Surface Science 411 (1998) 303–315
The structure and corrosion chemistry of bromine on Pt(111)
H. Xu, R. Yuro, I. Harrison *
Department of Chemistry, University of Virginia, Charlottesville, VA 22901, USA
Received 16 June 1997; accepted for publication 14 April 1998
Abstract
The corrosive adsorption of Br atoms on Pt(111) was studied by variable temperature STM, TPD and AES. Photoinduced
dissociative electron attachment to adsorbed CH Br at 193 nm was used to dose hot Br anions onto the surface at 90 K. Br formed
3
two ordered structures on Pt(111): (3×3) and (앀3×앀3)R30°. STM images showed that both structures can coexist at intermediate
coverage. Photofragmentation of CH Br produced Pt atom monovacancies on the Pt(111) surface which were attributed to
3
abstractive attack by the hot Br anions. The initial monovacancy etching efficiency of the adsorbing Br anions was high, roughly
33%. © 1998 Elsevier Science B.V. All rights reserved.
Keywords: Corrosion; Etching; Halides; Hot atom; Pt(111); Scanning tunneling microscopy (STM ); Surface photochemistry
1. Introduction
The surface chemistry of halogens on metals is
relevant to corrosion, catalysis and electrochemistry [1]. A particularly interesting adsorption mechanism for halogen containing molecules with
sufficiently high electron affinities is the possibility
of electron transfer or harpooning of the incident
molecules at long range (5–10 Å) from the surface
[2]. During approach to the surface, harpooning
may occur once the image potential pulls the
molecule’s electron affinity level beneath the Fermi
energy of the surface electrons. After harpooning,
the surface image potential serves to reel in the
nascent anion and energetic chemistry can ensue,
even at low substrate temperatures, as the substantial heat of adsorption is dissipated. A variety of
high energy non-adiabatic processes have been
* Corresponding author. Tel: +1 804 924 3639;
Fax: +1 804 924 3710; e-mail: [email protected]
observed during molecular halogen adsorption on
alkali metals, including UV/visible chemiluminescence, ejection of substrate electrons, and the
desorption of positive metal ions [3]. In this study
of the chemistry and structure of Br on Pt(111),
harpooning via photoinduced dissociative electron
attachment to adsorbed CH Br was employed to
3
dose hot Br anions onto the surface. Scanning
tunneling microscopy (STM ) revealed that the
adsorbing Br anions can efficiently abstract Pt
atoms from the surface and leave monovacancies
behind. Possible creation mechanisms and the thermal stability of the Pt(111) monovacancies are
discussed.
Most recent surface science studies of etching
by halogens have focused on issues relating to
semiconductor dry processing technologies [4],
rather than the etching of metals. For semiconductors, as well as for metals, there is evidence for
high energy chemistry as halogens adsorb.
Scanning tunneling microscopy shows that molec-
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PII: S0 0 39 - 6 0 28 ( 98 ) 0 03 3 8 -0
304
H. Xu et al. / Surface Science 411 (1998) 303–315
ular bromine adsorption patterns Si surfaces at
high temperatures [5–7]. Step edges are selectively
etched but vacancy islands may also be formed on
the terraces. Molecular beam/STM experiments
[8] indicate that when Br adsorbs on room tem2
perature Si(111), the Br atoms chemisorb in pairs
only about 70% of the time. For the remaining
adsorption trajectories, the surface abstracts only
a single Br atom from the impinging molecule and
the complementary atom escapes into the gas
phase.
Although the initial steps of halogen adsorption
on metals have rarely been investigated by STM
[9], a variety of surface structures have been
identified by electrochemical STM and low energy
electron diffraction (LEED). A (앀3×앀3)R30°
structure develops for Cl, Br and I on Ag(111),
and for Cl and Br on Cu(111), Pd(111) and
Rh(111) [1,10]. In these structures, all the halogens are believed to sit in three-fold hollow sites.
On Pt(111), low coverages of I adsorb in similar
fashion [11] but LEED studies [12,13], find that
Cl and Br form only (3×3) ordered structures.
Electrochemical STM images of Br on Pt(111)
have identified (3×3) [14], (4×4) [15] and (3×
3(앀3)/2) [16 ] structures. In the present study, in
which Br anions were dosed photochemically, Br
was found to form ordered (앀3×앀3)R30°
domains before the surface saturated in a (3×3)
monolayer.
2. Experimental
Experiments were performed in an ion pumped
ultrahigh vacuum system equipped for laser irradiation of the sample, STM, Auger electron spectroscopy (AES), thermal programmed desorption
( TPD) and photoproduct time-of-flight ( TOF )
spectroscopy to a quadrupole mass spectrometer.
The system operating pressure was less than
2×10−10 Torr. A schematic cross sectional view
of the chamber is given in Fig. 1. The sample
crystal sits in the center of a 12 in. diameter
spherical chamber and can be rotated to face the
radially distributed surface analysis equipment. A
Physical Electronics single pass cylindrical mirror
analyzer is used for AES and a twice differentially
pumped Extrel quadrupole mass spectrometer is
employed for TPD. The entire chamber sits on a
Newport optical bench with air suspension legs
for vibrational isolation.
The home built STM utilizes a Besocke inertial
approach [17] and is styled after the scuttling
‘‘Beetle’’ instruments developed by Comsa [18,19]
and Salmeron [20]. The STM is driven by electronics and software from RHK Technology. The
sample is cooled by a copper braid connection to
an Oxford Instruments heat exchanger which can
be operated with either liquid helium or liquid
nitrogen. Heating is by electron bombardment and
the sample temperature can be varied and stabilized over a temperature range from roughly 18 to
1350 K. In this study, liquid nitrogen was used for
cooling and the lowest attainable temperature was
85 K. STM images could be acquired at crystal
temperatures less than about 400 K after the STM
head was lowered onto the sample. A useful feature
of the UHV system was its ability to iteratively
perform STM and surface analysis techniques such
as AES and TPD without losing sample temperature control and hence control over the adsorbate
coverage and surface condition. This made for
relatively straight forward identification of species
which could only be ‘‘topologically’’ imaged by
STM.
The 5-mm diameter Pt(111) crystal oriented to
±0.5° was purchased from Metal Crystals and
Oxides. The surface was initially cleaned by cycles
of Ar+ ion sputtering, annealing in 1×10−7 Torr
of O at 800 K, and flashing to 1200 K to desorb
2
oxygen. Eventually, AES, TPD and STM showed
that the surface was clean of detectable contaminants. Thereafter, the surface could usually be
cleaned simply by dosing O and flashing until
2
clean O TPD spectra were obtained. The clean
2
surface exhibited a distribution of terraces widths
in the 50–400 Å range. Atomically resolved STM
images of the Pt(111) terraces displayed an apparent electronic corrugation of 0.1 Å. STM images
were recorded at 1 nA tunneling current with
sample bias voltages within ±1 V of the STM’s
tungsten tip and are reported here as raw data.
The STM current is rigorously dependent on a
convolution of the tip and sample electronic den-
H. Xu et al. / Surface Science 411 (1998) 303–315
305
Fig. 1. Cross sectional view of the UHV system for scanning tunneling microscopy and surface photochemistry. An (x, y, h) manipulator is used to position the sample towards different surface analysis/preparation stations.
sity of states near the Fermi level and evaluated
at the tip-to-sample separation distance [21]. In
the absence of accurate electronic structure calculations, it is common to make the expedient interpretative assumption that at constant bias the
tunneling is a function of separation distance alone
and the intrinsic adsorbate density of electronic
states does not vary appreciably from site to site
across the surface. In this manner, constant current
STM ‘‘images’’ of tip-to-sample distance versus
displacement across the surface are interpreted as
topological records of the ‘‘heights’’ of surface
features. However, it should be borne in mind that
this interpretation is rigorously incorrect and
electronic structure effects often dramatically
influence STM images. Fig. 2 shows an atomically
resolved image of a Pt(111) terrace for which the
nearest neighbor spacing is 2.77 Å and the
observed corrugation amplitude is 0.1 Å.
Br anions were dosed onto the surface by irradiating adsorbed CH Br with a 193 nm ArF excimer
3
laser at a surface temperature of 90 K. The laser
was operated at 10 Hz and the fluence of the 20 ns
laser pulses was kept under 2 mJ cm−2 to limit
306
H. Xu et al. / Surface Science 411 (1998) 303–315
Fig. 2. STM image of clean Pt(111) at room temperature showing atomic resolution. The image size is 23 Å×23 Å.
transient surface heating to less than 10 K (microsecond timescale) [22]. Dissociative electron
attachment (DEA) of photoexcited substrate
electrons to the adsorbed CH Br produced ener3
getic CH and Br− photofragments [23,24]. Most
3
of the CH species left the surface with transla3
tional energies of up to 2 eV but about 20% were
trapped on the surface [25] along with all of the
Br−. Annealing the surface to 300–325 K desorbed
undissociated CH Br and surface bound CH radi3
3
cals cracked or preferentially reacted with H to
form desorbing CH [26 ]. Without any coadsorbed
4
H from background gas adsorption, this procedure
should leave only Br and very little C (≤5% of
the Br) on the surface. In practice, there seems to
be sufficient adsorbed H available that virtually all
the trapped CH radicals hydrogenate since only
3
Br was detectable on the annealed surface by AES
or TPD. Br coverages were most easily calibrated
by TPD because electron stimulated desorption
and a low Auger cross section make quantitative
AES difficult for this element. Direct counting of
the Br coverage by STM was also problematic
because the high mobility of individual Br atoms
across the terraces prevented their imaging by
STM at the experimental temperatures of
T ≥85 K. Br atoms could only be observed by STM
s
Fig. 3. TPD spectra of 0.23 ML CH Br on Pt(111) before and
3
after photofragmentation at 193 nm. Desorption was monitored
at the mass to charge ratio of Br+, m/e=79. CH Br desorbs
3
molecularly at low temperature, while Br desorbs atomically
at 850 K.
when they were more strongly bound to surface
defects, steps or other Br atoms in island structures.
A Br coverage scale in TPD was established based
on the knowledge that full coverage of CH Br on
3
Pt(111) is 0.25 ML [27] and Br is retained on the
surface after CH Br photofragmentation. Hence,
3
the reduction of the CH Br TPD signal after laser
3
irradiation could be related to the Br photofragment coverage which desorbed atomically at
~850 K as shown in Fig. 3. However, two small
desorption features around 300 and 500 K may
also be observed in Fig. 3. The 300 K peak is
believed to derive from the sample mount but the
500 K peak may stem from HBr desorption [28].
Unfortunately, we were unable to unambiguously
confirm the later assignment by TPD at the parent
mass of HBr. If appreciable Br does evolve in this
manner prior to the main atomic Br desorption at
850 K the Br coverages reported here may be
systematically low.
H. Xu et al. / Surface Science 411 (1998) 303–315
307
3. Results and discussion
3.1. Br structures on Pt(111)
Fig. 4 shows STM images taken at 280 K of a
Br covered surface prepared by repeated cycles of
dosing and irradiating CH Br at 90 K and briefly
3
annealing to 300–350 K. The surface coverage of
Br as determined by TPD was 0.33 ML. The large
area scan of Fig. 4a shows a cascading series of
Br covered terraces which are separated by monatomic height steps. The Br overlayer exhibits a
(3×3) lattice periodicity. In the higher resolution
image of Fig. 4b, Br can be seen to exist in two
forms which image at different heights. The highest
lying Br atoms appear brightest. The distance
between nearest bright Br atoms is 8.3±0.2 Å
which is consistent with a (3×3) structure. The
bright and gray Br atom rows alternate on the
surface and their height difference is 0.2–0.3 Å.
Each bright Br has six nearest neighbors of gray
Br atoms. These data are consistent with the Br
atoms occupying two inequivalent sites. Earlier
electrochemical STM studies of the (3×3) structures of I [29,30] and Br [14] on Pt(111) have
determined that the halogens sit on both top and
bridge sites, rather than a single equivalent site.
The brighter, higher lying halogen atoms in the
STM images are assigned to sit at the top sites
[14,29,30]. LEED studies [12,13] have shown the
(3×3) Br structure to be stable over a range of
coverage.
At lower Br coverages it was possible to observe
a (앀3×앀3) structure which was missing from
previous LEED [12,13] and room temperature
electrochemical STM studies [14–16 ]. In the
(앀3×앀3) structure, all the Br atoms image at the
same height which suggests that they occupy equivalent sites. In analogy with Br on the other (111)
fcc metal surfaces [1], the Br atoms are assumed
to sit on the three-fold hollow sites. Fig. 5 shows
an STM image taken at 130 K and a Br coverage
of about 0.15 ML in which both (앀3×앀3) and
(3×3) island domains coexist. In the upper right
hand corner in particular, (앀3×앀3) islands are
found in which all the atoms are imaged with
equal intensity. The other ordered islands with
(a)
(b)
Fig. 4. STM images of 0.33 ML Br/Pt(111) at T =280 K. The
s
surface was prepared by repeatedly dosing and irradiating
CH Br at 90 K with intermediate annealing to 300–350 K. (a)
3
An 130 Å by 130 Å image showing the (3×3) Br overlayer
formed on top of the Pt(111) terraces. (b) A higher resolution,
48 Å×48 Å image shows that the (3×3) Br structure contains
two different types of Br atoms which image at different heights,
as bright and gray.
larger periodicity are of the (3×3) structure. It
can be seen that the atomic rows of the (앀3×앀3)
islands point 30° away from those of the (3×3).
308
H. Xu et al. / Surface Science 411 (1998) 303–315
sistent with the STM observations. Apparently, Br
atoms can occupy all three of the high symmetry
sites on Pt(111). In the (앀3×앀3) structure, Br
sits in three-fold hollow sites and all Br atoms are
imaged equivalently by STM. In the (3×3) structure, Br sits on both top and bridge sites which
showed markedly different heights in STM images.
The top site Br atoms imaged brighter than the
gray Br atoms at bridge sites. STM line scans at
1 nA current indicate that Br at top, bridge and
three-fold hollow sites are imaged at roughly 0.6,
0.3 and 0.2 Å further tip-to-sample separation than
the Pt(111) surface atoms. However, it should be
noted that the observed ‘‘height’’ in constant current mode of STM operation depends on both the
adsorbate electronic density of states and the
adsorbate distance from the tip.
Fig. 5. STM image showing coexisting (앀3×앀3)R30° and
(3×3) Br island domains at h =0.15 ML and T =130 K. The
Br
s
ordered islands with smaller periodicity have the
(앀3×앀3)R30° Br structure and those with larger periodicity
have the (3×3) Br structure. The image size is 166 Å×166 Å.
This further confirms that the two structures have
different atomic arrangements and suggests that,
indeed, the (앀3×앀3) structure is rotated 30° from
the underlying Pt(111) lattice.
Fig. 6 provides a structural model for the (3×3)
and (앀3×앀3)R30° Br adsorption on Pt(111) con-
Fig. 6. Models of the (앀3×앀3)R30° and (3×3) Br structures
on the Pt(111) surface which are consistent with the STM
observations.
3.2. Etching of Pt monovacancies by Br adsorption
The dark holes which are apparent in the STM
image of Fig. 5 are a consequence of CH Br photo3
fragmentation at low temperature. Excimer laser
irradiation of the clean Pt(111) surface produced
no such holes nor did thermal heating of adsorbed
CH Br which leads solely to molecular desorption.
3
Hole generation was found to be a characteristic
and very reproducible feature of CH Br photofrag3
mentation on Pt(111). Heating the surface past
950 K eliminated the holes and regenerated the
flat Pt(111) terraces.
Fig. 7a and 7b are typical small and large scale
STM images of the hole features after photofragmentation of CH Br and a brief annealing to
3
325 K. In these images taken at 240 K, the Br
coverage is h =0.14 ML and the hole coverage
Br
is h =0.02 ML. In Fig. 7a, the Pt(111) surface
H
lattice can be weakly resolved and Br atoms are
imaged as the bright round spots which can be
seen to congregate around the step edges and the
dark holes. Along the step edge, a second row of
blurred features can be observed which are attributed to weakly bound Br atoms moving rapidly
along the step edge. The dark holes in the surface
exhibit a characteristic triangular lateral shape and
image as depressions of roughly 1.0±0.3 Å depth.
The corners of the triangular holes are rounded
H. Xu et al. / Surface Science 411 (1998) 303–315
(a)
(b)
309
and the edge lengths are 5±1 Å. The large scale
image of Fig. 7b shows the holes to be relatively
uniformly distributed, although there may be some
tendency of the holes to form strings of several
units. The mobility of the holes at 300 K is low
because STM images of the surface hole features
after preparation at 90 K were not appreciably
altered after annealing the surface to 300 K for
several seconds. The size of the holes and their
characteristic shape suggests that they are Pt atom
monovacancies formed by Pt atom abstraction by
the hot Br− photofragments from photoinduced
DEA of adsorbed CH Br. The holes image as
3
slightly larger than one Pt atom and smaller than
a three Pt atom cluster. When an atom vacancy is
created on the surface, the substrate electron density will be redistributed and there will be some
smoothing of the electron density in towards the
surface [31]. As a result the monovacancies may
image with an enhanced effective size by STM.
Other potential assignments for the dark hole
features of the STM images are surface contaminants or tip artifacts. After CH Br photofragmen3
tation and annealing to prepare a brominated
surface, the most plausible impurity on the
Br/Pt(111) surface is carbon which has been shown
to exhibit bright (raised) features surrounded by
dark rings in STM images [32]. Subsurface carbon
may show as a depression but these are very
shallow features (~0.1 Å) [33]. Although AES did
not show any contaminants immediately after preparing the Br/Pt(111) surface, with time, some
CO could be seen to adsorb through the growth
of correlated C and O AES signals. However,
CO is mobile and cannot be imaged by STM at
the surface temperature of our experiments
(T ≥85 K ). Hence, we do not attribute the dark
s
holes in the STM images to any surface contami-
Fig. 7. (a) A high resolution STM image taken at T =240 K
s
showing the holes formed after photofragmentation of CH Br
3
on Pt(111). In this 84 Å×84 Å image, h
=0.02 ML and
holes
h =0.14 ML. The bright round features are Br atoms and the
Br
Pt lattice is only weakly resolved. A line scan across several of
the holes is shown. (b) A larger scale, 420 Å×420 Å, STM
image of the same surface. The holes are distributed fairly uniformly across the surface. Br atoms can be seen to decorate the
step edges.
310
H. Xu et al. / Surface Science 411 (1998) 303–315
nant effects. The hole features could be observed
over a wide variety of bias and current conditions
and the images were reproducible when scanned
over both ±X directions. The attainment of such
high resolution, reproducible and consistent STM
images eliminates the possibility that the dark
holes are tip artifacts. Finally, the appearance of
the holes features only after CH Br photofragmen3
tation argues forcibly against an assignment of the
holes as surface impurities or STM tip artifacts.
Annealing the 0.14 ML Br/0.02 ML hole/
Pt(111) surface of Fig. 7 to 600 K led to further
aggregation of the Br along the step edges and a
diminishment of the hole coverage. Coupled with
the observation that the flat surface can be restored
by heating beyond 950 K, this behavior suggests
that the monovacancy holes may aggregate into
vacancy islands or move to the step edges at high
temperatures and in so doing heal the surface.
Qualitatively similar behavior was observed by
Comsa and coworkers [34] in STM studies of
vacancy islands formed by Ar+ ion sputtering on
a Pt(111) surface. In the Comsa study, monovacancies produced by 600 eV Ar+ ion sputtering of
a 350–625 K surface aggregated into vacancy
islands. Interestingly, the smallest vacancy islands
were found to image with a depth slightly less than
one half the Pt(111) interlayer spacing (see their
Fig. 2), which is similar to the depth of the monovacancy holes reported here. In our STM images,
isolated step edges exhibit a height of 2.3±0.2 Å,
which represents a single Pt atom layer thickness,
but the monovacancies display a much smaller
depth of only 1.0±0.3 Å. We attribute the smaller
image depth of the monovacancies (and the smallest vacancy islands in Comsa’s images) to an STM
tip effect. It is likely that the tip terminates in a
single atom but has a relatively blunt understructure which prevents the tip from faithfully tracking
the depth contour of a single monovacancy without
allowing additional tunneling current to flow
between adjacent portions of the tip and the surface terrace. The additional tunneling pathways
lead to a reduced extension of the STM tip in the
constant current mode and hence to a reduced
monovacancy image depth. For larger vacancy
islands and step edges, the tip can get far enough
away from the upper terrace that tunneling on the
lower surface can once again proceed faithfully
through a single terminal tip atom. A full monatomic step difference will only be measured
between regions of upper and lower surfaces for
which the tunneling conditions are identical, conditions which cannot be met when imaging a monovacancy. In consequence, the assignment of the
1 Å deep holes of the STM images to monovacancies seems quite reasonable, particularly because
the potential impurities discussed above would
introduce much less corrugation to the surface
electron density.
It was easiest to observe the Pt monovacancies
on the terraces at low Br coverage when the mobile
terrace Br would tend to aggregate along the step
edges. At higher coverages, Br atoms could sometimes be seen to sit on or in the monovacancies as
in the case of the upper right most vacancy of
Fig. 7a. The Br atoms of Figs. 4 and 5 image with
a diameter of roughly 3 Å which is comparable to
the 2.77 Å diameter of the Pt surface atoms. If the
Br atoms are too big to perfectly substitute for a
Pt atom in a monovacancy they may ride up above
the Pt(111) surface plane. Nevertheless, the STM
line scan of Fig. 7a shows that Br at a monovacancy images at a lower height than the Pt(111)
surface. The relatively low image height is likely
an electronic effect because even Br atoms on the
undisturbed surface image just a few tenths of
ångströms above the surface plane despite their
apparent lateral diameter of 3 Å. Within the
ordered (3×3) and (앀3×앀3)R30° Br structures
of Figs. 4 and 5 some of the monovacancy sites
may be filled over and be difficult to discern at the
higher Br coverages. Monovacancy creation by
bromine adsorption may have gone undetected in
earlier STM studies of Br/Pt(111) surfaces prepared electrochemically [14–16 ] or by flame annealing and quenching in Br vapor at 1300 K [15]
2
because these works concentrated on the Br structures observable at high coverage. Alternatively, it
could be that monovacancy etching is a unique
characteristic of the adsorption of hot Br anions
prepared here by photoinduced DEA of CH Br.
3
In order to determine if the monovacancy etching was associated with the low temperature photochemical adsorption of Br anions or a subsequent
H. Xu et al. / Surface Science 411 (1998) 303–315
thermally induced process, some low temperature
STM images were taken prior to the 300 K annealing step which was normally used to rid the
surface of residual CH Br and any CH photofrag3
3
ments. Fig. 8 shows an STM image of a 120 K
surface taken immediately after partial photofragmentation of 0.28 ML of CH Br (i.e. with no
3
subsequent annealing). Although the image is
somewhat noisy, the Pt(111) lattice remains visible
even through the adsorbed layer comprised of
residual CH Br and surface trapped Br and CH
3
3
photofragments. The hole coverage is 0.023 ML
and the holes image with their characteristic triangular shape. Neither Br, CH nor CH Br are
3
3
resolved, presumably due to high surface mobility
or tip effects. Importantly, this STM image demonstrates that the substrate holes are created at low
temperature directly following the CH Br photo3
fragmentation.
The probability of creating monovacancies as a
function of Br coverage was investigated by counting the number of dark holes in the STM images
and measuring the Br coverage by TPD. Only
dark holes with the characteristic shape and size
of monovacancies or monovacancy strings were
Fig. 8. STM image taken at T =120 K and h =0.14 ML shows
s
Br
holes generated by CH Br photofragmentation prior to annea3
ling to higher temperatures. The Pt lattice is barely visible in
this 80 Å×80 Å image.
311
counted. Occasionally, shallow depressions
(0.1–0.2 Å) were observed and these were attributed to subsurface impurities (rather than Br atoms
trapped in a hole) and were not added to the hole
counts. Fig. 9 is a plot of the hole density versus
Br coverage. The hole coverage initially increases
with Br coverage and saturates after 0.07 ML of
Br. As mentioned above, monovacancies may be
filled over by Br atoms and so it may be that at
the higher Br coverages some of the monovacancies
escape counting by the method of Fig. 9.
Nevertheless, the initial slope of the graph can be
used to estimate the efficiency of monovacancy
creation under conditions where the Br coverage
segregates to the step edges and hole counting
should be most accurate. The initial etching efficiency of 0.33 holes/Br indicates that 33% of the
Br adsorption processes create a surface monovacancy. This is an impressively efficient etching
process.
3.3. Mechanism of monovacancy formation
The discovery of a low temperature Br induced
etching of Pt monovacancies on Pt(111) is somewhat surprising because there is substantial energetic cost associated with pulling a Pt atom from
Fig. 9. A plot of the monovacancy hole coverage versus Br
coverage.
312
H. Xu et al. / Surface Science 411 (1998) 303–315
the surface and creating a monovacancy and an
adatom. Using a Born–Haber cycle, the cost of
producing an adatom/monovacancy pair is estimated as roughly 1.1 eV given that the heat of
sublimation of Pt is 4.8 eV [35] and a Pt adatom
on Pt(111) is bound by 3.7 eV [36 ].
In this study, surface Br was generated photochemically through 193 nm laser induced dissociative electron attachment (DEA) of adsorbed
CH Br at a surface temperature of 90 K. The
3
calculated laser induced surface temperature
increase was less than 10 K (microsecond timescale) and there was no evidence for laser induced
thermal chemistry. No species other than CH
3
photofragments were observed to leave the surface
in laser triggered time-of-flight ( TOF ) spectra to
the quadrupole mass spectrometer. In particular,
Pt, Br, PtBr and PtBr were looked for in TOF
2
spectra but none were found to desorb. Photoninduced ejection of metal atoms has previously
been observed on Ag(111) surfaces at relatively
low laser fluence (although 10 times higher than
used here) [37,38]. The high kinetic energy of the
desorbing atoms was characteristic of plasmon
excitation and a plasmon coupling theory has been
advanced [39]. However, the experimental reports
dealt with surfaces which were substantially roughened in order to couple the incident light field to
the surface plasmon modes. In the present study,
the Pt(111) surface was atomically smooth and
monovacancies were seen to develop on the flat
terraces and not preferentially near the ‘‘rougher’’
step edges. These observations coupled with
absence of any photodesorbing Pt species discounts
the possibility of a plasmon mediated surface
etching mechanism. Furthermore, the Pt d-band
electronic structure leads to much greater damping
of surface plasmons on Pt surfaces as compared
to Ag ones so we consider a plasmon mediated
etching mechanism on Pt(111) to be highly
unlikely under our experimental conditions.
Alkanethiol self assembled monolayers (SAMs)
prepared by liquid or vapor phase deposition on
Au(111) surfaces [40] at ambient temperatures
have been shown by STM to exhibit single Au
atom deep vacancy islands (30–100 Å diameter)
which cover 5–30% of the surface area [41]. The
Au vacancy islands are produced during the
adsorption process and remain covered by SAMs
when imaged by STM. Although the mechanism
for forming these large vacancy islands remains
controversial, recent vapor phase deposition [42]
and photooxidation [43] experiments indicate that
it is adsorbate induced migration of Au atoms on
the surface which leads to the vacancy islands and
not a chemical etching process. Migration of Au
atoms is facile on the (111) surface at 300 K. The
(22×앀3) ‘‘herringbone’’ reconstruction of the
Au(111) surface has 5% greater atom density in
its topmost layer than the 1×1 unreconstructed
phase, yet transitions between the phases, which
require substantial mass transport, are quite easily
driven by adsorption or even the local electric field
from an STM tip [44]. There is some evidence
that the vacancy islands are associated with the
kinetics of adsorption and the ability of the SAMs
to locally lift the (22×앀3) herringbone reconstruction of the underlying Au(111) surface [42].
Unlike Au(111), the Pt(111) surface does not
reconstruct until temperatures of 1330 K are
exceeded [45] and this temperature is not known
to be reduced by the adsorption of any adspecies
other than Pt adatoms derived from a Pt gas phase
[46 ]. The enhanced energetic separation between
the reconstructed and (1×1) phases of Pt(111)
keeps this surface in a much more rigidly defined
state at low temperatures. In consequence, the
monovacancies formed by Br adsorption on
Pt(111) at 90 K are believed to be formed by a
chemical etching process rather than a mechanism
involving local reconstruction of the surface and
transport of surface substrate atoms in analogy to
the formation of the many atom vacancy islands
observed after alkanethiol SAM adsorption on
Au(111).
Let us now turn to the energetics of Br adsorption following 193 nm photoinduced DEA of
adsorbed CH Br. The dynamics and mechanism
3
of CH Br photofragmentation via DEA on
3
Pt(111) have previously been described in detail
[23,24,27]. Briefly, photofragmentation occurs
when a photoexcited substrate electron attaches to
an affinity level of physisorbed CH Br and the
3
resulting unstable anion dissociates to give CH +
3
Br− in some tens of femtoseconds. The affinity
(ad)
H. Xu et al. / Surface Science 411 (1998) 303–315
level of adsorbed CH Br lies beneath the vacuum
3
level and is stabilized by the surface image potential and polarization of the surrounding adsorbates
[23,24]. TOF studies of the 193 nm photofragmentation show that CH photofragments leave the
3
surface with translational energies of up to 2 eV
[27]. To the extent that momentum conservation
can be applied to the separating photofragments
at the position of the physisorbed CH Br−
3
(~2.5 Å from the surface image plane where surface forces are relatively weak), the nascent Br−
photofragments heading towards the surface
should have translational energies of up to
E =0.4 eV. The Br− photofragment so formed
T
will then gain its heat of adsorption as diagrammed
in Fig. 10 such that its total energy will be in the
1.8–2.2 eV range with respect to cold Br
on
(ad)
the CH Br/Pt(111) surface. This energy liberated
3
upon adsorption of the Br photofragment is more
than sufficient to abstract a Pt atom from the
surface (1.1 eV ). Hence, it is likely that monovacancy creation occurs when a Br photofragment
reacts to form a PtBr compound and the local
energy accumulated is sufficient to eject the com-
313
pound up onto the surface. Once PtBr is on the
surface the Br can trade its initial Pt binding
partner for an underlying surface atom and liberate
a Pt adatom onto the surface. Alternatively, the
local energy available after forming a PtBr bond
might be liberated by ejecting a neighboring Pt
atom up onto the surface. Unfortunately, the
significant mobility of Pt and Br adatoms at the
experimental operating temperatures prevented a
direct observation of the initial etching event by
STM. It is also true that the monovacancy/adatom
pairs are created on a surface already covered
with CH Br. However, after annealing to 325 K
3
to remove CH Br and any trapped CH photofrag3
3
ments from the surface, it was easy to observe by
STM the relatively immobile Pt monovacancies
and the Br atoms which had migrated to step
edges or had formed island clusters.
Density functional calculations indicate that the
equilibrium bonding of halogen atoms on jellium
metal surfaces is substantially ionic [47]. The simplest energetic calculations to approximately
describe the Br/Pt(111) system can be made if we
assume a full unit of charge is transferred during
Fig. 10. Schematic representation of Br adsorption diabats on CH Br/Pt(111).
3
314
H. Xu et al. / Surface Science 411 (1998) 303–315
Br adsorption. Fig. 10 schematically depicts the
energetics of Br atom adsorption onto a CH Br
3
covered Pt(111) surface and displays crossing Br
and Br− diabats. Let us write the Br−/surface
attraction as
V(z)=(W−EA)−(E +E ),
(1)
im
pol
where W=4.3 eV is the work function of
CH Br/Pt(111), EA=3.36 eV is the electron
3
affinity of Br and E is the surface image energy
im
E =e2/4z=3.6 eV Å/z,
(2)
im
where z is the distance from the surface image
plane (which is ~1.9 Å away from the Pt ion
cores) and E is the polarization energy of the
pol
ion by the surrounding CH Br adsorbates. The
3
last term is relatively short ranged but in the
CH Br plane can be estimated as about 1/3 the
3
Born ion solvation energy [48] of Br− in liquid
CH Br, or about 1.4 eV. Assuming E falls off
3
pol
rapidly with distance, the Br− diabat crosses the
covalent curve at z ~4 Å and electron transfer can
c
occur. Photoinduced DEA of physisorbed CH Br
3
is believed to happen somewhat closer to the
surface at about z =2.5 Å. The potential energy
DEA
released as Br− is adsorbed after DEA of CH Br
3
is roughly
V ={E* −E (z
)}−{E (z
)
DEA
ad
pol DEA
im DEA
−E (z )}#E −0.5 eV.
(3)
im c
ad
The first bracketed term is roughly the adsorption
energy of Br as measured in TPD for small coverages of Br on Pt(111) where adsorbate polarization
effects are negligible, E . Assuming a pre-exponenad
tial factor for first order desorption of 1013 s−1, a
Br adsorption energy of E =2.3 eV is required to
ad
give the atomic Br desorption at 850 K as indicated
in Fig. 3. In this manner, we may estimate V
DEA
as roughly 1.8 eV from Eq. (3). The nascent Br−
photofragments have up to 0.4 eV of translational
energy so that leaves from 1.8–2.2 eV of energy
available as Br− adsorbs on Pt(111) following
DEA of CH Br. This is essentially the same energy
3
as available during adsorption of atomic Br from
the gas phase and is close to double the energy
required to create a monovacancy/adatom pair on
Pt(111).
Alternatively, let us also consider the energetics
of a Br atom adsorbing on a clean Pt(111) surface
with work function of 5.8 eV. In this instance, the
Br affinity level is stabilized only by the image
attraction and charge transfer can occur at
z≤1.7 Å from the surface image plane. This is
closer to the surface than the 2.5 Å CH Br phy3
sisorption distance where Br− photofragments are
born by photoinduced DEA. Hence, when an
isolated CH Br molecule on Pt(111) photodissoci3
ates, although the Br− photofragment may
undergo charge exchange with the surface, the
photofragment should eventually accumulate
about the normal heat of adsorption for a Br atom
given that the majority of the Pt/Br bonding
interaction is developed for z≤1.7 Å.
Having established that dosing Br− via photoinduced DEA of CH Br is energetically quite similar
3
to dosing the surface with Br atoms from the gas
phase, we anticipate that monovacancy etching of
Pt(111) may also be effective using a Br atomic
beam. Indeed, molecular adsorbates which are
dissociated thermally or non-thermally to give hot
Br atoms which in the act of adsorbing can accumulate more than 1.1 eV of energy may have some
ability to etch monovacancies on Pt(111).
The finding that Br photofragments on Pt(111)
can etch surface monovacancies may be of some
concern to scientists interested in preparing alkyl
radical species via thermal or non-thermal dissociation of halogenated hydrocarbons [49]. If surface
radical species are generated in this way for use in
subsequent kinetic or spectroscopic studies it may
be important to recognize that the presence of not
only halogen fragments but also etched monovacancies or adatoms may influence the surface radical chemistry.
4. Conclusions
The structure of Br on Pt(111) was studied by
variable temperature STM following deposition of
Br by photoinduced dissociative electron attachment of adsorbed CH Br and subsequent desorp3
tion of all species other than Br. In accordance
with previous LEED and electrochemical STM
studies, a (3×3) Br structure was observed at high
coverage. At lower coverage, (앀3×앀3)R30° Br
island domains were observed for the first time.
H. Xu et al. / Surface Science 411 (1998) 303–315
An important observation was the formation of
monovacancies in the Pt(111) surface following
CH Br photofragmention. Apparently, hot Br
3
photofragments possess sufficient energy to etch a
monovacancy/adatom pair in Pt(111) at an energetic cost of about 1.1 eV. The initial monovacancy
etching efficiency of Br photofragments was 33%.
Etching occurred quite homogeneously across the
surface. The monovacancies were stable and immobile at 85–300 K but diminished in number upon
heating to 600–700 K. Annealing to 950 K
desorbed the Br and healed the surface of monovacancy holes.
Acknowledgements
The authors thank Miquel Salmeron and Frank
Ogletree of LBNL for their generous advice
on building an STM. Financial support from the
Department
of
Energy
grant
#
DEFG0595ER14563, the Jeffress Trust and the
University of Virginia is gratefully acknowledged.
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