How atoms move in the (1 × 1) to (1 × 2) surface reconstruction of fcc (110) planes

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Surface Science 182 (1987) L257-L262 North-Holland, Amsterdam

SURFACE

SCIENCE

LETTERS

HOW ATOMS MOVE IN T’HJ3(1 x 1) to (1 X 2) SURFACE RECONSTRUCf’ION OF fee (110) PLANES T.T. TSONG

and Qiaojun GAO

Physics Department, The Pennsylvania

State University,

University Park, PA 16802, USA

Received 10 November 1986; accepted for publication 15 December 1986

The result of a recent study of atomic steps in the (1 x 1) to (1 x 2) surface reconstruction of Pt and Ir (110) planes using ns pulsed-laser heating can only be understood by highly correlated jumps of atoms within a small [llO] atom-row. We argue in favor of a simultaneous jump of the entire small [llO] row of atoms, possibly stimulated by surface phonon waves propagating in the direction normal to, and also parallel to, the atom-row direction. Surface reconstruction of fee (110) planes evolves with long-range atomic diffusion, thus the transition occurs relatively slowly. Reconstruction of fee (001) planes evolves with short-range atomic movements, and the transition occurs very rapidly.

One important consideration in surface atomic reconstructions, in fact in any structure phase transition, is how atoms are transported to achieve the new structure. For example, in the (1 X 1) to (1 X 2) reconstruction of the fee (110) plane, Bonzel and Ferrer argue against the simple missing row model for the reason that the (1 X 1) to (1 X 2) reconstruction would have to evolve with long-range diffusion of surface atoms [l]. The transition temperature of this reconstruction is too low for the long-range diffusion to be possible. Thus they proposed a “saw tooth missing row model” for the (1 X 2) reconstructed surfaces in order to be consistent with mass diffusion data. However, recent direct observation by field ion microscopy [2,3] and ion scattering experiments (ICISS) [4], support the validity of the simple missing row model. Thus the question of how atoms are transported in this atomic reconstruction remains an unanswered question. This question was addressed by us in a recent experiment with a combination of field ion microscopy and a ns pulsed-laser heating technique [3]. With this technique, it is now possible to see with atomic resolution changes that occurred within - 5 ns by pulsed-laser heating of the surface. Thus it is possible to observe directly the atomic steps involved in the (1 x 1) to (1 x 2) reconstruction of the Pt and Ir (110) planes with - 5 ns time resolution. The reconstruction is seen to evolve with breaking of [llO] atom-rows into small [llO] atom-row fragments of two to a few atoms in size, and with lateral jumps 0039-6028/87/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

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T. T. Tsong, Qiaojun Gao / Atom movement in surface reconstruction

Fig. 1. (a) A (1 X 1) Pt (110) plane prepared by low temperature field evaporation. irradiation of one laser pulse of 5 ns width, two atoms are seen to move to the neighbor channel, forming a partially reconstructed (1 X 2) surface. (c) Another laser-pulse causes [IlO] atom-row to move to the neighbor surface channel. (d) A (1 X 1) Ir (110) plane. irradiation of one laser pulse, the lower two rows of atoms shift one step toward the plane

(b) By surface another (e) By edge.

(i.e., in the [llO] direction) and cross-channel jumps (i.e., in the [OOl] direction) of these .fragments. Jumps of single atoms are rarely seen. In fact, when a plane-edge [llO] atom-row contains less than - 10 atoms, these atoms are always seen to jump all together to the next surface channel within 5 ns. A few examples of these atomic jumps are shown in fig. 1. A question immediately arises: how each atom jumps in the reconstruction. 5 ns is still a very long period of time since the atomic vibrational period is less than 1 ps, and over 5000 atomic vibrations can occur in 5 ns. In this report we would like to examine this question more carefully based on the data we have presented recently [3,7]. Let us consider only the case of cross-channel jumps here. The same arguments, however, can be applied directly to lateral jumps of small [llO] atom-rows. One can envision three possibilities for a small [llO] atom-row at the edge of a (110) plane to move to the next surface channel. These are uncorrelated jumps of individual atoms, highly correlated jumps of these atoms, and a simultaneous jump of all these atoms. We will show here that the first mechanism is totally inconsistent with our observation. For this discussion, we will assume a potential barrier as shown in fig. 2. For our discussion details of the potential barrier are not essential. The potential

T. T. Tsong, Qiaojun Gao / Atom movement in surfacereconstruction

r t\

dils et

+*++++3---

row2

_--6

1

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-

%-

w33xom3’ Fig. 2. Schematic

diagram

for the discussion

of uncorrelated

jumps

of single atoms.

energy of an atom in row 1 is represented by e1 and in row 2 by e2. Obviously 1z2 1 > 1cl 1, otherwise (1 X 2) reconstruction will not occur. The potential energy at the maximum of the activation barrier is represented by eM. To simplify our discussion, we will assume that the edge effect is very small, so that an atom near the two ends of the atom row will experience the same potential barrier as other atoms. Now let us focus on one atom, say A, which sits in row 1 at t = 0. The probability that this atom will be found in row 2 after heating the surface to temperature T for a time t is represented by p2(t), and the probability that it remains in row 1 is represented by pr( t). p*(t) satisfies the following difference equation, &+

dt) =&)

+ dp,(t) (1)

where k,, and k,, are, respectively, the jump rate from row 1 to row 2, and the jump rate from row 2 to row 1 at temperature T. They are given by k,, = y. exp[ - (cM - c,)/kT],

(2)

k,,=v,exp[-(CM-q)/kT].

(3)

The sum of PI(t) and p2(t) is unity. The solution of eq. (1) satisfying the initial condition that at t = 0, p*(O) = 0 and ~~(0) = 1 is PA4

=

1 - exp( -Kr) 1 + exp[ -(er

- E2)/kT]

’

(4)

where K=vo{exp[ -(eM-

er)/kT]

+exp[-(EM--#T]}.

(5)

For (or - e2) z+ kT, we have p*(7)=

[l-exp(-KT)].

(6)

7 is the time period of the heating, which is - 5 ns in our experiment. When

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T.T. Tsong Qiaojun Gao / Atom movement in surface reconstruction

there are no atoms in row 1 at t = 0, the probability of having exactly n atoms in row 2 after heating to T for a time period of T is

(7) The probability that n = 0, i.e., no atom is seen in row 2 after irradiation of a laser pulse, is Co(O) = [l -P2C41f10,

(8)

and the probability that n = n,, i.e., all no atoms are seen in row 2, is P&%J

=A?+

(9)

In our experiment we adjusted the laser power just sufficiently to observe an atomic jump in every few laser pulses. We either observe no jump of atoms or observe a jump of the entire row of atoms. Thus this experiment gives a value of P,,(n,) of about 0.2 to 0.3, and P,,(n) = 0 for n # n,. From eq. (9) and the value of P,,(n,) we must have p*(r) # 0 or 1. On the other hand, from eq. (7) and the value of P,,(n # n,) we must have either ~~(7) = 0 or 1. These two outcomes, based on uncorrelated jumps of individual atoms and the experimentally observed P,,(no) and P,,(n # n,), contradict one another. Thus the initial assumption of uncorrelated atomic jumps is incorrect. We must conclude that in the (1 X 1) to (1 X 2) surface reconstruction of Ir and Pt (110) planes, atoms move either by highly correlated jumps, or by simultaneous jumps of small [llO] atoms-rows as illustrated in fig. 3. Even with the 5 ns time resolution of the pulsed-laser technique, it is still impossible to observe direct jumps of individual atoms if they are highly

(a)

(b)

w

Fig. 3. Two possible atomic jumping processes of (1 X 1) to (1 X 2) surface reconstruction of fee (110) planes. One by successive jumps of single atoms (a), and one by a simultaneous jump of the entire row of atoms (b).

T. T. Tsong, Qiaojun Gao / Atom movement in surface reconstruction

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correlated. One may need a time resolution of - 1 ps. This is unlikely to be achievable by the pulsed-laser heating technique since heat transport, or phonon transport, takes a time much longer than 1 ps. Thus the question of which of the two processes, i.e., highly correlated jumps of individual atoms or a simultaneous jump of the entire row of atoms, is the correct one probably cannot be answered by the pulsed-laser heating technique. A possible solution to the problem may be by molecular dynamics simulation. As far as we are aware, no observation of these types of atomic motion in molecular simulations has been reported. Also one may need an accurate form of atomic interaction in order for the simulation to be reliable. Experimental data on the atomic interaction is unlikely to be available in the near future. Here we would like to argue in favor of a simultaneous jump of the entire row of atoms based on some experimental evidence of a very strong adatom-adatom interaction at the closest distance in the [llO] direction. Kellogg [5] finds that when two Pt adatoms are deposited on the same surface channel of the Pt (110) plane, they will combine into a very stable nearest-neighbor pair which cannot be thermally dissociated. In fact, when the temperature is raised, this pair starts to attract atoms from the bulk and a very stable long [llO] atom-row is formed. We can also understand the (1 x 5) surface reconstruction of the Ir (001) plane in terms of very stable and strongly bound [llO] atom-rows. Six of such [llO] atom-rows will squeeze into five unit spacings of the (001) plane by very strong interaction between two neighboring atom-rows. This results in the formation of closely packed and buckled atom-row structure of the (1 X 5) structure showing quasihexagonal atomic arrangements [6]. With such a strong nearest-neighbor interaction of atoms in a [llO] atom-row, it is unlikely that an entire row of several atoms can dissociate one by one and jump to the neighbor surface channel in succession in 5 ns. It is much more likely that the entire [llO] row of atoms jump simultaneously into the neighbor surface channel. Such collective jumps of entire rows of atoms are stimulated by phonon waves, presumably soliton waves, propagating in the [OOl] direction. This type of atomic jump has not been discussed before either in experimental or computer simulation studies. It is interesting to consider the speed of structure phase transitions here. The (1 X 1) to (1 x 2) surface reconstruction of fee metals does evolve with relatively long-range motion of atoms. Although the atom-transport is made efficient by simultaneous jumps of small [llO] rows of atoms at relatively low temperature, the reconstruction still occurs relatively slowly. Complete reconstruction of a plane by heating of one laser pulse rarely occurs even when the plane is very very small. The (1 X 5) reconstruction of the Ir (001) plane on the other hand, involves only short movements of atoms. The reconstruction should be very fast. This is, in fact, in complete agreement with our observation [7]. A relatively large plane, of over 100 A in size, of the (1 X 1) structure can be completely reconstructed to the (1 x 5) structure by heating of one

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T.T. Tsong, Qiaojun Gao / Atom movement in surface reconstruction

laser pulse [7]. A partial reconstruction is sometimes observed, but only very rarely. One may therefore characterize two types of structure phase transition based on atomic movements: one evolves with a relatively long-range atomic diffusion which should be a slow process, and one evolves with very short-range atomic movements which should be a very fast process. This work was supported

by NSF.

References [l] [2] [3] [4] [5] [6]

H.P. Bonzel and S. Ferrer, Surface Sci. 118 (1982) L263. G.L. Kellogg, Phys. Rev. Letters 55 (1985) 2168. Q.J. Gao and T.T. Tsong, Phys. Rev. Letters 57 (1986) 452. H. Niehus and G. Comsa, Surface Sci. 151 (1985) L171. G.L. Kellogg, 33rd AVS Meeting, Baltimore, 1986. E. Lang, K. Muller, K. Heinz, M.A. Van Hove, R.J. Koestner 127 (1983) 347. [7] Q.J. Gao and T.T. Tsong, J. Vacuum Sci. Technol. June/July

and G.A. Somojai,

Surface

(1987), to be published.

Sci.