Jpn. J. Appl. Phys. Vol. 41 (2002) pp. 4932–4935
Part 1, No. 7B, July 2002c
2002
The Japan Society of Applied Physics

Scanning Tunneling Microscopy Study of Water Molecules
on Pd(110) at Cryogenic Temperature
Tadahiro KOMEDA1 , Hirokazu F UKIDOME1 , Yousoo K IM1 , Maki K AWAI1 ,Yasuyuki S AINOO1,2 and Hidemi S HIGEKAWA2
1 RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
2 Institute of Applied Physics and CREST, Japan Science and Technology Corporation (JST), University of Tsukuba, Tsukuba 305-8573, Japan(Received February 7, 2002; accepted for publication March 13, 2002)

We show scanning tunneling microscopy (STM) images of water molecules on Pd(110) at a cryogenic temperature of 4.7 K
which enables the observation of isolated water molecules. We observe water-induced features in the STM images in a lowcoverage region (∼ 0.02 ML), which can be classified in terms of height; namely, (A) 0.15 Å, (B) 0.3 Å and (C) 0.55 Å, observed
at Vs = −100 mV. In a high-coverage region, feature C dominates and forms a local c(2 × 2) structure. Since the orderedstructure is identical with the previously reported superstructure of bilayer water on Pd(110), feature C should correspond to atetramer of water molecules, a unit of bilayer water. Feature A is located on the Pd(110) row. Since feature A is the smallest
one, we consider that this corresponds to the monomer of the water molecule whose existence has been observed previously attemperatures below 10 K by electron energy loss spectroscopy (EELS) measurement. [DOI: 10.1143/JJAP.41.4932]
KEYWORDS: scanning tunneling microscopy (STM), water, Pd(110), gas adsorption, adsorption geometry

1. Introduction

2. Experimental

Water adsorption has been investigated using various surface science techniques.1–11) This is partially due to the risinginterest in combining the biological techniques and the processes of metal and semiconductor surfaces. Although a water molecule has a simple structure, its adsorption on metal
surfaces has special features that cannot be observed in otheradsorption systems. This is mainly due to the strong hydrogenbonding between water molecules that is of the same order asthat between the metal and the water molecule and makes theformation of a water cluster easier. Thus, a number of the previous experiments of water adsorption on metal surfaces havebeen executed on condensed water, where the direct interaction between the metal and the water molecule is masked.
However, the interaction between the water molecule and themetal surface is of great interest, since the bonding is formed
between a lone pair of a water molecule and metal orbitals
whose nature has not been well understood.
In the current report, we show scanning tunneling microscopy (STM) observation of the adsorption of watermolecules on the Pd(110) surface at cryogenic temperature.
The STM observation at liquid He temperature has recently
attracted attention, partially due to the success of measurement of inelastic tunneling spectroscopy (IETS) using STM
on a single adsorbate,12) which enables the direct measurement by vibrational spectroscopy of a single molecule. At thesame time, the diffusion of adsorbates is reduced at low temperature, which quenches the motion of the adsorbates before
they encounter other adsorbates and form clusters. The images we present in this paper clearly show a water molecule in
an isolated form, as well as a locally ordered structure whichcorresponds to the reported c(2 × 2) superstructures of bilayerwater. To our knowledge, this is the first direct observation ofa water molecule in an isolated form.

All experiments were performed using a low-temperature
STM (LT-STM, Omicron GmbH) with an electrochemicallyetched tungsten tip equipped in an ultrahigh vacuum chamber(< 3 × 10−11 Torr). The Pd(110) surface is kept at 4.7 K during imaging. For the dosing of gas molecules, the sample isdetached from the STM cold head and is placed in front of adosing tube with part of the sample holder being attached toa wobble stick which is pre-cooled with liquid N2 . The dosing process, which starts with the detachment of the sample
and ends with returning the sample to the STM head, can beaccomplished within 10 s at the fastest. Due to the short time
period required for dosing, the increase of the sample temperature is small; the maximum temperature is expected to be
below ∼ 50 K from the experiment of rare-gas adsorption.
3. Results
A typical topological STM image of the Pd(110) surface
is shown in Fig. 1, where a submonolayer coverage of water molecules is dosed on the surface (∼ 0.02 ML). The inset
image shows a clean Pd(110) surface observed prior to thedosing of water molecules. The Pd row in the [11̄0] direction,
along which Pd atoms are closely packed, follows the diagonal direction from upper left to lower right. Both images areshown in the identical crystallographic direction. The waterinduced features are observed as protrusions and bright spots
in the image. The variation of these features in the image
under a bias voltage and with different conditions of the tip
apex, such as that of being coated with a water molecule, issmall compared with other simple adsorbates. For example,
a CO molecule on Pd(110) is imaged either as a protrusion
or as a dip depending on the bias voltage.13) In addition, CO
is observed as a dark feature when the tip has picked up a
CO molecule and the surface is imaged with the molecule atthe tip apex.13) In the case of the water molecule observation,such contrast reverse is not observed.
We observe three water-induced features in the image

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T. KOMEDA et al.

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C

B

A

50

Fig. 1. STM topographic image of submonolayer water molecules adsorbed on Pd(110) surface. Water molecules are dosed at a temperature
below 50 K, and STM observation is executed at the sample temperatureof 4.7 K. Imaging conditions are Vsample = −100 mV and Itunnel = 1 nA,
and the length-scale is shown in the figure. Inset shows a clean Pd(110)
surface obtained prior to dosing of water molecules (area 45 × 45 Å2 ). The
crystallographic direction is identical for both images, and the [11̄0] direction corresponds to the direction from upper left to lower right. Marks A-C
correspond to three features induced by the dosing of water molecules,
whose typical heights are 0.15 Å, 0.3 Å, and 0.55 Å, respectively.

shown in Fig. 1. They can be classified by height and we refer to them as features A, B, and C with nominal heights of
0.15 Å, 0.3 Å, and 0.55 Å, respectively, observed at Vsample =−100 mV. The observed heights listed above are too small tobe the actual height even for a water monomer regardless of
its bonding configuration. The STM tip is basically tracing
the contour that gives the same density of states.13) Since thedensity of states induced by water adsorption near the Fermi
level is expected to be small, the observed height of the water molecule is much smaller than the actual height of themolecule.
The observed height shows a slight bias voltage dependence. Feature B shows a decrease in height with increasing
amplitude of the bias voltage; its height is 0.38 Å at Vsample =−20 mV and 0.3 Å at Vsample = −100 mV. On the other hand,feature C shows an increase in height with voltage; it is 0.45 Å
at Vsample = −20 mV, and 0.55 Å at Vsample = −100 mV. The
difference of the bias-dependent height variation should correspond to the difference of the electronic states detected by
the STM tip. The height of feature A does not show strong
dependence on the bias voltage, which is ∼ 0.15 Å.
Next we show a high-resolution image of features A and C.
A magnified image of feature A is shown in Fig. 2. Pd(110)rows along the [11̄0] direction can be seen in the perpendicular direction, where the distance between the rows is 3.9 Å.
The water-induced bright area is elongated along the row; thelength of the bright area is 7–8 Å along [11̄0] but its widthis less than 3 Å. There are bright spots on the row, but theirposition along the row cannot be detected with the resolution
obtained.
A high-resolution image of feature C is shown in Fig. 3.

Fig. 2. High-resolution image of feature A which has the characteristicheight of 0.15 Å (area 28 × 28 Å2 , Vsample = −60 mV and Itunnel = 1 nA).
The feature at the Pd row ([11̄0] direction) is visible in the vertical direction. The elongated water feature is located on the Pd row.

Fig. 3. High-resolution image of feature C with a typical height of 0.55 Å
(area 75 × 75 Å2 , Vsample = −50 mV and Itunnel = 1 nA). The feature at
the Pd row ([11̄0] direction) is visible in the diagonal direction. The featureinduced by the water molecule has a circular shape with a bright and dark
contrast in the head and tail of the molecule. The center of the feature islocated at the trough of the Pd row.

This particular image is observed with the tip apex coatedwith water molecules. We intentionally scanned the surface
at a reduced gap distance so as to pick up a water molecule.
The reasons we consider that the image is obtained with thewater molecule on the apex of the tip are as follows. First, thecorrugation of the atomic structure of the Pd(110) substrate
is significantly higher than that with a bare metal surface.
Second, the IETS spectra show distinct features even whenmeasured on a bare metal area.14, 15) The image obtained with

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T. KOMEDA et al.

of which is marked A in Fig. 4. Although the domain size ofthe local ordering is small, the superstructure coincides withthe reported ordered structure of water on Pd(110) that has thec(2 × 2) structure. In the lower panel, we show a schematicof the c(2 × 2) structure proposed for water molecules onPd(110).3, 5) It is based on a model in which a bilayer of water molecules forms a stable bonding configuration. The large
and small gray circles with gradation correspond to oxygen
atoms of the water molecules in the top and the second layer,
respectively. The zigzag-shaped chain of protrusions marked
A in Fig. 4 corresponds well with the positions of the watermolecules in the top layer.
The correlation of the heights observed for feature C in
Fig. 1 and the clusters with the local ordered structure in Fig. 4
suggest that feature C is the unit of the bilayer structure of water molecules.
4. Discussion

Fig. 4. Topographic image of water molecule on Pd(110) obtained under
the conditions of Vsample = −60 mV and Itunnel = 1 nA. Clusters of watermolecules on the bare Pd(110) surface with the height of 0.55 Å can beobserved. The ordered c(2 × 2) structure of water molecules on Pd(110)proposed in a previous report is shown in the schematic model in the lower
panel. The large and small circles with gradation correspond to the oxygen atoms in the top and the second layers of water, respectively (bilayer
model). The gray circles correspond to the substrate Pd atoms. The zigzag
features observed in the left-panel marked A correspond well with the water molecules in the top layer.

water molecules on the tip apex, however, has an especiallyhigh resolution for the determination of the bonding site of
the water molecule.15)
The image has a characteristic ring shape with bright anddark contrast in the head and the tail of the molecule along
the [11̄0] direction. It is evident that the center of the ring is
positioned on the trough of the Pd row, although its position
along the Pd row is difficult to determine.
When a substantial amount of water molecules are dosedon the surface, water clusters are formed on the surface whose
typical image is shown in Fig. 4. The amount of dosed watermolecules is larger than that in Fig. 1, but the coverage is stillin the submonolayer region, and both clusters and the barePd(110) surface are observed. In addition to the increase in theamount of dosed water molecules, the substrate temperature
at the time of dosing is higher than that in the case in Fig. 1
due to the increase in the dosing time period. Both conditions promote the collision of the impinging water moleculeswith each other and thus the formation of clusters of watermolecules.
The protrusions have a height of 0.55 Å that agrees withthat of feature C in Fig. 1. The crystal direction is the sameas that in Fig. 1, and we can identify chain-like water-induced
features along the [11̄0] direction. In addition, zigzag-shaped
structures can be identified inside the cluster, a clear example

We discuss water cluster formation and the origin of the
STM features observed for a low-coverage surface. The clusters of water molecules are considered to be formed in a process in which the impinging molecules migrate on the surface
before their motion is quenched. Thus we expect more water
clusters on the surface if a greater amount of water molecules
are dosed at a higher substrate temperature. The difference
between the water-induced feature shown in Fig. 1 and that inFig. 4 could be due to this mechanism; the surface in Fig. 4
appears to contain more water clusters. The chain structure in
Fig. 4 is possibly the initial stage of the formation of a wide
domain of c(2 × 2) structures which has been reported previously.3, 5)
The mechanism underlying the initial formation of a onedimensional chain structure instead of the two-dimensional
structure is not clear at this stage. However, the mechanism
might be similar to the case of chain structure formation of
CO on a Pd(110) surface.13) A one-dimensional (1D) CO
chain structure along [11̄0] with a two-time periodicity isformed before the formation of a two-dimensional c(2 × 2)
structure. For the CO case, we have learned that the hoppingof CO is restricted on the Pd row and the inter-row hopping
apparently has a higher barrier, which contributes to the formation of the 1D structure. By analogy, the chain structureformation of water molecules might suggest anisotropic hopping of water molecules along the Pd row.
In the structures proposed for a bilayer of watermolecules,3, 5, 10) the unit is considered to be a tetramer thatis composed of three water molecules in the second layer and
one water molecule in the top layer at the hollow position of
the second layer. An isolated tetramer corresponds to feature
C in Fig. 1 based on the observed height. In addition, the center of the bright spot of feature C is located on the trough of
the Pd(110) row. The water molecules at the top layer are located at the trough in the model shown in Fig. 4,3, 5) which isconsistent with the case of feature C.
Both features A and B in Fig. 1 are observed to be lower inheight than feature C. Thus, we consider that the number ofwater molecules contained in features A and B is smaller thanthat of the tetramer, i.e., four. Since we did not observe any
water-induced feature smaller than feature A, it is likely thatfeature A corresponds to the monomer of the water molecule.

Jpn. J. Appl. Phys. Vol. 41 (2002) Pt. 1, No. 7B

The existence of the water monomer on Cu(100) and Pd(100)surfaces has been reported at the temperature of 10 K.1) If
we assume that the STM detects the position of the oxygen
atom of the molecule, the bonding site proposed for Cu(100)and Pd(100) is consistent with the STM images of feature A
shown in Fig. 2 where the bright spots are located on the Pd
rows.
Although feature A is most likely the monomer of a watermolecule, we cannot reject the possibility that it corresponds
to a dimer of water molecules. One of the reasons is the elongated shape of feature A shown in Fig. 2. The protrusion has
the length of 7–8 Å along the [11̄0] direction and the widthof ∼ 3 Å. As the bonding site is expected to be the on-top position of the Pd atoms, the elongated shape along the [11̄0]
direction can easily be explained if feature A is a dimer inwhich two water molecules are combined along the Pd row.
However, the question remains why a smaller unit corresponding to a monomer cannot be identified at all in Fig. 1.
One possibility is that the impinging water molecules are
originally dimers. To confirm this possibility, it is necessary
to dose water molecules at an even lower temperature. The
modification of the dosing system for this purpose is underway.
5. Summary
We have shown STM images of water molecules onPd(110) at a cryogenic temperature of 4.7 K. The reduced
diffusion length of the adsorbate at low temperature contributes to the quenching of the lateral motion of impinging
molecules before the formation of clusters of adsorbates. To
our knowledge, an isolated water molecule was imaged by
STM for the first time in the current study. We observe threewater-induced features in STM images in a low-coverage region (∼ 0.02 ML), which can be characterized by their height;
namely, (A) 0.15 Å, (B) 0.3 Å and (C) 0.55 Å, observed at
Vs = −100 mV. In a high-coverage region, feature C dominates and forms a local c(2 × 2) structure. Since the orderedstructure is identical with the previously reported superstructure of bilayer water on Pd(110), feature C is thought to correspond to a tetramer of water molecules, which is a unit ofT. KOMEDA et al.

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bilayer water. The magnified image of the isolated feature
C shows that its center is located on the trough of Pd(110),
which is also consistent with the structural model proposed
for the bilayer water with a c(2 × 2) structure. Feature A
is located on the Pd(110) row. Since feature A is the smallest feature, we currently consider that this corresponds to themonomer of a water molecule whose existence has been observed previously at a temperature below 10 K by electron energy loss spectroscopy (EELS) measurement. However, theimage is elongated along [11̄0] with a bright area of ∼ 7–8 Å;thus the possibility that feature A corresponds to a dimer ofwater molecules still remains.
Acknowledgments
Financial support for the present work was provided (inpart) by a Grant-in-Aid for Scientific Research on Priority
Areas “Surface Chemistry of Condensed Molecules” from
the Ministry of Education, Culture, Sports, Science andTechnology, Japan.

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