Deposition of lead iodide films on Rh(1 0 0) electrodes from colloidal solutions - The effect of an iodine adlayer

Article abstract of DOI:10.1016/S0039-6028(03)00839-2

The growth of PbI₂ precipitates on single crystal substrates from colloidal solutions has been investigated with in air scanning tunneling microscopy and synchrotron-based X-ray photoelectron spectroscopy. The PbI₂ growth on Rh(1 0 0) results in nano-clusters with lateral dimensions between 30 and 60 ?, consistent with earlier reports. However, the growth of PbI₂ on a well-ordered iodinated Rh(1 0 0), denoted as (√2 × √2)R45°-I, leads to atomically smooth PbI₂ films having a hexagonal symmetry with lattice constant identical to the bulk value of 4.5 ?. The heteroepitaxy is believed to be effected by the atomic iodine monolayer that helps to accommodate large lattice mismatch between PbI₂ and Rh surface with short-range van der Waals interaction.

Full text of DOI:10.1016/S0039-6028(03)00839-2

Surface Science 540 (2003) 274–284
www.elsevier.com/locate/susc
Deposition of lead iodide films on Rh(100) electrodes
from colloidal solutions––the effect of an iodine adlayer
a
a,
b
b,
*
Chia-Huei Yang , Shueh-Lin Yau *, Liang-Jen Fan , Yaw-Wen Yang
a
Department of Chemistry, National Central University, ChungLi 320, Taiwan, ROC
b
Synchrotron Radiation Research Center, Hsinchu 300, Taiwan, ROC
Received 8 February 2003; accepted for publication 22 May 2003
Abstract
The growth of PbI2 precipitates on single crystal substrates from colloidal solutions has been investigated with in air
scanning tunneling microscopy and synchrotron-based X-ray photoelectron spectroscopy. The PbI2 growth on
ꢀ
Rh(1 0 0) results in nano-clusters with lateral dimensions between 30 and 60 A, consistent with earlier reports. However,
the growth of PbI2 on a well-ordered iodinated Rh(1 0 0), denoted as ð 2 ꢀ 2ÞR45°-I, leads to atomically smooth PbI2
p
p
films having a hexagonal symmetry with lattice constant identical to the bulk value of 4.5 A. The heteroepitaxy is
ꢀ
believed to be effected by the atomic iodine monolayer that helps to accommodate large lattice mismatch between PbI2
and Rh surface with short-range van der Waals interaction.
Ó 2003 Elsevier B.V. All rights reserved.
Keywords: Scanning tunneling microscopy; X-ray photoelectron spectroscopy; Lead; Halides; Epitaxy; Growth; Iodine; Rhodium
1. Introduction
the sequence of I–Pb–I planes thus defines a mo-
lecular layer. The unit cell dimension is a ¼ 4:557
ꢀ
Lead iodide (PbI2) is a wide bandgap semicon-
ductor (Eg ¼ 2:32 eV) and has been a semicon-
ductor material for use in solid state X-ray and
gamma-ray detectors [1,2]. The thin film fabrica-
tion of lead iodide material is expected to open up
its application. Heavy metal iodides are structur-
ally similar to transition-metal dichalcogenides.
Their basic structure repeat unit is a hexagonally
close-packed layer of metal ion sandwiched be-
tween two layers of iodide ions. For lead iodide,
and c ¼ 6:979 A [3]. Each atomic plane in the
molecular layer is consisted of strongly bonded
atoms of the same element, but the bonding be-
tween adjacent molecular layers is due to weak,
van der Waals force [4]. Owing to this structural
anisotropy, it is possible to grow layer-structured
PbI2 materials over a large lateral dimension [5].
The structure of small-scale PbI2 materials has
been investigated by transmission electron mi-
croscopy (TEM) and atomic force microscopy
(AFM) [6,7]. Both results indicate preferential
formation of single-layered, platelet-like nano-
clusters with lateral dimensions ranging from 1 to
6 nm, depending on the solution concentration.
TEM data also point to the existence of stacking
* Corresponding authors. Tel.: +886-3-4227151-5909; fax:
+886-3-4227664.
E-mail address: philyau@cc.ncu.edu.tw (S.-L. Yau).
0039-6028/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved.
doi:10.1016/S0039-6028(03)00839-2

C.-H. Yang et al. / Surface Science 540 (2003) 274–284
275
faults i.e., a few percent of polytypism of 4H, 6R,
and 12R present besides predominant 2H structure
film on ordered iodine-pre-dosed Rh(1 0 0). The
van der Waals interaction derived from the pre-
dosed iodine adlayers is believed to be responsi-
ble for the ensuing growth of atomically flat PbI2
film.
ꢀ
[6]. AFM results reveal an averaged 10 ꢁ 1 A
height for PbI2 clusters deposited on mica, sug-
gesting a seemingly large interlayer expansion by
as much as 40% [7]. Meanwhile, an ab initio cal-
culation predicts that in-plane lattice constant can
decrease by 10% accompanied by the increase of
interlayer distance [8]. It suffices to say that the
structure of the supported PbI2 thin film is still not
completely certain.
2. Experimental
2.1. Preparation of iodine-coated Rh(100) sub-
strate
One major benefit expected from the current
nano-materials research is the ability to engineer
the bandgap of semiconductor materials through
quantum confinement effect. Previous studies suc-
cessfully demonstrated a blueshift of UV absor-
bance peaks for the decreasing size of PbI2
clusters, produced from a simple solution prepa-
ration procedure [5–12]. When engineering this
class of bandgap materials, the importance of
controlling individual property and assembling
them in an orderly fashion cannot be overstated.
As a layer-structure material, the growth of PbI2
film is subject to some certain constraints. For
instance, the film thickness can only be increased
The Rh(1 0 0) electrodes were made by melting
one end of an ultrapure Rh wire (/ ¼ 0:8 mm)
with a hydrogen and oxygen torch. With a high
melting point of 1923 °C and the readiness for
oxidation, it was rather difficult to melt the rho-
dium to prepare high-quality single crystal Rh
beads. The as-prepared Rh beads have a mean
diameter of ꢂ2 mm. To deposit iodine overlayers,
we annealed the Rh(1 0 0) electrode with a hydro-
gen torch for 5 min and then quickly transferred
the electrode into an iodine-containing glass cell.
During the operation, the whole setup was purged
constantly with ultrapure nitrogen gas.
ꢀ
at a multiple of 7 A, the thickness of a complete
The iodine molecules dissociatively adsorb on
the Rh(1 0 0) surface. The number of iodine ad-
layers and their surface morphology depended
sensitively on the iodine deposition conditions
such as partial pressure of iodine, N2 flow rate, size
and temperature of Rh electrodes, the distance
between the Rh and iodine sources, etc. No single
recipe for growing iodine monolayer could be ex-
actly specified. However, a higher flow rate and a
larger separation from the iodine source to the
electrode tended to favor the growth of iodine
monolayer. Typically, we placed the annealed
Rh(1 0 0) electrode 1 mm away from the iodine
source under a high N2 flow rate. This procedure
was usually successful in making well-ordered
iodine adlattices. Once the Rh electrodes were
coated with iodine atoms, their surfaces appeared
hydrophobic and essentially passivated toward
further oxidation or adsorption of organic con-
taminants in air. Similar iodine pretreatment has
been applied for growing ordered films on noble
transition-metal electrodes of Pt, Ir, Pd, and Au
[14–17].
charge-neutralized I–Pb–I layer. As for in-plane
dimension, whether there exists a size-propensity
for the PbI2 clusters is still a subject of discussion
due to the uncertainty in assigning the observed
optical transitions [6]. Due to a large disparity in
the surface free energies between dichalcogenide
and the metal substrate, it will be of great interest
to engineer a heteroepitaxy by altering the free
energy of the metal surface through the adsorption
of foreign materials [13].
In the present paper, we report a joint scanning
tunneling microscopy (STM) and X-ray photo-
electron spectroscopy (XPS) study of the thin PbI2
films grown from solution onto different
substrates. STM is utilized to characterize the real-
space atomic structures of the films. High-resolu-
tion synchrotron based XPS work addresses the
chemical structure and species distribution of the
lead iodide precipitate. We found a striking dif-
ference of PbI2 growth upon changes of sub-
strates––from nano-clusters occurring on a clean
Rh(1 0 0) to an atomically smooth, crystalline

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C.-H. Yang et al. / Surface Science 540 (2003) 274–284
2.2. Preparation of PbI2 solutions
Hz in high-resolution scans to minimize the influ-
ence of thermal drift. The spatial measurements
from STM results typically had an error of ꢁ5%.
All the chemicals, including lead nitrate, po-
tassium iodide, and acetonitrile were of ultrapure
grade and used without further purification. Triply
distilled Millipore water was used to prepare all
solutions. We adopted the procedure reported in
the literature to prepare PbI2 nanoparticles [5–9].
This method involved adding 5 ml 0.01 M
Pb(NO3)2 to 100 ml actonitrile, followed by the
injection of an aliquot of 2.5 ml 0.05 M KI under
constant stirring. The resultant solution appeared
to be pale yellow but transparent. An iodine-
modified Rh(1 0 0) electrode was then dipped into
this PbI2 solution for 1 min or longer, depending
on the type of material to be studied. The Rh(1 0 0)
electrode was then dried with a nitrogen steam
before mounting onto the STM cell.
According to several previous studies, this
preparation procedure produced PbI2 nano-crys-
tallites with lateral dimensions grouped around
1.2, 1.8, and 2.9 nm [5]. These species were thought
to be responsible for the prominent absorption
features in the optical spectrum at 3.42, 3.95, and
4.80 eV, respectively. In addition to acetonitrile, a
number of organic solvents, for example, 2-pro-
panol and pyridine, were tested as the co-solvent
in making PbI2 colloidal solutions. It appeared
that polyiodide ions and metallic hydroxide species
formed in wet chemistry could cause complications
in characterizing PbI2 nano-clusters [11]. Further-
more, pyridine could replace iodide inside the co-
ordination sphere of Pb2þ ions and thus modified
the optical properties of the nano-clusters [12].
2.4. X-ray photoelectron spectroscopy
All the photoemission data were acquired from
an ultra-high vacuum (UHV) end station hooked
up to BL12A wide range spherical grating mono-
chromator beamline (WR-SGM) at Synchrotron
Radiation Research Center. This unique beamline
was equipped with a set of six spherical gratings to
deliver soft X-ray photons with energy ranging
from 15 to 1500 eV. The mu-metal UHV chamber
housed a differentially pumped sputter ion gun for
sample cleaning, an angle-integrated VG CLAM4
energy analyzer, an angle-resolved VG ADES 400
energy analyzer, and a load-lock sample transfer
mechanism. A combined energy resolution of the
beamline and energy analyzer was estimated to be
better than 0.2 (0.5) eV for 200 (700) eV photon
energy in use here. All the XPS spectra presented
here were first normalized to photon flux by di-
viding recorded XPS signal with photocurrent de-
rived from a gold mesh situated in front of the
sample. Binding energy scale in all the spectra was
calibrated against three Rh core levels spanning a
wide energy range: Rh 4p3=2 at 47.27, Rh 3d5=2 at
307.2, and Rh 3p3=2 at 496.5 eV. [18,19]. Because
the Rh single crystal electrode employed for STM
measurement was rather small, ꢂ3 mm diameter,
another larger Rh(1 0 0) single crystal with a di-
ameter of 10 mm was used for photoemission
measurement. This single crystal was cleaned by
the flame-annealing and the good crystallinity was
confirmed by STM before the deposition of the
PbI2 thin films.
2.3. STM measurements in air
The STM was a Nanoscope-E (Santa Barbara,
CA) and the tip was made of tungsten (diameter 0.3
mm) prepared by electrochemical etching in 2 M
KOH. Cleaning was done by rinsing with water
and acetone prior to use. The scanner was an A-
head from digital instruments. The maximal scan
size was 500 ꢀ 500 nm and the piezo was calibrated
against a highly oriented pyrolytic graphite sample.
Thermal drift appeared to be a main obstacle in
obtaining the correct measurements of interatomic
spacing. Thus, we normally used a scan rate of 40
3. Results and discussion
3.1. In air STM imaging of iodine adlayers on
Rh(100)
Fig. 1a shows a constant-current, topography
scan of a Rh(1 0 0) electrode coated with one
monolayer of iodine atoms. Well-defined features
of steps and terraces, typical of single crystal

C.-H. Yang et al. / Surface Science 540 (2003) 274–284
277
surfaces, are obvious. The steps are found to in-
tersect at a near 90° angle, a feature commonly
observed on a fcc(1 0 0) surface. Nearly all the
steps are parallel to the [0 0 2] and [0 2 0] directions,
along which the close-packed atomic rows
p
p
of ð 2 ꢀ 2ÞR45°-I lie (vide infra). Fig. 1b shows
the atomically resolved STM image of the terrace,
indicating
attributable to the uppermost iodine adlayer. The
a
highly ordered square array,
close-packed atomic protrusions are aligned 45°––
rotated from the atomic rows ([0 1 1] and [0 1 1]
ꢁ
directions) of the Rh(1 0 0) substrate. The spacing
measured between two nearest-neighboring iodine
atoms is 0.38 nm, which corresponds nicely to the
p
theoretical value ( 2 times the spacing of Rh atom
0.268 nm). Thus, this STM result indicates the for-
p
tent with that reported previously [20].
p
mation of a ð 2 ꢀ 2ÞR45° iodine lattice, consis-
This iodine structure is highly ordered with very
few vacancy defects within the adlattice. It is
worthwhile mentioning that for all the studies of
iodine adsorption on Pt(1 1 1), Pt(1 0 0) and
Ir(1 1 1) [14–16], vapor phase dosing always pro-
duces fewer vacancy defects and domain bound-
aries in the iodine overlayers than those produced
by adsorption from KI solutions [21]. All the
atomic features appear to be equally bright with
the corrugation of 0.04 nm, a typical value found
for iodine adatoms chemisorbed on transition-
metal surfaces like Pt, Rh, Ir, etc. Fig. 1c presents
p
p
a ball model for the Rh(1 0 0)-ð 2 ꢀ 2ÞR45°-I
structure with all iodine atoms residing at identical
4-fold hollow sites. This model necessitates similar
intensity for all iodine atoms in the STM image, as
indeed observed in Fig. 1b.
In addition, the STM results reveal an unusual
surface feature, marked by a white arrow in Fig.
1a. A careful examination reveals that it stretches
for nearly 60 nm before vanishing into the terrace.
Consequently, this feature would be better defined
as a dislocation defect, rather than a step ledge,
although it exhibits a height difference of 0.25 nm.
In comparison, we hardly notice this type of defect
on iodine-modified Pt(1 1 1) and Rh(1 1 1) elec-
trodes prepared in a similar manner [14–16]. The
cause of this unique phenomenon is not clear at
present. These dislocation defects persisted after
the deposition of PbI2 films.
Fig. 1. (a) In air STM image of an iodine-modified Rh(1 0 0)
surface, where the white arrow marks a dislocation defect. (b)
Atomically resolved image of (a). (c) A ball model corresponds
to the atomic arrangement in (b).

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C.-H. Yang et al. / Surface Science 540 (2003) 274–284
As noted by previous researchers, the layer
thickness of iodine on Rh(1 0 0) largely hinged on
the conditions under which they were deposited. A
van der Waals interaction is presumed to be re-
sponsible for the growth of iodine multilayers.
Detailed structural investigation will be presented
elsewhere. The formation of iodine multilayers on
noble transition-metals, such as Ir and Rh, is a
rule, rather than an exception [15,16].
3.2. STM imaging of the surface morphology of
PbI2 films
As described in Introduction Section, the in-
teraction among dissimilar Pb and I layers in PbI2
is of strong electrostatic force in origin, but the
interlayer interaction among a complete, charge-
neutralized PbI2 monolayer is due to weak, van
der Waals force. The data, including those not
presented here, show the formation of iodine
multilayer interacting via weak van der Waals
force is possible. We will now explore the possi-
bility of growing an atomically smooth PbI2 film
supported on iodine monolayer through a utiliza-
tion of van der Waals interaction. Fig. 2a shows an
STM image obtained for an I-modified Rh(1 0 0)
electrode soaked for 1 min in a PbI2-containing
solution that is also comprised of water and
acetonitrile. The smoothness of the film is evident,
and the image is different from those for iodine
monolayer and iodine multilayer. Several ques-
tions can be raised regarding this finding. What
chemical species are responsible for the observed
image and what kind of the depth distribution
these species have? What happens to the atomic
iodine layer laid down prior to the PbI2 deposi-
tion? Does this iodine layer become incorporated
into I–Pb–I layer structure or does this iodine layer
exist as a separate entity from I–Pb–I? Obviously,
the answers to these questions cannot be provided
by STM alone. We thus digress slightly and invoke
synchrotron-based high resolution XPS data to
shed light on these questions.
Fig. 2. (a) In air STM topographic scan showing the surface
morphology of a PbI2 film deposited on an iodine-modified
Rh(1 0 0). (b) Another topography scan obtained in a separate
experiment, where D and S, respectively, represent dislocation
and isolated islands. (c) A cross-section profile along the dotted
line in (b).
3.3. XPS measurements
eV. The short solution immersion time was used to
ensure a PbI2 monolayer structure that allows the
I–Rh interface to be examined. The spectrum
Fig. 3a shows an XPS survey spectrum for the
PbI2 thin film taken with a photon energy of 700

C.-H. Yang et al. / Surface Science 540 (2003) 274–284
279
core levels appear at 618.8 and 138.4 eV, respec-
tively, in good agreement with the literature values
of 619.5 and 138.7 eV for the bulk PbI2 [18,19].
When the PbI2 thin film is briefly annealed to 200
°C in vacuum, the PbI2 film decomposes to evolve
iodine into the gas phase while leaving behind a
metallic Pb species, evidenced by its binding en-
ergy shift to 136.8 eV. This XPS result confirms the
formation of PbI2.
High-resolution I 4d photoemission measure-
ment was performed to discern the iodine species
because of the involvement of both iodine and
iodide in the preparation method. I 4d rather than
I 3d core level is preferred because of its narrower
lifetime width [22] and a better energy resolution
of the beamline at the lower energy end. Fig. 3b
shows an I 4d core level spectrum for the as-pre-
pared PbI2 thin films taken at a photon energy of
200 eV. A non-linear least squares fitting is per-
formed using a constant background in conjunc-
tion with a line shape function generated by a
convolution of the Gaussian function with the
Doniach–Sunjic function broadened by a finite
lifetime. The spin orbit splitting in the doublet is
fixed at 1.70 eV and a branching ratio of the
doublet is optimized at 0.65, close to its statistical
value of 0.67. In a given spectrum, the Gaussian
with (CG), Lorentzian width (CL) and Doniach–
Sunjic asymmetric parameter (a) are allowed to
vary in the fits but are constrained to be the same
for all the peaks. A consistent set of parameters
emerges from fitting the spectra of four different
angles. The best fit parameters are as follows:
CG ¼ 0:672 eV, CL ¼ 0:233 eV, and a ¼ 0:035, in
line with earlier reports of iodine adsorption on
Ag(1 1 1) and Ge(1 1 1) [22,23]. Most importantly,
the fitting reveals the presence of two I compo-
nents with a major peak at I 4d5=2 ¼ 49.1 eV and a
minor one at I 4d5=2 ¼ 49.9 eV. The intensity ratio
between two peaks is 4:1.
Fig. 3. (a) A survey XPS spectrum for PbI2 thin film taken at a
photon energy of 700 eV. The peak assignments are designated
with the labels displayed nearby. (b) High-resolution I 4d core
level spectrum for PbI2 thin film. The spectrum was acquired at
40° emission angle with 200 eV photons. A non-linear least
squares fit reveals the presence of two I components denoted in
thin lines. The resultant curve is expressed as solid line that runs
through raw data (symbols). A minute contribution by the
substrate Rh 4p5=2 signal at 47.3 eV can become noticeable for
an emission angle approaching surface normal.
contains a series of peaks with the labels of the
peak assignments included for the ease of refer-
ence. It is found that, besides a small amount of
carbon and oxygen contaminants, the PbI2 thin
film is relatively clean even prepared from a
somewhat contaminant-prone solution environ-
ment. The near complete absence of K 2p doublets
at 293 and 295.8 eV is in accord with the predic-
tion based on the large solubility product con-
stants of KI and KNO3. The I 3d5=2 and Pb 4f7=2
The structure of as-prepared PbI2 thin film is
discussed in light of the finding of two I 4d core
levels. The I 4d5=2 core level of 49.1 eV is attributed
to the iodide bonded to Pb ions, due to the larger
screening of negative ion. In comparison, the I 4d5=2
core level of 49.9 eV is assigned to the chemisorbed
iodine atom, with binding energy higher than that
found for Iad on Ag(1 1 1), 48.9 eV [22]. The

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C.-H. Yang et al. / Surface Science 540 (2003) 274–284
separate entity and on top of which a single PbI2
monolayer forms. This picture of PbI2 thin film is
in accord with the STM results that we shall return
to now.
support of this assignment comes from the inten-
sity analysis. For a substrate covered by a homo-
geneous adsorbate layer of thickness d, the
attenuation of photoelectron signal for the sub-
strate can be described as
3.4. STM imaging of the atomic structures of PbI2
films
Is ¼ I0;seꢃd=k cos h
s
where Is and I0;s are the intensities of photoelectron
peaks for the adsorbate-covered substrate and
clean substrate, respectively; h is the angle between
the substrate surface normal and electron collec-
tion direction; and ks is the inelastic mean-free-
path that refers to the photoelectrons traversing
through the said adsorbate. If one unreconstructed
It is critical to select a proper set of parameters
when performing STM imaging. Strong interac-
tion between the scanning probe and the surface
can result in the etching of the PbI2 films. On the
one hand, we usually employed a bias voltage of
more than 500 mV and a feedback current of lower
than 5 nA to minimize the etching effect. On the
other hand, we also explored the etching phe-
nomenon to measure the thickness of the mono-
layer and multilayer PbI2 film because the removal
of filmy PbI2 proceeded in a layer-by-layer fash-
ion. An artificially created step is typically 0.7 nm
deep, which agrees with the lattice constant along
the c-axis of PbI2. Although a previous ab initio
study claims that both intralayer contraction and
interlayer expansion occur for the Pb6I12 cluster
[9], we found no evidence supporting this claim for
the thin film materials. For layered semiconduc-
tors, the phenomenon of tip-induced etching
is ubiquitous. Reported systems include WSe2,
MoS2, HgI2, PbI2, and others [25–28]. The etching
event can be due to mechanical or electrochemical
force; however, we will refrain from a further
discussion because this issue falls outside the pre-
sent context.
By and large, the deposition of PbI2 does not
lead to a major change in the surface morphology.
Well-defined steps and terraces preexisting on the
iodine-modified Rh substrate are still preserved.
The uniformity of the terraces seems to suggest
that the PbI2 film is ordered. Indeed, high-resolu-
tion scans of the PbI2 terraces reveal clearly re-
solved atomic features, as shown in Fig. 4a and b.
The imaging parameters are 500 mV and 5 nA
and these atomically resolved structures are stable
against prolonged imaging, unless the bias voltage
decreases to 200 mV or lower. The symmetry of
these two structures is essentially hexagonal, de-
spite the presence of some inevitable distortion
(ꢁ5°), owing to the thermal drift of the piezo tube.
p
p
PbI2 monolayer is formed on top of ð 2 ꢀ 2Þ-
R45°-I, the elemental ratio between iodide and
iodine is expected to be around 1.6:1, based on the
respective lattice constants. In the aforementioned
equation, the exact value of the k term is always
plagued with some uncertainty. For the present
case, the situation is further complicated by the
presence of mixed Pb and I layers. Specifically, k
parameter now refers to how the photoelectrons of
specific energy are attenuated as they pass through
I and Pb layers. This parameter is not readily
available and we thus model the PbI2 on Iad system
as a homogeneous medium of constant layer
spacing l. A layer-attenuation factor, eꢃl=k, can be
immediately calculated as 0.72, or k=l ¼ 3, from
the observed 4:1 intensity ratio (ð1 þ 0:722Þ=0:723 ¼
4:1). For lower iodide layer, a value of 0.722 is
expected since it locates below the top iodide and
the Pb layers. Similar arguments apply to the
bottom iodine layer and give rise to a value of
0.723 for this layer. The distance from Iad layer to
the top Iꢃ layer, 3l, should be close to the c-axis
ꢀ
value of the bulk PbI2, namely, 7 A. Thus, the k
ꢀ
value is also equal to 7 A, a very realistic value
[24], given the rather crude approximation used.
Moreover, under the constraint of a short k pa-
rameter, a significant fraction of Iad signal can be
still observed, suggesting the thin nature of the
PbI2 film. It is most likely to be a monolayer film
because each additional PbI2 layer will increase the
ꢀ
film thickness by a hefty amount of 7 A. In brief
summary, XPS data unambiguously show the
formation of PbI2 on Rh–I surface. The high-res-
olution I 4d data indicate that Iad layer exists as a

C.-H. Yang et al. / Surface Science 540 (2003) 274–284
281
The average nearest neighbor spacing of 0.45 nm is
slightly larger than the van der Waals diameter
(0.43 nm) of an iodine atom but agrees quite well
with the lattice constant of bulk PbI2. Moreover,
the image in the Fig. 4a features radiative hexag-
onal trenches, denoted by the dotted lines, along
the closed-packed atomic rows of the hexagonal
array. The trenches are mainly the dislocation
defects that separate two neighboring hexagonal
domains.
Fig. 4b shows the presence of two rotational
domains that are separated by a winding domain
boundary highlighted by a dotted trace. These two
domains are rotated 45° with respect to each other
and one of their closed-packed atomic rows are
p
parallel to the close-packed direction of ð 2 ꢀ
p
flat and exhibit a long-range order. Fig. 4c shows a
2ÞR45°-I, i.e., [0 1 0]. The films are all atomically
p
p
model of the PbI2/ð 2 ꢀ 2ÞR45°-I system, where
PbI2, maintaining at its bulk lattice constant,
stacks on top of the atomic iodine layer. Atomic
iodine layer is represented with solid circles and,
for clarity, only the lower iodide layer (hollow
circles) is depicted in the model with the upper
iodide and Pb2þ layer sandwiched in-between
omitted. Due to the lattice mismatch between the
hexagonal PbI2 structure and the square lattice of
the I–Rh(1 0 0) substrate, it is concluded that an
incommensurate adsorption structure results,
leading to a corrugated surface. (Sentences in red
are misplaced, kind of odd here, considering de-
letion? If so, change However to Moreover in
the next sentence) However, from the atomic
smoothness of the PbI2 film, it can be argued that
the films are formed through a sequential deposi-
tion of Pb2þ and Iꢃ layers, rather than a deposition
via PbI2 particles. A deposition via particle accu-
mulation will inevitably lead to films exhibiting
considerable surface roughness.
One ordered PbI2 domain is highlighted and
reproduced in Fig. 5a. Also presented in Fig. 5b
and c are the profiles of two sectional cuts along
the solid and dotted lines in Fig. 5a. The height
difference between markers 1 and 2 in Fig. 5b is
0.11 nm, suggesting that the vacancy defect is one
atom deep and a small corrugation of 0.04 nm is
usually obtained. Fig. 5c shows the corrugation
along the dotted line and the resultant profile
Fig. 4. (a) and (b) are two atomically resolved STM images,
showing the arrangements of the uppermost iodide layer at
different locations of the sample. The ball model in (c) depicts
the relationship between the iodine adlayer and PbI2 film. The
imaging parameters are 500 mV and 5 nA.

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C.-H. Yang et al. / Surface Science 540 (2003) 274–284
Fig. 5. (a) An STM image highlighting the atomic arrangement
in the PbI2 film, where (b) and (c) are corrugation profiles along
the solid and dotted lines in the image.
Fig. 6. In air STM images of Rh(1 0 0) surfaces exposed to a
PbI2-containing water and acetonitrile colloidal solution for 2
(a) and 5 (b) min.
provides a typical intensity variation along a
closed-packed direction. This difference in atomic
corrugation can stem from unlike chemical nature
or registries of adsorbates. In the present situation,
the atomic features are due to the uppermost layer
of iodide ions of the PbI2 film. Because it is not
possible for all the iodide ions to have identical
registries with respect to the underlying square
lattice of ð 2 ꢀ 2ÞR45°-I, the presumed sub-
strate net. The corrugations of iodide ions thus
reflect the physical height difference among atoms
in the film.
To discern the role played by the chemisorbed
iodine layer in governing the growth of lead iodide
films on Rh(1 0 0) surfaces, we examined the PbI2
deposited onto a bare Rh(1 0 0) electrode with the
STM. Fig. 6 shows two topographic scans ob-
tained for two individual samples prepared in
separate experiments. The results in a and b were
obtained for Rh(1 0 0) soaked in PbI2-containing
colloidal solutions for 2 and 5 min, respectively.
Compared to the surface morphology of iodine-
modified Rh(1 0 0) in Fig. 5, the surfaces in both
p
p

C.-H. Yang et al. / Surface Science 540 (2003) 274–284
283
cases appear much rougher, featuring protruding
islands preferentially located at the terraces. The
averaged diameter of protruding features is about
force is weak and short-range and the iodine
atoms, located at identical 4-fold hollow sites, es-
sentially shield the charge corrugation due to the
metal surface. The smoothened surface corruga-
tion can then facilitate the layer growth of PbI2
because the interlayer interaction among com-
pleted PbI2 unit is also van der Waals force. In
contrast, the layer growth of PbI2 on bare
Rh(1 0 0) will be difficult, given the presence of
lattice mismatch between PbI2 lattice and the Rh
substrate as well as the large atomic corrugation of
the Rh surface.
ꢀ
ꢀ
30 A in a and 60 A in b. The relative corrugation
ꢀ
heights of these nano-clusters are 7 and 2.5 A for
the case of a and b, respectively. The former cor-
responds to the ideal height for a PbI2 layer, but
the origin of the latter is difficult to determine.
Theoretically, it is possible to use STM to discern
the chemical natures of the protruding islands. The
spectroscopy mode, invoking the dI=dV relation-
ship, allows the differentiation between these two
species. In reality, the presence of water layer on
the PbI2 sample and/or the tip prohibits us from
obtaining reproducible results. However, an addi-
tional observation that these protruding features
can be dissolved by water rinsing suggests that
these features are due to water-soluble PbI2, in-
stead of water-insoluble iodine.
Because of the non-directional character of ionic
bonding in these films, the spatial arrangements of
ions are theoretically dominated by factors, such
as lattice matching between the films and the
substrate and the electronic properties of the films.
There are many studies addressing the geometric
effect. For example, the heteroepitaxial growth of
ionic films of MgCl2 on several transition-metal
surfaces has been examined by using low energy
electron diffraction [33]. It is concluded that the
geometric matching between substrate and adsor-
bate plays a pivotal role in controlling the growth
of the MgCl2 films. Similar findings are reported
for heteroepitaxial growth of CuCl and CuBr,
zinc-blend structure ionic compounds, on MgO-
(0 0 1) with rock-salt structure [34]. In addition to
this geometric effect, Tasker first hypothesized the
need to have zero dipole moment perpendicular to
a substrate as a necessary condition for the for-
mation of a stable ionic crystal surface [35]. This
idea seems to be applicable in the present case
because of the involvement of a completely charge-
neutralized PbI2 layer. Based on the STM results
in Figs. 4 and 5, it is believed that the most stable
The dramatic difference between PbI2 thin films
deposited on bare Rh(1 0 0) and on iodine-modi-
fied Rh(1 0 0) is to be discussed here. The PbI2 film
grown on the iodine-modified Rh surface exhibits
atomic-scale smoothness, whereas the PbI2 grown
on Rh surface shows nano-sized clustering fea-
tures. We speculate on the following reasons for
this difference. Previous studies show iodine atoms
covalently bond to Ni [29], Cu [30] and Rh [31]
substrates, as evidenced by the fact that iodine-
metal bond length is equated to the sum of iodine
covalent radius and the metal metallic radius.
Room temperature adsorption of molecular iodine
on Rh(1 0 0) leads to a saturation structure of
p
p
ð 2 ꢀ 2ÞR45°-I at a coverage of 0.5 ML and the
distance between two nearest-neighboring iodine
ꢀ
atoms is 3.80 A, in-between covalent diameter
(2.67 A) and van der Waals diameter of 4.3 A.
Saturation coverages of iodine atoms on a metal
surface are thought to be limited by the iodine–
iodine repulsive interaction rather than bonding
interactions with the metal surface. Our observa-
tion of surface passivation once iodine layer is
adsorbed on various metals agrees with the earlier
reports in which iodine layer also passivates highly
reactive semiconductor surfaces like Si [32] and Ge
[23]. Taken together, we argue that the overlayer
growth on top of iodine layer seems to proceed via
van der Waals interaction. The van der Waals
configuration of the films can consist of Iꢃ–Pb2þ
–
ꢀ
ꢀ
Iꢃ tri-layers sequentially stacked normal to the sur-
face normal. Although each layer is charged, there
is no net dipole moment in the surface normal.
4. Conclusion
A pre-dosed iodine adlayer on Rh(1 0 0) is
found to exert a marked effect on the deposition of
PbI2 from colloidal solutions prepared with water
and acetonitrile. Atomically resolved STM images

284
C.-H. Yang et al. / Surface Science 540 (2003) 274–284
p
p
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reveal a highly ordered ð 2 ꢀ 2ÞR45° iodine
overlayer with atomically flat terraces spanning
100 nm wide. This structure serves as the substrate
onto which a crystalline PbI2 film is deposited via a
simple immersion process. STM imaging reveals a
hexagonal lattice with a lattice constant of 0.45
nm, attributable to the uppermost anionic iodide
layer of PbI2 because the Pb2þ layer lying under
the iodide layer cannot be directly imaged. In view
of lattice-matching requirement during thin film
growth, the formation of a thin film with a hex-
agonal symmetry on a 4-fold symmetric structure
is somewhat unexpected. The XPS result unam-
biguously establishes a stacked structure in which
the PbI2 layer is located on top of the iodine layer.
The iodide anions of the PbI2 layer is believed to
interact with the pre-dosed iodine through van der
Waals force. In contrast, dipping a bare Rh(1 0 0)
electrode in PbI2-containing colloidal solutions
results in a deposition of nano-flakes. The flakes
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Acknowledgements
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This work is supported by the National Science
Council of the Republic of China under contract
no. NSC 90-2119-M-008-002. We thank K.S.
Liang of SRRC for a fruitful discussion on the
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