Jump processes in surface diffusion - Bilkent University - Faculty of ...

42 G. Antczak, G. Ehrlich / Surface Science Reports 62 (2007) 39–61
Fig. 6. Distribution of atomic weights of individual atoms field evaporated
from Ir(110) surface [6]. Top: after deposition of Ir atom. Bottom: after
deposition of W atom. Center: After deposition of W atom, followed by heating
and observed cross-channel diffusion event; material desorbed is iridium.
Fig. 7. Atomic weight distribution of material field evaporated from first
Ir(110) layer [6]. Left: after cross-channel diffusion has occurred, a W atom
is detected in the first substrate layer. Right: when no cross-channel motion was
detected, iridium is desorbed.
cross-channel motion was discovered, the latter with a
lower activation energy. In cross-channel diffusion they again
envisioned a dumbbell intermediate, as shown in Fig. 8(b),
in which a pair of atoms sat across the ridgeline. When
this decomposes, the adatom can move back to its original
channel, or else it can move into the channel wall, placing
a lattice atom in the adjacent channel. Additional simulations
of diffusion across channels on a Lennard-Jones fcc crystal
were done by Mruzik and Pound [12]. On the (110) plane,
cross-channel motion again occurred by exchange with a lattice
atom, but movement on the (113) plane was along the channels.
Garofalini and Halicioglu [13] did similar estimates on the
Pt(110) plane at both low and high temperatures. At lower
temperatures both iridium and gold atoms diffused along the
channels, but at higher temperatures exchange took place for
iridium, and a platinum lattice atom appeared in the next
channel. Gold at this temperature also formed a dumbbell
with an atom from the channel wall, but returned to continue
diffusion in its original channel, so diffusion was really onedimensional.
More experimental work was reported in 1982 by
Wrigley [14], who examined the temperature dependence of
cross-channel motion, as shown in Fig. 10, with an activation
energy of 0.74 ± 0.09 eV and a prefactor 1.4(×16 ±1 ) ×
10 −6 cm 2 /s. Data were obtained at only five temperatures,
and keeping in mind the low prefactor, must be viewed with
some doubt. What was not clear at this point was the reason
for the magnitude of the prefactor. Was it associated with the
complicated mechanism, or were the measurements not detailed
enough?
At essentially the same time, Tung and Graham [15] looked
at self-diffusion on various surfaces of nickel. The behaviour
of the (110) plane varied depending on whether it had been
cleaned thermally, or had been subjected to field evaporation
in hydrogen gas. Diffusion measurements were made after the
hydrogen treatment and some field evaporation, as well as after
thermal cleaning only, and gave the results shown in Fig. 11. For
both treatments self-diffusion was two-dimensional on Ni(110);
however, diffusion after thermal treatment led to very low
diffusion prefactors ∼10 −7 –10 −9 cm 2 /s, with an activation
energy of 0.23 ± 0.04 eV for in-channel movement and 0.32 ±
0.05 eV across the channels. After hydrogen treatment, the
characteristics for in-channel motion were a diffusion barrier
of 0.30 ± 0.06 eV and a prefactor of 10 1 cm 2 /s; for crosschannel
diffusion the barrier was lower, 0.25 ± 0.06 eV, with
a prefactor of 10 −1 cm 2 /s. In the temperature range 80–90
K, cross-channel motion predominated. On Ni(331) and (113),
however, movement was always one-dimensional. Tung [16]
also briefly studied self-diffusion on Al(110), determining
the temperature for the beginning of atom movement. Twodimensional
diffusion was observed on the (110) plane, at a
temperature of 154 K for both directions, leading to an estimate
of 0.43 eV for the activation energy. It should be noted that
the diffusion characteristics, both for nickel and aluminum
(110) are uncertain, and the data have been reanalyzed [17].
Nevertheless, these observations firmly established crosschannel
motion on these surfaces.
Another five years later, in 1986, Kellogg [18] measured
the rates of in- and cross-channel self-diffusion on Pt(110)
over a range of temperatures. From an Arrhenius plot, shown
in Fig. 12, he found an activation of 0.72 ± 0.07 eV with a
prefactor of 6 × 10 −4 cm 2 /s for diffusion along the channels,