Jpn. J. Appl. Phys. Vol. 41 (2002) pp. 4976–4979
Part 1, No. 7B, July 2002c2002
The Japan Society of Applied Physics

Characteristic Configuration of Cis-2-butene Molecule on Pd(110)
Determined by Scanning Tunneling Microscopy
Yasuyuki S AINOO, Yousoo K IM1 , Hirokazu F UKIDOME1 , Tadahiro KOMEDA1 ,Maki K AWAI1, ∗ and Hidemi S HIGEKAWA†
Institute of Applied Physics and CREST-JST, University of Tsukuba, Tsukuba 305-8573, Japan
1 RIKEN (Institute of Chemistry and Physics), 2-1 Hirosawa, Wako, Saitama 351-0198, Japan(Received March 15, 2002; accepted for publication April 25, 2002)

Adsorption of cis-2-butene molecules on the Pd (110) surface was studied by scanning tunneling microscopy (STM) at 4.7 K
for the first time. In contrast to the fact in the previous work that trans-2-butene molecules are adsorbed at the on-top siteof the Pd(110) surface with its C=C double bond being almost parallel to the [11̄0] axis, cis-2-butene molecules were foundto be adsorbed slightly off the on-top site. In addition, the C=C bond was not parallel to the [11̄0] axis, but to four directions
equivalent to the structure of the Pd(110) surface. Furthermore, at high bias voltages for the STM measurement, the cis-2-butene
molecule became unstable and moved in two different manners around the palladium atom to which the molecule was bonded,suggesting the existence of two different mechanisms underlying the motion on the surface. [DOI: 10.1143/JJAP.41.4976]
KEYWORDS: scanning tunneling microscopy, Pd(110), hydrocarbon, adsorption

1. Introduction
One of the most attractive targets in recent surface scienceis the identification of the adsorbed isomeric molecules on
the surface, and the clarification of how the chemical reactions of molecules depend on their configurations on the surface. In addition to the isomeric structures, their adsorption
configuration on material surfaces, in terms of factors such asadsorption site and molecular orientation, plays an importantroles in chemical reactions, such as catalysis, corrosion andetching. For example, during catalytic processes, the hydrogenation reaction of hydrocarbons is strongly influenced by
the configurations of the related molecules on the surface.1, 2)
Therefore, in order to understand and control the molecular
reactions on material surfaces, it is extremely important toanalyze the adsorption structures of isomeric molecules at thesingle molecular level.
Scanning tunneling microscopy (STM) is known to havepowerful potential in this field. Namely, instead of information averaged over various molecules, it can provide us information for individual molecules. Since electronic structures
are probed by STM measurement, the molecular configuration can be analyzed in detail. In fact, recently, the adsorptionconfiguration of the trans-2-butene molecule and its chemical
reaction into the 1, 3-butadiene molecule induced by inelastic
tunneling was studied by STM at the single molecular level.3)
Configurations of the two molecules were clearly discriminated and determined.
As is well known, the 2-butene molecule is a kind of hydrocarbon and has trans-2- and cis-2- isomeric structures (Fig. 1).
Therefore, when the configuration of the another one, cis-2butene, is determined, this material becomes an intriguing
candidate for the study described above. In this paper, we
present our recent STM results for the configurations of theadsorbed cis-2-butene molecules on Pd(110) at 4.7 K. A completely different adsorption characteristic from that of trans2-buten was observed for cis-2-butene.
∗ E-mail address: maki@postman.riken.go.jp† E-mail address: hidemi@ims.tsukuba.ac.jp

Fig. 1. Schematics of (a) trans-2- and (b) cis-2-butene molecules.

2. Experimental
Experiments were performed using a low-temperature
STM system (LT-STM, Omicron GmbH) with double thermal
shield cups equipped in an ultrahigh vacuum (UHV) chamber. In general, since the activation barriers for surface diffusion or rotation for molecules are below 100 meV, their mean
residence time at 77 K is below 10−7 s.4) Therefore, a highcoverage molecular surface is prepared for STM imaging of
the surface in order to reduce molecular motions. However,this method is not suitable in this case because it becomes difficult to analyze the characteristic of the isomers at the single
molecular level. In addition, in the case of hydrocarbons on
the catalysis surface, there exists a process of thermal dehydrogenation.2, 4) Therefore, all measurements were performed
at 4.7 K.
3. Results and Discussion
A clean and flat Pd(110) surface, with a 20-nm-wide terrace, was prepared by a standard polishing technique, Ar+
sputtering and annealing cycles. Base pressure was < 3 ×10−11 . The butene molecules (99% purity for both trans-2and cis-2-butene molecules) were introduced onto the surface through a variable leak valve (< 2.2 × 10−10 Torr). A

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mechanically sharpened and field emitted tungsten wire wasused as a STM tip. Sometimes the STM tip can be modified
with molecules in order to be able to observe the adsorbed
molecules and substrate more clearly.3, 5) However, in the caseof cis-2-butene, adsorbed molecules became unstable with themodified STM tip. Therefore, in order to minimize the interaction between tip and sample for the precise determinationof the adsorbed molecular configurations, all images were obtained with a nonmolecular terminated bare tip in this study.
Figures 2(a) and 2(b) show a typical STM image of trans2-butene and cis-2-butene molecules adsorbed on the Pd(110)
surface and a magnified image of the surface. Here, both the2- and cis-2-butene molecules are co-adsorbed. A trans-2butene (labeled “T”) is known to be imaged by STM as having a dumbbell-like pair of protrusions.3) It is adsorbed at theon-top site on the Pd(110) surface with the C=C bond being

Fig. 2. (a) STM image of trans- (T) and cis-(C) 2-butene molecules(Vs = −20 mV, It = 1.0 nA). (b) A magnification of (a). (c) Schematics of the molecular arrangement in (b).

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almost parallel to the [11̄0] axis, and two CH3 related parts
are considered to be imaged brightly, and to have a dumbbelllike structure. On the other hand, cis-2-butene (labeled “C”)
was observed in completely different shape. Namely, it has an
asymmetric gourd-like shape, and one side on the long-axis isbrighter than the other one. This assignment was confirmedby the observation of the surface with the adsorption of thepure cis-2-buten molecules. In this paper, we call the brighter
side the “head” of the molecule. Another remarkable point isthat the cis-2-butene molecules appeared with their long-axes
in the four different directions on the same surface as indicated by the schematic in Fig. 2(c). the single small spots inFig. 2(a) are possibly considered to be impurities.
Regarding the structures of trans-2- and cis-2-butene
molecules, it is difficult to explain the observed differences described above by only the geometric difference inthe molecular structures. A possible explanation is that themolecular orbitals are modulated by the interaction between
CH3 groups in the molecules and the substrate. In order toclarify this point, let us first determine the adsorption site.
If it were possible to image both the adsorbed molecules
and the atomic structure of the substrate together, the detailsof the adsorption site could be determined easily. However,corrugation of the molecules is about 0.1 nm, while the corrugation associated with the close-packed metal surface is of
the order of 0.0001–0.001 nm.6) Thus, it is difficult to visualize both the adsorbed molecules and the substrate atomicstructure together. There are two useful techniques for determining the adsorbed sites. One is to estimate the positions of molecules by comparing them with a grid mesh of
the clean substrate surface obtained for the same size area.
In this case, adsorption sites of the trans-2-butene molecules
can be used as the reference. Namely, as mentioned above,trans-2-butene molecules are adsorbed at the on-top site ofthe substrate.3) However, distortion of the STM image dueto the piezo creep during scanning may affect the results obtained with this method. Another method is digital filteringwith the Fourier transfer conversion technique.6, 7) This is avery simple technique for enhancing periodic structures suchas substrate atoms. However, it also highlights some noise frequency components around the periodic structure. Therefore,
in order to determine the adsorption site of the cis-2-butene
and its configuration precisely, we applied these two independent techniques simultaneously, i.e., we performed double checking to determine the relative positions of adsorbed
molecules and substrate atoms.
Figure 3(a) shows an STM image of a clean Pd(110) surface. Each atom of the surface can be clearly observed. Figure3(b) is an STM image with the adsorbed molecules after treatment by Fourier transfer conversion technique. The substrateperiodic structure is enhanced, and is in good agreement withthat in Fig. 3(a). After comparison of Figs. 3(a) and 3(b), thegrid mesh of the clean substrate surface is drawn over Fig.
3(b), which is shown in Fig. 3(c). In Fig. 3(c), each cross ofthe mesh indicates the Pd atomic position, on-top site. For example, the center of a trans-2-butene molecule indicated by T
is on an on-top site.
The first remarkable point is that the cis-2-butene molecular STM image is located off from the on-top site of the palladium atom. In addition, the long-axis of the image is not parallel to the atomic row ([11̄0] axis), in contrast to the case of

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Y. S AINOO et al.

Fig. 3. (a) STM image of a clean Pd(110) surface. (b) STM image withadsorbed molecules after treatment by Fourier transfer conversion technique. (c) STM image of trans- (T) and cis-(C) 2-butene molecules witha grid mesh indicating the Pd(110) substrate surface lattice. (d) Schematics of the molecular configurations. Hatched areas drawn on the molecules
correspond to the STM images in (c). STM conditions are Vs = −20 mV,
It = 0.50 nA.

the trans-2-buten molecule. The orientation of the long-axisof the two cis-2-buten molecules shows two equivalent directions as compared to the crystal axis of the Pd(110) surface,resulting in the four equivalent orientations in Fig. 2(b).
Since the paired protrusions of the trans-2-butene cannot
be explained by the model of observing the single C=C double bond, the dumbbell-like STM image of the trans-2-butene
is related to the electronic structure of the two CH3 parts ina molecule.3, 8) On the other hand, as is shown in Fig. 1(b),in a cis-2-butene molecule, two CH3 parts exist on the sameside of the C=C double bond axis. Therefore, overlap of thetwo CH3 related orbitals in a molecule may form the gourdlike shape in the STM image. A schematic of the proposed
adsorption model is shown in Fig. 3(d). The hatched structures of Fig. 3(d) correspond to the molecular STM images inFig. 3(c). For trans-2-butene, the adsorption model in ref. 3 isFig. 4. Bias dependent STM images of trans- (T) and cis-(C) 2-butene
molecules adsorbed on Pd(110). (a) Vs = +15 mV, It = 1.0 nA. (b)Vs = +80 mV, It = 1.0 nA. (c) Vs = +150 mV, It = 1.0 nA. (d)Vs = +200 mV, It = 1.0 nA.

shown. Theoretical calculation is necessary to further clarify
the configuration of the cis-2-butene molecule.
Next, we studied the difference in the interaction between
the adsorbed molecules and the substrate for trans-2- and cis2-butene. Figure 4 shows a series of bias dependent STM images of the coadsorbed trans-2- and cis-2-butene isomers (tunneling current: 1.0 nA, ∼ 6.4 nm2 ). The asymmetric gourd-Jpn. J. Appl. Phys. Vol. 41 (2002) Pt. 1, No. 7Bshape labeled “C” and the dumbbell-like shape labeled “T”
represent cis-2-butene and trans-2butene molecules, respectively, similar to those in Fig. 2.
At low bias voltages, all adsorbed molecules were stable
and could be observed clearly (Fig. 4(a)). However, when
the bias voltage was raised to Vbias = 80 mV, the contoursof almost all the cis-2-butene molecules became fuzzy andsome of them exhibited an L-like shape as indicated by arrows “L” in Fig. 4(b) with a schematic model. Furthermore,
at Vbias = 150 mV (Fig. 4(c)) and Vbias = 200 mV (Fig. 4(d)),
strangely modulated structures appeared as indicated by arrows “S”. These phenomena are not caused by the contact between the STM tip and molecules because the tunneling current was fixed in these measurements. Namely, for the sametunneling current, the tip-sample distance increases with anincrease in the bias voltage. Thus, the observed changes in
the molecule images are considered to be due to the motionof the molecules under the STM tip during the scan. In fact,
the STM scan was started from the bottom left and continued along the horizontal axis (from left to right then low tohigh), and the observed change appeared during one of thescans. Trans-2-butene molecules remained stable throughout
this experiment.
Regarding of the observed molecular change, there exist
at least two steps with the increase in bias voltage. In the firstone shown in Fig. 4(b), the cis-2-buten molecule becomes unstable and exhibits an L-like shape. However, since the “head”
of the molecule is kept on the same on-top site, this motion
seems to be a type of rotation with the head position being
the axis; however, in the second one shown in Figs. 4(c) and4(d), the “head” part changes its position, suggesting a kind

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of hopping process in this case. These results suggest the existence of two different energy transfer mechanisms for thecase of cis-2-butene. A detailed study of these motions is now
in progress and will be published elsewhere.9)
4. Conclusions
In conclusion, the adsorption of cis-2-butene moleculeson the Pd (110) surface was studied by STM at 4.7 K forthe first time. Completely different molecular configurations
from those of trans-2-buten molecules were observed. In addition, at high bias voltages for the STM measurement, the cis2-butene molecule became unstable and moved in two different manners around the palladium atom to which the molecule
was bonded, suggesting the existence of two different mechanisms of the motion on the surface.
This work was supported by a Grant-in-Aid for Scientific
Research from the Ministry of Education, Culture, Sports,Science and Technology, Japan.

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