Surface Science 426 (1999) 212–224
www.elsevier.nl/locate/susc
Interaction of bromine with Ni(110) studied by scanning
tunnelling microscopy
T.W. Fishlock a, J.B. Pethica a, A. Oral a, R.G. Egdell b, *, F.H. Jones b
a Department of Materials, Oxford University, Parks Road, Oxford OX1 3PH, UK
b New Chemistry Laboratory, Oxford University, South Parks Road, Oxford OX1 3QT, UK
Received 4 November 1998; accepted for publication 29 January 1999
Abstract
The adsorption of bromine on Ni(110) has been studied by scanning tunnelling microscopy (STM ). At low
bromine coverage ‘butterfly’ structures are observed in STM, built up from ‘pairs of pairs’ of greyscale maxima. Two
alternative models for the butterflies are discussed, the most plausible of which involves location of dissociated Br
atoms in two-fold hollow sites flanked by pairs of Ni adatoms.
At higher coverages the discrete butterfly structure breaks down and adatom pairs form a disordered arrangement
on the surface. Annealing a Br-saturated surface to 200°C for 1 h leads to formation of a well-ordered p(3×2)
reconstruction that produces sharp superstructure spots in LEED. The superstructure involves alternating rows with
atomic sequences Ni–Br–Br–Ni–Br–Br–Ni and Br–Ni–Ni–Br–Ni–Ni–Br.
The binding of Br butterflies to Ni(110) at low coverage is too strong to allow facile manipulation with the tip of
the STM. © 1999 Elsevier Science B.V. All rights reserved.
Keywords: Bromine; Interaction; Ni(110); Nickel; Scanning tunnelling microscopy (STM )
1. Introduction
There is a growing interest in the interaction of
halogens with simple metal surfaces because of the
possibility of exploiting halogen etching processes
in the fabrication of metal interconnects in microelectronic devices [1,2]. However, our own interest
in the area derives from the fact that halogens
adsorbed on metal surfaces represent systems with
good prospects for the study of manipulation of
individual atoms on surfaces at room temperature
in a scanning tunnelling microscope (STM ). The
controlled and reproducible modification of surfaces at the atomic scale by STM offers long term
* Corresponding author.
E-mail address: [email protected] (R.G. Egdell )
prospects of technological application in a wide
range of areas. STM can also be applied to probe
physical and chemical processes on an extremely
local scale [3]. A crucial requirement in this area
is an understanding of the forces required to
initiate atomic motion. Clarke and coworkers [4,5]
studied variation of STM image corrugation height
with tunnel current. ‘Roll off ’ in the corrugation
at high current was attributed to the action of
strong tip–surface forces during the accumulation
of STM images. Molecular dynamic simulations
were shown to provide a quantitative description
of the roll off behaviour. In the earlier work, tip
induced motion of halogen atoms produced
by dissociation of 1-chloro-2-bromoethane on
Cu(100) was observed under the conditions where
corrugation roll off was observed [5]. Elsewhere,
0039-6028/99/$ – see front matter © 1999 Elsevier Science B.V. All rights reserved.
PII: S0 0 39 - 6 0 28 ( 99 ) 0 03 2 3 -4
T.W. Fishlock et al. / Surface Science 426 (1999) 212–224
Altman and coworkers have found that imaging
of Br and Cl on Cu(100) surfaces is difficult at
low coverage [6–8]. This may be due to tip induced
motion of halogen atoms, although there is also
the possibility that thermal fluctuations inhibit
observation of the adsorbate atoms. The present
paper deals with the bromine/Ni(110) adsorbate system. The binding of Br is expected to
be stronger in this system than for halogens on
Cu(100), and in addition the row-like structure of
the Ni(110) surface gives scope for exploring
anisotropy in tip induced atomic motion.
There are surprisingly few STM studies involving halogen adsorption on metals. Most of the
work to date has concentrated on close-packed
(111) faces of Ag [9–11] and Cu [12]. However,
recent studies of Br and Cl adsorption on Cu(100)
in UHV by Altman and coworkers [6–8] have
revealed well-defined halogen overlayers in a
c(2×2) reconstruction, with formation of a CuCl
or CuBr overlayer at higher coverage. Halogen
adsorption on Cu(100) [13–15] and Ag(100) [16 ]
has also been studied in an electrochemical environment. To our knowledge there has been no
previous structural study of Br on Ni(110) by
STM or indeed by LEED or other diffraction
techniques.
By contrast, the Ni(110) surface has proved a
popular substrate for the study of other adsorbate
systems: the range of atomic species studied
includes O [17–20], S [21], C [22], N [23], H [24]
and Au [25], as well as a number of co-adsorbate
systems [26–28]. Molecular adsorption of CO [29]
and C H [30] has also been studied with sufficient
6 6
resolution to allow determination of the binding
site. Chlorine adsorption on Ni(110) has been
studied extensively in this laboratory by STM and
LEED [31]. In agreement with Shuxian et al. [32],
it was found that even though chlorine appears to
chemisorb dissociatively, the Cl adatoms remain
bound in pairs in adjacent two-fold hollow sites
at room temperature. Assignment of the greyscale
maxima in the images to Cl atoms was made by
analysis of atomically resolved STS data [31]. The
Cl atom pairs had a strong propensity to migrate
to step edges after mild annealing, where they were
strongly bound and not amenable to manipulation
with the tip of the STM.
213
Here we present a scanning tunnelling microscopy study of the interaction of molecular bromine with Ni(110) at room temperature. The main
emphasis is on the nature of species formed at low
coverage, although we have also investigated
ordered structures formed at half monolayer coverage. At low coverages the chemisorbed bromine
forms a basic structural unit imaged as two pairs
of greyscale maxima in STM. The pairs are oriented with their axes parallel to one another to
form what can be described as a ‘butterfly’ structure. Attempts were subsequently made to manipulate the butterfly structures with the STM tip, but
it was found that disruption of the Ni substrate
and tip degradation was induced before the Br
could be moved.
Images recorded at higher bromine coverages
display marked differences depending on sample
treatment. High coverage images recorded immediately after dosing again show pairs of greyscale
maxima oriented along the [001] direction, but the
additional doubling up of the pairs observed at
low coverages is no longer a predominant feature.
Annealing the sample after a high bromine dose
reveals a well-ordered and well-characterised
p(3×2) surface reconstruction.
2. Experimental
A semicircular Ni(110) crystal 10 mm in diameter and 3 mm thick (Metal Crystals Inc.,
Cambridge, UK ) was mechanically polished with
diamond paste down to 0.25 mm and then ultrasonically rinsed in double distilled water. The crystal
was mounted on a Ta sample-holder using Ta wire
clips. All the experiments were performed with a
commercial Omicron UHV–STM operating at
room temperature. Electrochemically etched tungsten tips were used in all experiments. The STM
is mounted in an ion- and turbo-molecular pumped
UHV chamber with a base pressure of 4×
10−11 mbar. The UHV chamber is equipped with
facilities for X-ray photoelectron spectroscopy
( XPS), low energy electron diffraction (LEED)
and argon-ion sputtering. Annealing is performed
by radiative heating on the rear side of the mount.
All temperatures quoted in the present work relate
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T.W. Fishlock et al. / Surface Science 426 (1999) 212–224
to the temperature measured on a thermocouple
spot welded to the sample manipulator close to
the sample plate, in the standard Omicron
arrangement.
The initial cleaning of the surface was achieved
with repeated cycles of Ar ion sputtering (1.4 kV,
10 mA, 30 min) and annealing (700°C, 20 min) until
LEED patterns indicated a well-ordered (1×1)
surface, and no impurities could be detected by
XPS or STM imaging.
An electrochemical Ag/AgBr–CdBr /Pt cell was
2
used as a UHV-compatible molecular bromine
source [33]. Two such sources were used one after
the other and the results were found to be reproducible between them. The sources were mounted
in line of sight of the sample in its LEED/XPS
position and could be advanced to within 2 cm of
the sample during Br dosing. Before use the sources
were extensively outgassed to ensure that there
was no O or HBr present in the impinging Br
beam. Characteristic O induced features were
found to be absent from STM images. The Br flux
in the impinging beam could not be measured
directly, but the Br coverage as gauged by XPS
and STM was found to be proportional both to
the electrochemical cell current×time (typical cell
currents were 0.1–0.2 mA) and to the rise in background pressure×time. Surface coverages were
ultimately estimated directly from the STM images
themselves.
3. Results and discussion
3.1. Low Br coverage studies
The (110) surface is the most open of the low
index surfaces for ccp metals. The surface involves
close packed rows running along the [11: 0] direction, with an atomic separation of a/앀2 within the
rows and a between rows, where a is the ccp lattice
parameter (3.52 Å). The surface atomic density is
therefore 앀2/a2=1.14×1015 Ni atoms/cm2. Fig. 1
shows an STM image of a flat terrace of the sort
that dominated the STM images, although atomic
scale steps were occasionally observed scattered
across the surface to give an average terrace width
Fig. 1. 30 Å×32 Å image of a typical terrace area on the clean
Ni(110) surface (tunnel current 5 nA, sample bias −0.14 V ).
The corrugation amplitude along the close packed rows is
approximately 0.1 Å. The high symmetry directions are indicated. All other figures have images aligned in essentially the
same direction unless otherwise indicated.
of about 500 Å. Optimal atomic resolution along
the close packed Ni rows was achieved at a relatively high sample bias of −0.14 V, although
images of adsorbate induced features were more
usually acquired at lower sample biases around
−0.01 V. The tunnelling conditions that give optimal images of adsorbates are not therefore ideal
for imaging of the Ni substrate.
Low, but significant, concentrations of defects
were observed in the close packed rows, sometimes
involving a small aggregate of vacancies. These
vacancies could be distinguished from adventitous
C adsorbate atoms (which were also imaged occasionally) by the fact that the maximum greyscale
change corresponded to the substrate atomic positions rather than the hollow sites of the (110)
substrate. Fig. 2 shows a high resolution image of
one cluster involving three Ni vacancies, now taken
with tunnelling conditions that were subsequently
used to image adsorbates. An important feature
of this image is that there is suppression of tunnelling not just in the upper rows from which the Ni
T.W. Fishlock et al. / Surface Science 426 (1999) 212–224
215
Fig. 2. 24 Å×24 Å image of Ni(110) showing the presence of
three Ni vacancies in the close packed rows whose positions are
indicated with an ×. Tunnel current 4 nA, sample bias
−0.01 V.
atoms are removed, but also in the adjacent subsurface rows.
The exposure of the clean Ni(110) surface to a
very low dose of molecular bromine results in the
formation of an adsorbed species imaged as two
discrete pairs of maxima. The feature can be
described as having a ‘butterfly’ shape. Fig. 3a and
b are images recorded after a nominal bromine
dose of 5×10−9 mbar s showing respectively one
and two butterfly features: the surface coverage is
estimated to be about 2.3×1013 Br atoms/cm2 at
this stage (see below for discussion of calibration
of surface coverage). The greyscale maxima of the
butterfly lie inset from the close packed [11: 0] rows
of the substrate. Two close packed rows run into
the butterfly so that the overall separation between
the greyscale maxima along the [001] direction is
just over twice the terrace periodicity along this
direction (7.04 Å). The separation between the
maxima along the [11: 0] direction is twice the close
packed separation (4.98 Å). There is pronounced
suppression of greyscale intensity in the vicinity of
the maxima of the butterfly. This is strongest
between the two central rows, offset outward from
the positions of the topographic maxima along the
Fig. 3. Images of Ni(110) after bromine exposure to give surface
coverage of 2.3×1013 Br atoms/cm2. Tunnel current 1 nA,
sample bias −0.01 V. (a) 50 Å×49 Å area containing a single
butterfly feature. (b) 50 Å×46 Å area containing two butterfly
features. The close packed rows are highlighted in (b), thus
allowing location of the adsorption site relative to these rows.
[11: 0] direction. ‘Doubled’ features of the sort
found in the present work often arise from multiple
tip effects. At the outset we rule out this possibility.
Butterfly features alone were observed at low bromine exposure on Ni(110) with seven different tips
and with two different STM control systems. The
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T.W. Fishlock et al. / Surface Science 426 (1999) 212–224
butterflies were of a consistent size and always
oriented in the same direction relative to the
crystallographic directions of the surface regardless
of the scan direction (see below, Fig. 6). Finally,
Br adsorption on substrates of different symmetry,
notably Cu(100), never gave rise to butterfly
features.
In the simplest picture, the greyscale maxima
of the butterflies are interpreted as bromine atoms,
which occupy two-fold hollow sites between the
close packed rows. The greyscale maxima do not
coincide exactly with the centres of the hollows,
but previous experience with simulation of STM
[34] images highlights the danger of assuming that
image maxima correspond exactly to the lateral
position of adsorbate induced features. Each atom
pair can be envisaged to arise from dissociation of
a single Br molecule. However, it is very difficult
2
to reconcile this interpretation with the apparent
distribution of the Br pairs on the surface: for low
exposures the probability of a second Br molecule
2
impinging on the surface in the vicinity of a preexisting Br atom pair is very low. Given that the
2
thermal mobility of the pairs is low at room
temperature (see below), there is no obvious mechanism for pairing of the putative bromine pairs
into the pairs of pairs that constitute the butterfly
structures.
We therefore suggest that the greyscale maxima
correspond to Ni adatoms in two-fold hollow sites
adjacent to a dissociated Br atom, each pair being
bound to a single Br atom. The appearance of the
four maxima in the butterfly arises from the dissociation of Br into two Br atoms and enhanced
2
greyscale intensity of two Ni atoms adjacent to
each of the two Br atoms. This interpretation of
the STM images is in some ways similar to that
proposed for O on Ni(110) [17–20] and Cu(110)
[35], where added row structures are formed upon
oxygen chemisorption and the Ni or Cu adatoms
in two-fold hollow sites appear as maxima in the
STM images. In these systems the chemisorbed O
occupies bridge sites. However, Br is much larger
than O. The covalent radius for Br (1.14 Å) is very
similar to the metallic radius of Ni in the bulk
metal (1.25 Å) and thus Br is more easily accommodated in the larger hollow sites. In contrast to
the O–Ni(110) system, it does not appear that the
Ni adatoms are selectively removed from step
edges. Instead the adatoms derive from the close
packed rows at the centre of the butterfly. This
accounts for the strong suppression of the tunnelling current in the vicinity of the Br adsorbate: the
proposed Ni vacancies in the butterfly appear very
similar to intrinsic vacancy defects such as those
of Fig. 2. However, electronic structure changes
associated with Br adsorption may also contribute
to the overall appearance of the butterfly features.
The Ni vacancies create a highly local ‘missing
row’ structure within the Ni(110) surface. Missing
row reconstructions are of course a major feature
of the surface structural chemistry of heavier fcc
metals [36 ] and of adsorbate systems based thereon
{e.g. for O on Pd(110) [37,38] or Rh(110)
[39,40]}. A relatively low energy is required to
transfer an atom from a close packed row to an
adjacent two-fold hollow site on Ni(110), even
though the intrinsic 1×1 reconstruction is more
stable than a missing row reconstruction on the
bare surface. The ease of formation of Ni vacancies
will of course be enhanced by the binding to the
chemisorbed Br atoms. A schematic structural
model for the butterflies is shown in Fig. 4a. In
contrast to Cl on Ni(110) [31], the Br atoms
deriving from dissociation of a given Br molecule
2
do not occupy immediately adjacent two-fold
hollow sites. A simple rationale for this observation is provided by the fact that the two Br atoms
cannot simultaneously maintain coordination by
the four surrounding Ni atoms from the close
packed rows and extend their coordination number
by 2 due to the Ni adatoms unless they are
separated by at least 2a long the [001] direction.
Surface coverages quoted in the rest of this
paper are based on the assumption that each
butterfly contains two Br atoms. At low coverage
it is easy to count the number of butterflies appearing in a sequence of large area images, whilst
higher coverages are derived assuming that the
flux from the deposition source is proportional to
the electrochemical cell current and that the sticking coefficient stays constant. Unfortunately, the
strongest Br core line – Br 3d – overlaps the Ni
3p peak and it is not possible to quantify Br
coverage by XPS at low coverage. However, the
higher coverages derived from Br 3p peaks in XPS
T.W. Fishlock et al. / Surface Science 426 (1999) 212–224
217
After room temperature adsorption, the butterflies have no tendency to cluster. However, gentle
heating of surfaces with coverages between
0.5×1013 Br
atoms/cm2
and
3.0×1013 Br
atoms/cm2 promoted aggregation of butterflies.
For example, Fig. 5a and b show typical images
Fig. 4. (a) Schematic representation of structure of butterfly
feature induced by room temperature adsorption of Br on
2
Ni(110). Dark spheres are Br, light spheres are Ni. The Ni
adatoms which appear as greyscale maxima in STM are highlighted by arrows. The positions of Ni vacancies are indicated
with an ×. (b) Representation of modified butterfly structure
occasionally observed after annealing Br-exposed surface, with
increased separation between BrNi units along the [001] direc2
tion. Details are as in (a).
are in line with the independent determinations
by STM.
Large area STM images taken immediately after
a small exposure to bromine show a random
distribution of butterflies. Unlike chlorine
adsorbed on Ni(110), the Br induced features
show no propensity to adhere to step edges [30].
This difference may be understood by recognising
that the driving force for migration of Cl to step
edges is that at step sites the coordination number
n of Cl adsorbate atoms increases from n=5
characteristic of two-fold hollow sites on the terraces to n=6. The presence of two adatoms adjacent to Br allows a coordination of n=7 even on
the terraces and there is no preferential adhesion
at step edges.
Fig. 5. Images of Ni(110) after bromine exposure to give surface
coverage of 2.3×1013 Br atoms/cm2 with subsequent anneal to
100°C for 10 min. Tunnel current 1 nA, sample bias −0.01 V.
(a) 50 Å×50 Å area showing aggregation of units along [11: 0]
direction. (b) 33 Å×29 Å area showing aggregation of units
along [001] direction.
218
T.W. Fishlock et al. / Surface Science 426 (1999) 212–224
after annealing a surface with a coverage of
2.3×1013 Br atoms/cm2 to 100°C for 10 min. The
butterfly species are still observable after annealing
but now form adsorption islands of between two
and eight units. The most likely mechanism for
aggregation involves diffusion of the BrNi units
2
along the [11: 0] direction in the troughs between
the close packed rows. It is remarkable that in this
process the pairs of BrNi units appear to remain
2
bound together. The aggregation ultimately
depends on diffusion of a complete (BrNi ) dimer
22
until the mobile butterfly unit finds itself adjacent
to another butterfly along either [11: 0] or [001]
directions. The butterflies then remain bound in
pairs until further diffusing butterflies impinge on
the pair to form a larger aggregate.
Some images provide evidence for diffusion
along the [001] direction. Thus Fig. 6a shows an
image of a 2.5% Br covered surface after annealing
to 100°C for 10 min. Four of the bromine doublepair ‘butterfly’ features are observed. However, the
structure of at least one of these butterflies differs
from those in Fig. 3. As highlighted by the guidelines in the figure, three close packed rows now
run through the butterfly and the separation of
the greyscale pairs along the [001] direction is 3a=
10.56 Å. A schematic of this new butterfly structure
is shown in Fig. 4b: the evolution between the two
structures of Fig. 4 appears to involve diffusion of
a BrNi unit across a close packed row into
2
adjacent two-fold hollow sites. It is an incidental
feature of Fig. 6a that the resolution along the
close packed rows is much higher than in Fig. 3,
thus allowing more definitive assignment of the
adsorption sites. Fig. 6b shows an image of the
characteristic butterfly aggregate of Fig. 6a taken
with a rotated scan direction. This figure serves to
illustrate that the appearance of the butterfly structure is independent of scan direction and rules out
the possibility that the structure arises from
multiple tip effects.
3.2. High Br coverage studies
Adsorption experiments were extended to coverage regimes up to about 0.6×1015 Br atoms/cm2,
at which point the surface saturates. No ordered
Fig. 6. (a) 60 Å×60 Å image of Ni(110) after bromine exposure
to give surface coverage of 2.3×1013 Br atoms/cm2 with subsequent anneal to 100°C for 10 min. Tunnel current 5 nA, sample
bias −0.01 V. Four Br butterfly features are visible. The guidelines highlight positions of close packed [11: 0] rows and [001]
rows, showing that the greyscale maxima along the [001] direction has increased compared with Fig. 3. (b) Image as in (a)
but with scan directions rotated. The butterfly features are
unchanged, demonstrating that the butterfly structure does not
arise from multiple tip effects.
T.W. Fishlock et al. / Surface Science 426 (1999) 212–224
superstructures were evident in LEED after room
temperature adsorption and STM images revealed
that increasing coverage simply involved increasing
numbers of disordered butterflies. At the highest
coverages discrete ‘pairs of pairs’ were still found
in the images but now along with individual
BrNi units and aggregates into trimers and larger
2
clusters. There was also evidence for extensive
roughening of the surface with formation of etch
pits. This is illustrated in Fig. 7a and b which
shows two typical large area images for a surface
with a coverage of 0.51×1015 Br atoms/cm2. It
should be emphasised that local BrNi units with
2
the two greyscale maxima remain a characteristic
feature of the image. As expected, the rough and
disordered surfaces formed by room temperature
Br adsorption gave diffuse LEED patterns with
no indication of superstructure spots.
Annealing samples with a high bromine coverage leads to major restructuring and reordering of
the surface. Fig. 8a is an STM image recorded
after a bromine dose giving a coverage of about
0.6×1015 Br atoms/cm2 (i.e. just over half a monolayer) and a subsequent anneal to 200°C for
20 min. There is no indication from XPS or mass
spectroscopy of pronounced Br desorption under
this mild annealing. However, major mass transport has occurred on the surface and the topography is now dominated by rounded ridge-like
features running along the [001] direction. This is
attributed to adsorbate induced facetting of the
substrate. The ridges have typical width of about
30 Å in the [11: 0] direction and are at least 250 Å
long in the [001] direction. The top of the ridges
flattens off from sloping sides and some are topped
by two ridges, 10 Å apart, running down the length
of each rod. Atomic structure along the ridges was
never well-resolved, possibly due to the very large
dynamic range in the z-direction (the height from
trough to peak of the rods is approximately 5 Å)
disrupting the tip. The degree of long range order
of the adsorbate induced facets was not sufficient
to produce discernible features in LEED and the
substrate spots remained weak and diffuse at this
stage. After further annealing at 200°C the ridgelike features gradually gave way to flat terraces,
separated by monatomic steps. For annealing
periods of the order of 30 min at 200°C, ridges
219
Fig. 7. (a) 400 Å×400 Å image of Ni(110) with a Br coverage
of 0.51×1015 Br atoms/cm2. Tunnel current 3 nA, sample bias
−0.01 V. Br pairs are again in evidence and the surface appears
to be markedly etched, forming pits and troughs in the Ni substrate. (b) 400 Å×400 Å image of a different area of the surface.
Other conditions are as in (a).
and terrace structures co-exist (Fig. 8b). Finally,
prolonged annealing at 200°C for periods in excess
of 1 h yielded a surface completely dominated by
220
T.W. Fishlock et al. / Surface Science 426 (1999) 212–224
Fig. 9. 50 Å×50 Å image of Ni(110) after a bromine exposure
to give a surface coverage of 0.6×1015 Br atoms/cm2 and subsequent anneal to 200°C for 60 min. Tunnel current 3 nA, sample
bias −0.01 V. The (3×2) cell is highlighted and the two
different types of atomic row are identified.
Fig. 8. (a) 600 Å×600 Å image of Ni(110) after a bromine
exposure to give surface coverage of 0.6×1015 Br atoms/cm2
and subsequent anneal to 200°C for 20 min. Tunnel current
3 nA, sample bias −0.01 V. The surface topography is dominated by rod-like features directed along the [001] direction.
(b) 400 Å×365 Å image of Ni(110) after a bromine exposure
to give surface coverage of 0.6×1015 Br atoms/cm2 and subsequent anneal to 200°C for 30 min. Tunnel current 3 nA, sample
bias −0.01 V. Rod-like formations now co-exist with flat Ni
terraces supporting a (3×2) surface reconstruction.
terraces and monatomic steps. Again these transformations did not involve significant Br
desorption.
Fig. 9 is an STM image from such a surface.
The image reveals a p(3×2) reconstruction1 with
alternating rows (designated x and y in Fig. 9)
running along the [11: 0] direction. The different
rows each contain different numbers of greyscale
maxima. The (3×2) periodicity found in STM is
accompanied by a well-defined (3×2) superstructure pattern in LEED (Fig. 10). The separation
between the rows is the bulk lattice parameter and
the alternation therefore doubles the periodicity
along [001]. The type x rows contain very strong
greyscale maxima separated by 3a/앀2 thus tripling
the periodicity along the [11: 0] direction, whereas
the type y rows contain pairs of weaker maxima,
with a single vacancy in the ×3 cell. The maxima
in the type y rows are offset by a/앀2 along the
[11: 0] direction from those in the type x rows. The
(3×2) surface reconstruction appears to represent
a thermodynamically stable structure for the half
monolayer Br coverage, since even after further
annealing at 200°C the LEED pattern remains
1 The nomenclature (3×2) rather than (2×3) is employed
for consistency with Refs. [23,41,42].
T.W. Fishlock et al. / Surface Science 426 (1999) 212–224
Fig. 10. LEED pattern for(3×2) Br induced reconstruction on
Ni(110). Beam energy is 70 eV.
unchanged and no other reconstructions are
imaged by the STM. However, after annealing at
300°C, complete desorption of Br is apparent with
a reversion to the (1×1) structure of the bare
substrate.
One possible schematic model for the (3×2)
reconstruction is depicted in Fig. 11. This involves
alternating rows with atomic sequences Ni–Br–
Br–Ni–Br–Br–Ni (type x rows) and Br–Ni–Ni–
Br–Ni–Ni–Br (type y rows). In each case it is
proposed that the greyscale image maxima correspond to Ni positions. Overall the surface coverage
is half a monolayer of Br, i.e. there are equal
numbers of Br and Ni atoms in the on-top rows.
221
The type y rows correspond to complete filling of
two-fold hollow sites of the bare surface by the
BrNi units that make up the butterflies found at
2
low coverage. The type x rows involve added rows
with a stacking sequence of Br and Ni atoms
similar to that found in the CdCl layer structure
2
of bulk NiBr , i.e. Br–Br–Ni–Br–Br–Ni–, etc.
2
However, the overall reconstruction is better envisaged to involve substitutional replacement of Ni
by Br. Its formation must involve major disruption
of the bare surface and pronounced redistribution
of Ni. This idea is reinforced by the observation
that the (3×2) reconstruction only emerges after
prolonged annealing, during which the surface has
passed through the extensively stepped and ridged
morphology represented by Fig. 8.
The proposed structure has some limited similarity to the (2×3) N induced reconstructions
found on Ni(110) [23], Cu(110) [41] and Ag(110)
[42]. All involve row-like structures running along
the [11: 0] direction. The reconstructions on
Ni(110) and Ag(110) are based around NM units
2
(M=Ni or Ag), but in this case the M adatoms
occupy 2/3 of the two-fold hollow sites between
close packed rows: two complete added rows of
metal adatoms alternate with an empty row. The
adatoms are bound into NM units by N atoms
2
occupying alternate pseudo-three-fold sites in the
{111} microfacets that flank the added rows. The
N induced (3×2) reconstruction on Cu(110) also
involves pairs of added rows, but in this case the
N atoms occupy bridging sites. Again the larger
size of Br compared with N probably accounts for
the preference in the current system for occupation
of two-fold hollows.
Despite these relationships with previously
known reconstructions, it is extremely surprising
that the half monolayer of Br does not form an
alternative 2×2 reconstruction with alternating –
Ni–Br–Ni–Br– and –Br–Ni–Br–Ni– rows: this
structure avoids adjacent Br atoms and would be
expected to be energetically favoured on simple
electrostatic grounds. However, there was no evidence from LEED or STM for this alternative
reconstruction.
3.3. Nano-manipulation
Fig. 11. Schematic representation of the p(3×2) Br induced
reconstruction on Ni(110). Dark spheres are Br, light spheres
are Ni.
After characterisation of the Ni(110) surface
with a very low Br exposure, attempts were made
222
T.W. Fishlock et al. / Surface Science 426 (1999) 212–224
to move or break up the bromine butterflies using
the STM tip. Firstly small areas of typical size
60 Å×60 Å were imaged in the normal way to
locate a suitable butterfly. Once the object to be
manipulated was located, the tip was moved to it
with the normal feedback conditions active. The
manipulation method attempted in this work used
a so-called ‘drag’ technique, which involves changing the feedback control conditions. The feedback
between the z piezo position and the current is
still active during the ‘drag’ operation, but a new
tunnel current and tip voltage are selected. The
drag operation simply involves attempting to dislodge and move an object by sweeping the tip
across it whilst the drag parameters for tunnel
current and sample bias are active. Normal feedback conditions are then re-employed and an
image is taken to reveal the extent of motion
induced by the tip.
Many attempts were made to move one of the
adsorbed ‘butterfly’ species using the drag mode.
Due to the anisotropic nature of the Ni(110)
surface, the tip was moved across the butterfly
along [001], [11: 0] and [11: 1] directions, both
through the centre and over the ‘wings’. The initial
feedback parameters entered for the drag mode
were a tunnel current of 10 nA and a tip voltage
of 0.01 V. No motion of butterfly features was
observed under these conditions. Successively
higher tunnel currents were then used coupled with
lower and lower sample bias voltages (both negative and positive) in order to bring the STM tip
closer to the adsorbate features during the drag
operation. The most extreme conditions involved
a tunnel current of 50 nA and a tip voltage of
0.005 V. Dragging the tip across the surface under
these conditions failed to move the butterflies, but
it did cause major disruption to the Ni surface.
Lines of ‘debris’ along the route of the tip motion
during drag mode were sometimes imaged, possibly created by adatom desorption from the tip or
tip fragmentation under the extreme tunnelling
conditions. Any further increase in the tunnel
current or further decrease of the sample bias
resulted in tip failure and the laborious process of
tip conditioning had to be re-initiated. We conclude
that the BrNi unit evident in STM images is very
2
stable at room temperature and not amenable to
tip induced motion.
4. Concluding remarks
The adsorption of Br on Ni(110) has been
2
studied by STM. The initial adsorption is dissociative. The two Br atoms deriving from an incident
Br molecule remain in close proximity, although
2
not in adjacent hollow sites as was found to be
the case for Cl on Ni(110) [31]. Each Br adsor2
bate atom is stabilised by Ni adatoms etched from
nearby close packed [11: 0] rows and it is the
adatoms that appear as greyscale maxima in STM.
The complete ‘butterfly’ unit therefore involves
two Br atoms, four Ni adatoms and four Ni
vacancies. The butterflies appear to be strongly
bound units and remain intact after mild thermal
annealing, even though such treatment allows thermal diffusion along the [11: 0] direction.
A saturated layer of Br on Ni(110) remains
disordered after room temperature adsorption, but
thermal annealing promotes formation of a novel
(3×2) reconstruction which contains an ordered
arrangement of two different row structures. One
of these may be thought to represent saturation
coverage of the BrNi units that constitute each
2
‘wing’ of a butterfly.
The reasons why Br appears to etch Ni atoms
out of close packed rows but Cl does not are not
obvious to us at present: in fact, simple considerations of mean bulk bond energies suggest that a
converse pattern of behaviour might be found.
Removal of an Ni atom from a row breaks seven
Ni–Ni bonds. When the atom is transferred to a
hollow site next to a Br atom, as depicted in
Fig. 4a, we form four new Ni–Ni bonds and one
new Ni–Br bond, so that overall one Ni–Br bond
replaces three Ni–Ni bonds. This will be energetically favourable if the Ni–Br bond strength is at
least three times that of the Ni–Ni bond. We have
the following bulk enthalpy changes [43]:
Ni(s)Ni(g) DH=429.7 kJ/mol
NiBr (s)Ni(g)+2Br(g) DH=865.6 kJ/mol
2
NiCl (s)Ni(g)+2Cl(g) DH=977.6 kJ/mol.
2
T.W. Fishlock et al. / Surface Science 426 (1999) 212–224
In the second reaction, formation of each Ni
atom breaks six Br–Ni bonds so the
mean bond strength is 865.6/6 kJ/mol=
144.3 kJ/mol. Similarly, the mean Ni–Cl bond
strength is 162.9 kJ/kJ/mol. In the first reaction,
the coordination number is 12, but breaking each
bond frees up two Ni atoms We thus have a mean
Ni–Ni
bond
energy
429.7×2/12 kJ/mol=
71.6 kJ/mol. Thus neither Cl nor Br satisfy the
simple condition for etching Ni atoms out of close
packed rows. Of course it is dangerous to pursue
comparisons between mean bulk bond energies
and surface bond energies too far. However, the
condition is satisfied for both Cl and Br if the Ni
vacancies are displaced by one atomic position
away from the halogen adsorbate along the close
packed rows, so that the vacancy position no
longer flanks the Ni atom in the hollow site.
However, these simple thermodynamic considerations suggest that Cl should show an even greater
propensity to remove Ni atoms from the close
packed rows. However, the etching process must
involve breaking of the halogen–halogen bond in
synchrony with removal of Ni atoms from the
close packed rows and it is possible that Br etching
is favoured because of the lower contribution to
the overall activation energy from breaking a Br–
Br bond (192.9 kJ/mol ) compared with breaking
a Cl–Cl bond (242.6 kJ/mol ).
Molecular dynamic simulations [34,44] reproduce the behaviour of Cl on Ni(110): on terraces
2
a Cl molecule dissociates into two Cl atoms,
2
which remain bound in adjacent two-fold hollow
sites. Close to step edges, the Cl adatoms are
drawn along the troughs between close packed
rows until eventually the Cl adsorbate atoms are
bound at the step. We are currently attempting to
extend these simulation procedures to the more
complicated behaviour displayed by Br on
Ni(110).
Acknowledgements
The equipment and a studentship to T.F. were
funded by EPSRC. T.F. is also grateful to the
Worshipful Company of Ironmongers for the
award of a scholarship.
223
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