Chemistry Reference
In-Depth Information
ingredient was the large diffusion anisotropy (i.e., fast azimuthal diffusion around
the island perimeter as atoms arrive from the wetting layer vs. slower radial diffusion
toward the island center). Some other intriguing non-classical characteristics of the
rings are as follows: they form only on islands with radius larger than some mini-
mum size 50 nm, their width is constant
20 nm (independent of the initial island
radius), and the azimuthal speed is at least one order of magnitude faster than the
radial speed toward the center. None of these characteristics are well understood. At
higher temperatures (
T
∼
240 K), multilayer height rings (four or five layers high)
are seen [
39
] that indicate even higher mobility of the wetting layer.
A typical example of how rings form and how this is used to estimate the speed
of the wetting layer is shown in Fig.
3.2
a-d for the case of growth on the amorphous
wetting layer on (7
=
80 nm) four- or five-layer islands form
at 240 K (because the stability difference between odd and even height islands is less
for larger lateral sizes). The four-layer islands become five-layer (and eventually
seven-layer) islands but over longer times than for smaller islands grown at lower
temperature (
×
7). First, several large (
∼
180 K [
12
]). After growing the islands at 240 K, the surface is cooled
to 180 K to be able to follow the ring evolution with the STM since the evolution is
slower at this lower temperature. A four-layer island is shown in Fig.
3.2
. The scale
is 500
∼
250 nm
2
in Fig.
3.2
b-d, which are obtained
after 9, 14, 25min, respectively, from the completion of Fig.
3.2
a. The ring com-
pletes the perimeter with constant speed 0.05 nm/s. (A movie that was made from
18 successive images collected over an hour demonstrates that the ring advances at
constant speed.) For such motion to be initiated it is necessary for the density of
this amorphous wetting layer to be higher than the metallic density of Pb(111). The
crucial role of the density is seen in stepwise coverage experiments (i.e., small depo-
sitions
500 nm
2
in Fig.
3.2
a and 250
×
×
1ML increase the wetting layer density, then mass is transferred to
the stable islands, and the density falls below the critical coverage but the next
θ
∼
0
.
θ
deposition restarts the transfer process). This speed of the rings is a lower bound
to the speed of the wetting layer. As measured in SPA-LEED experiments [
38
] and
as required by the model [
39
], the incoming wetting layer runs quickly over the
facet planes of the Pb islands, but to move to the top it needs to overcome a barrier
at the island edge. This barrier was measured to be 0.32 eV [
38
] and a barrier of
similar magnitude was deduced from the simulations [
39
]. The combination of the
rate to overcome this barrier and the intrinsic speed of the wetting layer defines the
observed speed of the rings.
Figure
3.3
a shows large Pb islands that have nucleated on top of the dense Pb
α(
√
3
×
√
3
). The temperature is 180 K and a total amount 2.9ML
was deposited in two steps, 1.6 and 1.3ML. The image scale is 500
)
phase (noted as
α
445 nm
2
.
×
×
The islands are bigger (than on the (7
7)) and resemble “continents” since they
extend across the whole surface in irregular meandering shapes. Rings are seen on
four-layer islands transforming them into five-layer islands, which is the preferred
height for this interface [
40
]. The larger lateral extent of these islands and the greater
number of rings at the perimeter confirm higher mobility of Pb on
α
noted before
180 nm
2
area with smaller
islands to illustrate that after the ring grows (bottom left island), the region between
the islands is in the SIC phase with slightly less than
(than on the (7
×
7)). Figure
3.3
b shows a smaller 180
×
≈
4
/
3ML. Figure
3.3
cshows
