Journal of The Electrochemical Society, 146 (3) 1019-1027 (1999)
1019
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A Comparison of Atomic Layers Formed by Electrodeposition of
Selenium and Tellurium
Scanning Tunneling Microscopy Studies on Au(100) and Au(111)
Thomas A. Sorenson, Tedd E. Lister, Boaming M. Huang, and John L. Stickney*,z
Department of Chemistry, University of Georgia, Athens, Georgia 30602-2556, USA
Structures formed by the electrodeposition of atomic layers of chalcogenides Se and Te, on Au(100) and Au(111), are described
and compared. Each element, on each surface, forms a low coverage structure, consisting of atoms packed simply in high coordinate sites at distances just above their van der Waals diameter. As coverages are increased above this level, structures composed of
chalcogenide atom chains or rings are formed. It is proposed that these chains or rings have significant molecular character, involving orbital overlap of adjacent chalcogenide atoms. Mechanisms are described to account for the formation of these chains and
rings. Discussion is also presented concerning the appearance of triangular phase boundaries for both chalcogenides on Au(111).
In the case of Se, isolated triangles, about 4-6 nm on a side are distributed across the surface, whereas a network of triangular phase
boundaries is observed in the deposition of Te. The triangular phase boundaries in Se appear to result from the nucleation of
domains in different threefold sites on Au(111). For Te, however, it is proposed that the triangular domains and phase boundaries
are the result of Te atoms being too large to form an extended (!w3X!w3)R308 structure.
© 1999 The Electrochemical Society. S0013-4651(98)09-061-2. All rights reserved.
Manuscript submitted June 6, 1998; revised manuscript received August 30, 1998. This was Paper 849 presented at the 1997 Joint
International Meeting of The Electrochemical Society and the International Society of Electrochemistry, Paris, France, August 31September 5, 1997.
The significance of chalcogenide surface chemistry is clear. The
interaction of oxygen with metal surfaces is one of the most important reactions industrially, and has been studied proportionately. Sulfur surface chemistry is widely important, as well. Sulfur is used to
passivate, promote, and protect metal and semiconductor surfaces.1-11 The surface chemistry of the other chalcogenides (selenium12,13 and tellurium14-17) are relatively less important, and their
surface chemistry has only begun to be investigated. This article presents a comparison of scanning tunneling microscopy (STM) studies
of electrochemically formed atomic layers of these chalcogenides,
on Au(100) and Au(111).
Recently, chalcogenide surface chemistry has been the subject of
an increasing number of studies concerned with the formation of
thin film materials. Monolayer assemblies of organosulfur molecules have been extensively studied and reviewed18-20 and studies on
analogous organoselenium monolayers have begun to appear.21,22 In
addition, the formation of chalcogenide atomic layers by electrodeposition has been investigated with the objective of growing thin
films of chalcogenide compound semiconductors by the electrochemical analog of atomic layer epitaxy (ALE).8,23-35
ALE involves the use of surface-limited reactions to deposit single atomic layers of elements, in a cycle, to form thin films of a compound.36-40 In electrochemistry, surface-limited reactions are referred
to as underpotential deposition (upd). UPD occurs when the first
atomic layer of an element is more stable on an electrode or substrate,
than subsequent layers of the element. UPD is the result of the formation of a surface compound, and is driven by its corresponding free
energy of compound formation.41-43 The result is that the first atomic layer of an element is deposited at an electrochemical potential
prior to (under) that needed to form a bulk deposit of the same element. The electrochemical formation of selenium30,34,35 and tellurium28,29,31-33 atomic layers on gold single crystals has been investigated using scanning probe and ultrahigh vacuum (UHV) surface
analytical techniques. Comparison of these two elements indicates
some striking similarities and notable differences. The following
study examines the structures formed electrochemically by these two
elements on Au(100) and Au(111) single-crystal surfaces using STM.
Experimental
All solutions were made with reagent grade chemicals. The
house-distilled water line was fed through a Nanopure filter system,
* Electrochemical Society Active Member.
z E-mail: [email protected]
resulting in water with a resistivity greater than 18 MV cm. The
deposition solutions were 1 mM SeO2 (Johnson Matthey) and
0.25 mM TeO2, with 20 mM H2SO4 (Baker Analyzed), pH 2.2, as
the supporting electrolyte. All potentials were referenced to a
Ag/AgCl (3 M KCl) reference electrode.
The gold single-crystal electrodes were prepared by orienting
with Laue X-ray back-diffraction, using a two-axis goniometer, followed by mechanical polishing with successively finer grits, down to
0.3 mm. The goniometer and polishing system were obtained from
Southbay Technologies. Following mechanical polishing, the single
crystals were electrochemically polished at constant current in a
cyanide bath. Prior to each STM experiment, the single crystals were
cleaned in concentrated nitric acid and annealed in a methane/oxygen flame.
A Digital Instruments Nanoscope III electrochemical scanning
tunneling microscope was used for all STM experiments. Both in air
and in situ studies were performed, and no obvious differences
where detected. Deposits imaged in air were formed in an electrochemical H-cell, using a three-electrode potentiostat. Coulometric
data was obtained using an integrator circuit built into the potentiostat using the geometric area of the electrode surface without compensating for double layer charging or variation from the geometric
surface area. An STM electrochemical cell consisting of a compartment containing a Ag/AgCl reference electrode (Bioanalytical Systems) and an Au auxiliary electrode, was used for the in situ studies.
A small solution trough from the ref-aux compartment to the single
crystal was used to make electrical contact.44 Tungsten tips, etched
at 12 VAC in 1 M KOH, were used for all experiments. Tips used for
in situ imaging were coated with polyethylene.
Results and Discussion
Cyclic voltammetry.—Cyclic voltammograms are shown in
Fig. 1 for Au(100) and Au(111) crystals in 20 mM H2SO4, contain1
ing HSeO2
3 (Fig. 1a and b) and containing HTeO2 (Fig. 1c and d),
respectively. A comparison of the voltammetry for the two crystal
planes reveals some minor differences, for a given solution, but for
the most part it is very similar. There are even significant similarities
between the voltammograms in the different solutions (Fig. 1). For
instance, each voltammogram shows a well-defined initial reductive
feature at about 0.3 V, labeled peak A. The charge corresponding to
the A peak for selenium is about twice that for tellurium, however.
In terms of monolayers, the coverages corresponding to the A peaks
for tellurium were 0.32 and 0.27 monolayers on Au(100) and
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Au(111), respectively (based on a four-electron process) from measurement by an integrator circuit in the potentiostat. For selenium,
the coverages were 0.87 and 0.62 monolayers, respectively. Reversing the scan direction resulted in an oxidative stripping peak centered at about 0.55 V for tellurium and 0.77 V for selenium. While
both systems exhibit a large degree of irreversibility, selenium was
significantly more so, based on the 0.45 V separation of the deposition and stripping features, vs. 0.25 V for tellurium.
The nature of peak A is somewhat ambiguous. In the case of tellurium (Fig. 1c and d), it can be considered to be the result of an
underpotential deposition process, where the first atomic layer is
more stable than subsequent tellurium. In the case of selenium, it is
not as clear. Equilibrium potential measurements for electrodes coated with bulk selenium or bulk tellurium show potentials of 0.35 and
0.2 V, respectively. Peak A for tellurium is just positive of the tellurium equilibrium potential, and thus corresponds to an underpotential process. Peak A for selenium, however, is below the corresponding equilibrium potential, and thus corresponds to an overpotential process. This process for selenium is very similar to underpotential deposition, in that it results in the formation of an ordered
atomic layer on the single-crystal substrate. However, at the observed potentials, bulk Se is thermodynamically stable as well, and
1
Figure 1. Cyclic voltammogramsof HSeO2
3 and HTeO2 in 20 mM H2SO4 (pH 2.2). Scan rate: 5 mV/s. Solid line: first scan; dashed line: second scan; dash-dot
1
1
line: third scan (a, top left) HSeO2
on
Au(100),
(b,
bottom
left) HSeO2
3
3 on Au(111), (c, top right) HTeO2 on Au(100), (d, bottom right) HTeO2 on Au(111).
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Journal of The Electrochemical Society, 146 (3) 1019-1027 (1999)
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S0013-4651(98)09-061-2 CCC: $7.00 © The Electrochemical Society, Inc.
is only prevented from depositing to a significant extent by slow
deposition kinetics.
Both the selenium and tellurium systems exhibit a second surface-limited reduction feature, labeled B, prior to the onset of bulk
deposition. For tellurium, the B peaks occur at approximately 0.07 V
and are significantly smaller than the corresponding A peaks. Quantification of these features is complicated by their convolution with
bulk Te deposition, however, it appears that between 0.5 and 0.6 total
monolayers have been deposited by the time the potential is scanned
to 0.05 V. Reversing the scan results in an oxidative stripping peak
around 0.42 V. The B peak for selenium is better separated from the
bulk deposition wave and significantly larger than for tellurium.
Coulometric results indicate that by scanning the potential down to
0.0 V, a total of between 2.5 and 3 monolayers of selenium have
deposited. The existence of a surface-limited peak that results in the
deposition of greater than a monolayer is unexpected, in general, for
a upd process. The peak does occur at a large overpotential, and
there is no distinguishable oxidative stripping feature, besides bulk
stripping, assignable to it. It appears that the peak does not correspond to the formation of some surface structure with increased stability but is, again, the result of slow kinetics. The slow kinetics is
also demonstrated by the oxidative stripping features in the voltammetry. As progressively more selenium is deposited, the stripping features shift to a more positive potential. The final scans in
Fig. 1a and b show that the bulk stripping peak is shifted positive and
the stripping features of peak A appear as a shoulder on the positive
side of the bulk stripping peak. There is some overpotential associated with depositing these first three monolayers of Se, however,
subsequent deposition is even more difficult. A more detailed discussion of the voltammetry of selenium on gold has been previously published.34
Au(100).—The first structure observed for both selenium and tellurium on Au(100) was a 0.25 coverage p(2X2), and Fig. 2 is an in
situ STM image of the Te version. The adlayer atomic rows are in
the same direction as the substrate atomic rows, the [110], on the
Au(100) surface. The interatomic spacing was 0.6 nm, twice the
interatomic spacing of the substrate surface. Analogous selenium
structures have previously been published.34,35 The p(2X2)-Te
structure was formed by scanning to 0.2 V, through peak A (Fig. 1c),
whereas the p(2X2)-Se structure was observed at more positive
potentials, just into peak A (Fig. 1a), or about 0.37 V. A model structure of the p(2X2) unit cell is presented in Fig. 3, where the ad-
Figure 2. STM image of Au(100)p(2X2)-Te formed at 10.20 V. Tunneling
conditions: Vbias 5 28.3 mV, it 5 3.2 nA.
sorbed tellurium atoms are drawn at their van der Waals diameter of
0.44 nm. 45 Formation of a homogeneous p(2X2) structure was difficult to accomplish for both the selenium and tellurium systems,
defects and other higher coverage features were frequently present
as well. In Fig. 2, for instance, a series of point defects and zigzagging chains of Te atoms are clearly displayed. Figure 3 is a schematic drawing of the p(2X2), and presents a structure proposed to
account for the chains. The Te chains appear to be phase boundaries,
resulting from the intersection of offset p(2X2) domains. The
domain structures differ by only a !w
2 shift in the high coordinate
sites used by the Te atoms, resulting in the Te atoms moving closer
together, changing from their normal spacing of twice the substrate
spacing to !w
3 times that spacing. One such boundary is observed in
Fig. 2 as a zigzagging chain, moving from the lower right to the
upper left. There are also phase boundaries where a chain is not
formed, where the interatomic spacing at the boundary is actually
larger, !w
10 times the substrate distance. There is also an example in
Fig. 2, starting at about the 5 nm mark on the x axis, and intersecting the right-hand y axis just above the 10 nm mark. This phase
boundary does not show up as well as those where the shift is to a
!w
2 distance, however it is evident if you follow a row of atoms perpendicularly across this boundary.
Further deposition resulted in a 0.33 coverage Au(100)(2X!w
10)
structure for both selenium and tellurium (Fig. 4a and b). This structure consists of zigzagging chalcogenide chains, where the distance
between individual atoms in the chain is !w
2, and the inter-chain spacing is !w
10. A model for this structure is presented in Fig. 5. This structure was formed by scanning through the selenium A peak (Fig. 1A),
to 0.35 V, and by scanning to the plateau between peaks A and B, 0.15
V, for tellurium. It is interesting that the chains that make up the
(2X!w
10) structure are identical to the !w
2 chain phase boundary discussed above with respect to the p(2X2) structure (Fig. 2 and 3). The
fact that chalcogenide chains form on the surface is not surprising, as
the crystalline form of tellurium, and the gray metallic allotrope of
selenium, both consist of a network of similar chains.46
In both cases, Se and Te, the conversion of the initial p(2X2) to
the (2X!w
10) involve the formation of an increasing number of the !w
2
phase boundaries (Fig. 3), until the (2X!w
10) results as a closepacked layer (Fig. 5). The mechanism of this conversion may be
something like a zipper. That is, as the coverage increases, the interatomic pressure builds. In order to incorporate more atoms than the
Figure 3. Model of the proposed structure of Au(100)p(2X2) at 0.25 coverage. The observed defects, missing atoms, and domain boundaries, are also
shown.
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0.25 coverage of the p(2X2), the structure must change. Keeping
high coordinate sites and pushing atoms closer together, results in
initiation of a !w
2 phase boundary (Fig. 2 and 3). The atoms are initially near their van der Waals diameter, where there is probably very
little orbital overlap of adjacent chalcogenide atoms. When they are
pushed into a !w
2 interatomic spacing, they are forced to interact,
overlap, initiating the formation of a chain molecule. This point of
view suggests that the p(2X2) atoms have a significant mobility, and
readily shift registry with the substrate, so that where initially there
was one domain of the p(2X2) structure, as the chain grows, two out
of phase domains must result. Chain growth may be initiated by
deposition of a few atoms into an existing p(2X2) domain. Chain
growth, however, involves a higher density of atoms than the p(2X2),
and may continue by scavenging nearby atoms. The result is a
decreased number of atoms available in the remaining domains of
Figure 5. Model of the proposed structure for the Au(100)(2X!w
10).
p(2X2). This helps explain the atomic defects and the presence of the
!w
10 phase boundaries (Fig. 2 and 3), both of which represent lower
coverages than the p(2X2) domains.
At higher coverages, selenium forms a 0.50 coverage c(2X2)
structure (Fig. 6). This structure consists of a square arrangement of
selenium atoms, probably in high coordinate sites, with an interatomic spacing of !w
2 (Fig. 7). The c(2X2) was formed by scanning
the potential to the valley past peak A (Fig. 1a), 0.30 V. Coexisting
with the c(2X2) in Fig. 6 are bright clusters that increase in frequency as still more selenium is deposited. Analysis of these bright clusters shows them to be square planar Se8 rings, formed on the surface.34 While the rings appear to be sitting higher on the surface than
the atoms in the surrounding c(2X2), the height difference between
the rings and the c(2X2) atoms was found to be only about 0.1 nm,
suggesting that they are not a second layer of Se, as was initially
thought. The increased brightness of the rings may be an electronic
Figure 4. STM images of the Au(100)(2X!w
10 ) at 0.33 coverage. (a, top)
Au(100)(2X!w
10)-Se. Tunneling conditions: Vbias 5 50 mV, it 5 4.0. (b, bottom) In situ image of Au(100)(2X!w
10 )-Te formed at 0.17 V. Tunneling conditions: Vbias 5 225 mV, it 5 1.3.
Figure 6. STM image of the Au(100)c(2X2)-Se. Tunneling conditions:
Vbias 5 53 mV, it 5 4.0.
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Figure 9. STM image showing the Au(100)(3X!w
10 )-Se.
Figure 7. Model of the proposed structure of the Au(100)c(2X2)-Se.
effect, as the electronic structure of the selenium atoms is significantly perturbed in converting between the relatively isolated Se
atoms in the c(2X2) to what appear to be Se8 molecules on the Au
surface. An alternative possibility to an electronic effect is presented
in Fig. 8. A comparison is made, suggesting that in the c(2X2) structure the atoms are all laying in their high coordinate sites, however,
when the Se8 molecule is formed, there is no simple way to get all
its atoms into high coordinate sites and it essentially floats on top,
thus giving it its increased intensity. In addition, it is generally
observed in STM images, the higher the packing density, the brighter
the images appear, as there is less dark space between atoms in the
rings, and the feedback modes used may have a tendency to accentuate close-packed high points.
At still higher coverages, the rings become close packed (Fig. 9).
Evidence from low energy electron diffraction (LEED) and STM
suggest that the unit cell most closely corresponding to this structure
is a (3X!w
10 ). That is, the periodicity of the rings is about three times
the substrate, for rings lined up in a row. However, it appears that
successive rows are shifted by one substrate atomic distance, resulting in the !w
10 distance in the unit cell.34 A proposed structure is
shown in Fig. 10. Selenium rings have previously been observed in
surface studies of Se on highly oriented pyrolytic graphite (HOPG)13
although the number of atoms reported per ring was four to six in
that study. Analogous S8 rings have previously been observed to
form on gold,7,47 as well. As with the chains observed in the
(2X!w
10 ) structure (Fig. 4 and 5), Se8 rings are found in three stable
allotropes of selenium, although in puckered boat configurations.46
The fact that the rings observed here are distorted from the stable
boat configuration is understandable, considering the obvious interaction between the rings and the gold surface.
Figure 8. Model showing the adsorption sites of the Se8 rings vs. other proposed structures, which have atoms placed in high coordinate sites.
Scanning into the second feature in the voltammogram for tellurium on Au(100) results in a structure that has been reported as a
(!w
2 X!w
5 ).29 Attempts to image this in situ have been complicated by
the onset of three-dimensional tellurium deposition and no images
with satisfactory atomic resolution have yet been obtained. That threedimensional growth occurs can be understood by noting the position
of this second voltammetric feature on the foot of the bulk Te deposition wave (Fig. 1c). That is, as the (2X!w
10) converts to the higher cov-
Figure 10. Model of the proposed structure for the Au(100)(3X!w
10)-Se at
0.89 coverage.
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erage structure, bulk tellurium deposition is beginning as well. There
is no evidence that Te8 rings are formed at higher coverages.
Au(111).—Both selenium and tellurium form Au(111)
3X!w
3 )R308 structures that exhibit large triangular features. The
(!w
tellurium !w
3 was formed by scanning through the A peak, to 0.20 V
(Fig. 1d), while the selenium !w
3 was formed by scanning to the to
the tip of the A peak, 0.30 V (Fig. 1b). In both images (Fig. 11a and
b) the interatomic spacings ranged from 0.49 to 0.52 nm, in good
agreement with the !w
3 distance on Au, 0.50 nm. Images of the selenium !w
3 exhibited triangular domain boundaries, randomly over the
surface (Fig. 11a). The triangles themselves were between 4 and
6 nm in size.30,48,49 In the case of tellurium, the triangles appeared
as a network of phase boundaries (Fig. 11b). The dimensions of
those triangles ranged from 3.3 to 3.6 nm, consistent with a (12X12)
periodicity observed using both LEED and STM (3.48 nm).27,29,50 A
Figure 11. STM images of the Au(111)(!w
3 X!w
3 )R308 structures. (a, top)
Au(111)(!w
3 X!w
3 )R308-Se showing the triangular domain boundaries. Tunneling conditions: Vbias 5 27 mV, it 5 6.0 nA. (b, bottom) In situ image of
the Au(111)(12X12)-Te formed at 10.20 V. Tunneling conditions: Vbias 5
212 mV, it 5 3.0 nA.
structure proposed to account for this periodicity is shown in Fig. 12.
The LEED pattern previously observed for this surface27 consisted
of hexagons of spots at the !w
3 positions. The spots were split in the
(NXN) directions, with a distance of one-twelth of that corresponding to the clean Au surface (1X1). The conclusions drawn were that
a (12X12) unit cell was present, with a high degree of (!w
3X!w
3 )R308
character. STM images of this surface, such as that shown in
Fig. 11b, clearly evidence the twelve periodicity as well as the !w
3.
In the proposed model, the darker lines with the twelve periodicity
result from a phase shift in the !w
3 structure. That is, tellurium atoms
in a particular triangular domain differ from those in neighboring triangular domains by only a phase shift. For a given domain, at the
phase boundaries, instead of the next atom being !w
3 times the AuAu distance away (0.50 nm), it is two times, or 0.58 nm away, corresponding to the next closest high coordinate site. A probable reason for this shift is that the atoms are just a little too big, they end up
pushing each other out of the !w
3 sites when the domain gets above
a certain size. As mentioned above, the van der Waals diameter for
Te is about 0.44 nm, suggesting that there should be room in the !w
3
structure (0.50 nm between site), however, it is hard to quantify the
amount of crowding required to achieve this structure.
A recent article has suggested that the actual coverage may be
significantly higher than the one-third coverage proposed here, more
like 0.42 coverage, and that the observed (12X12) modulation in the
intensity is actually an electronic effect, or caused by very minor
shifts in the atomic positions of a hexagonal close-packed adlayer of
Te atoms.33 The central difference between that work and the structure shown in Fig. 12 is the coverage. Presently, studies are underway to try and differentiate between the two possibilities.
The triangular domains in the selenium layers (Fig. 11a) also
appear to result from a phase boundary. The van der Waals diameter
for Se is smaller (0.40 nm) than that for Te, and crowding is probably not a factor. However, a large number of islands of the Se !w
3
structure begin to grow at very low coverages, probably less than 0.1
monolayer. The atoms appear to have a low mobility, so the domains
or islands remain until the coverage grows and the domains are
forced together, resulting in a random array of phase boundaries as
the islands meet. As the surface approaches the ideal !w
3 coverage of
one-third, the number of phase boundaries decrease, and the surface
becomes essentially a single domain.30 The last remnants of the
phase boundaries are these triangles. This growth process appears to
indicate that the surface mobility and exchange current for the Se
deposition process is very low. That is, the Se atoms do not move
much, until the coverage is high enough that some domains get
pushed into other sites, erasing the phase boundaries.
There appear to be great similarities in the triangular features in
the two surfaces shown in Fig. 11, and there are. They both result
from phase boundaries between domains of a (!w
3 X!w
3 )R308 struc-
Figure 12. Model of the Au(111)(12X12)-Te structure.
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Figure 13. STM image of the Au(111)(!w
3 X!w
7 )-Se, or Se8 ring structure,
coexisting with the Au(111)(!w
3 X!w
3 )-Se. Tunneling conditions: Vbias 5
29.5 mV, it 5 4.4 nA.
ture. The reasons for the phase boundaries, however, appear different for the two elements. In the case of the (12X12) Te structure, the
atoms are too big and a periodic array of phase boundaries is created to relieve the stress of overcrowding, caused by Te atoms which
are too big to form a single domain of the !w
3 . In the Se structure, the
triangular domains are the smallest phase shifted domains that can
exist, before they are completely wiped out, as the coverage is
increasing. They appear to be a byproduct of a low surface mobility
for the depositing Se atoms.
Four distinct selenium structures were described above for the
Au(100) surface, culminating with the growth domains of Se8 ring.
On the Au(111) surface, by the time the (!w
3 X!w
3 )R308-Se structure
was formed, domains of Se8 rings, analogs to those on Au(100),
were observed (Fig. 13), again indicating the irreversibility in the
Figure 15. In situ STM image showing the two domains of the
Au(111)(3X3)-Te structure formed at 10.05 V. Tunneling conditions: Vbias 5
260 mV, it 5 7.0 nA.
selenium deposition process. The rings coexist with domains of the
(!w
3 X!w
3 )R308 structure until they covered the whole surface. That is,
it was not possible to form a full surface covered with only the
(!w
3 X!w
3 )R308-Se structure, as some of the eight-ninths coverage Se8
ring structure would form as well.
The dimensions of the rings are about 0.58 nm on a side, based on
the model structure shown in Fig. 14, leading to a Se-Se interatomic
spacing of 0.29 nm. There appears to be little difference between the
Se8 rings formed on Au(111) and those formed on Au(100), except
that those on (111) are frequently deformed into a slight diamond
shape, as might be expected when trying to place square rings on a
hexagonal surface. The nominal unit cell that could accounts for the
close-packed structure in Fig. 14, would be a (3X!w
7 ).
The Te deposit formed by scanning to 0.05 V is displayed in
Fig. 15. The structures corresponding to this image are not yet clear.
On the left, chains are evident, similar to those seen on Au(100)
(Fig. 4b), however, most of the surface is covered with a roughly
hexagonal lattice, with an apparent (3X3) unit cell.
The interatomic distances measured along individual rows in the
hexagonal structure varied from 0.43 to 0.50 nm, depending on the
direction measured, while the interatomic distances of atoms in the
chains varied from 0.34 to 0.36 nm. Measuring between the chains,
in the direction of the gold substrate atomic rows, indicates a distance of between 0.85 to 0.95 nm, again somewhat dependent on the
direction measured. Given that the variability was the result of thermal drift, noticed in this experiment, two structures are proposed in
Fig. 16a and b to account for the domains in Fig. 15. Both are fourninths coverage (3X3) structures. LEED patterns,27 ex situ STM
images,29 and in situ AFM images32 corresponding to a four-ninths
coverage (3X3) have all been previously reported in the literature.
While significant variation exists between the observed distances
and the ideal 3X3 distance of 0.87 nm, this can, again, probably be
accounted for by the drift. Further studies of this surface are underway. No rings were observed.
Conclusions
Figure 14. Model showing the Au(111)(!w
3 X!w
7 )-Se at 0.89 coverage.
Significant similarities exist between the atomic layers of selenium and tellurium formed by electrodeposition on Au(100) and
Au(111). Both selenium and tellurium form p(2X2) and (2X!w
10 )
structures on Au(100). Further deposition of selenium resulted in
two higher coverages structures, a 0.50 coverage c(2X2) and a 0.89
coverage (3X!w
10 ), consisting of square planar Se8 rings. On
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1026
Journal of The Electrochemical Society, 146 (3) 1019-1027 (1999)
S0013-4651(98)09-061-2 CCC: $7.00 © The Electrochemical Society, Inc.
Au(111) selenium and tellurium both form one-third coverage
(!w
3 X!w
3 )R308 structures, exhibiting triangular domains surrounded
by phase boundaries. These domains exhibit a (12X12) periodicity
in the tellurium structure, whereas with selenium, the triangular
domains are more heterogeneous in size and distribution, usually
occurring as isolated features. Further deposition of selenium again
results in the formation of Se8 ring structure, a (3X!w
7 ), very similar
to the Au(100)(3X!w
10)-Se structure. Tellurium, upon further deposition, forms a mixed surface, composed of domains of two fourninths coverage Au(111)(3X3)-Te structures. Chains are observed in
one of these (3X3)-Te structures. Rings and chains are observed in
several of the selenium and tellurium structures, consistent with the
fact that stable allotropes of these elements also consist of rings and
chains. The periodic arrays of phase boundaries, seen in the
(12X12)-Te structure, appear to result from the Te atoms being too
big to fit in an extended domain. The phase boundaries then act as a
kind of stress relief. On the other hand, the isolated triangles
observed with the Se !w
3 adlattice occur because there is too little surface mobility, and multiple domains, formed during the initial stages
of deposition, are only slowly removed with the increasing coverage
of Se. The observed triangles are then the last remains of phase shifted domains. This low surface mobility is consistent with the slow
electrodeposition kinetics observed.
Acknowledgments
This work was supported in part by the Navy, Office of the Chief
of Naval Research, under grant no. N00014-91-J-1919, and by the
National Science Foundation, under grant no. DMR-9017431. Their
assistance is gratefully acknowledged.
The University of Georgia assisted in meeting the publication costs of this
article.
References
Figure 16. Model showing the two domains of the Au(111)(3X3)-Te structure. (a, top) Au(111)(3X3)-Te chains and (b, bottom) Au(111)(3X3)-Te
hexagonal.
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