Rept. Prog. Phys.
a,
759 (1988); T.T. Tsong, Surface Sci. Rept. 8 , 127 (1988).
2. H. F. Liu and T. T. Tsong, J. Appl. Phys.
s,
1334 (1986).3 . G. L. Kellogg and T. T. Tsong, J. Appl. Phys.
51,
1184 (1980).4. See Fig.7(g) to (i) of T. T. Tsong, Phys. Rev.
6 ,
417 (1972).5. D. W. Bassett, Thin Solid Films 48, 237 (1978).
6 . P. R. Schwoebel and G. L. Kellogg, Phys. Rev. Lett. 61, 578 (1988).
