Surface Science 536 (2003) L403–L407
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Surface Science Letters

Observation of cis-2-butene molecule on Pd(1 1 0) by cryogenic
STM: site determination using
tunneling-current-induced rotation
Y. Sainoo a,b, Y. Kim b, T. Komeda b,*, Maki Kawai b, H. Shigekawa aa

Institute of Applied Physics, CREST, University of Tsukuba, Tsukuba 305-8573, Japanb
RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
Received 15 November 2002; accepted for publication 10 April 2003

Abstract
We have investigated the bonding conﬁguration of an isolated cis-2-butene molecule (CH3 –CH@CH–CH3 ) on thePd(1 1 0) surface using a cryogenic STM at the sample temperature of 4.7 K. For the precise determination of thebonding site of the molecule, we utilize a novel method of rotating the molecule by injecting tunneling electrons into
the molecule. We have observed the rotation of the molecule around the terminal site of a single Pd atom with hopping
between four equivalent bonding sites when tunneling electrons with the energy of 170 meV were dosed. The bondingbetween the cis-2-molecule and the Pd(1 1 0) surface is considerably through p-bonding at the carbon double-bond,which makes bonding on the terminal site favorable. However, the analysis of the images of rotating molecule reveals
that the C@C is shifted from the terminal site and located at the oﬀ-symmetry position.
Ó 2003 Elsevier Science B.V. All rights reserved.
Keywords: Scanning tunneling microscopy; Alkenes; Palladium; Tunneling; Surface electronic phenomena (work function, surface
potential, surface states, etc.)

The determination of bonding conﬁguration ofhydrocarbon molecules has attracted attentions
for a long time which is partially due to the factthat the adsorption site and the orientation ofadsorbates play a key role in the catalytic reaction,*
Corresponding author. Present address: Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba, Sendai 980-8577, Japan. Tel.:+81-22-217-5368; fax: +81-22-217-5404.
E-mail address: komeda@tagen.tohoku.ac.jp (T. Komeda).

such as hydrogenation reaction of hydrocarbons[1,2]. In addition, recent rising interest for thehybridization between bio-molecules and semiconductor or metal surface should demand more
information of the bonding conﬁguration of large
molecules.
It has been demonstrated scanning tunnelingmicroscopy (STM) has a high capability in observing adsorbed molecules in a real space. Furthermore, STM can manipulate individual atoms
and molecules. Especially recent studies havedemonstrated that the vibrational excitation of0039-6028/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0039-6028(03)00564-8

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adsorbates caused by tunneling electrons can be
the energy source in producing certain selective
dynamic motions and reactions [3–6].
In this paper, we study the bonding conﬁguration of cis-2-butene molecule (CH3 –CH@CH–
CH3 ) on Pd(1 1 0) at 4.7 K with a novel method ofrotating the molecule by the injection of tunnelingelectrons. We successfully rotate the cis-2-butene
molecule around a single Pd terminal site, andfour stable bonding conﬁgurations are identiﬁedaround the single Pd terminal site. The images arewell explained if we assume a model where the cis2-butene molecule adsorb on a single Pd atom witha p-bond manner, though the center of the carbon
double-bond is shifted from the terminal site of aPd atom.
A clean Pd(1 1 0) surface was prepared by thestandard cleaning technique with a repetitive Arþ
sputtering and annealing cycles. The molecules(99% purity) were dosed through a dosing tube
located close to the sample surface at the substratetemperature below 50 K. A mechanically sharpened and ﬁeld-emitted tungsten wire was used as
the STM tip. All STM measurements were performed at 4.7 K.
Fig. 1 shows the STM image of the Pd(1 1 0)
surface where isolated cis-2-butene molecules areadsorbed. On the same surface, trans-2-butene
molecules are dosed in addition as reference molecules. The Pd rows are in the direction from upper-left to lower right. The images of cis-2-butene
molecules appear as oval protrusions. Furthermore the oval is asymmetric and has bright region(head) and dark region (tail) as an internal structure. Thus there exists a single symmetry line in theimage of the cis-2-butene molecule, which is along
the long axis of the oval.
With the angle of the image of Fig. 1 where Pdrows are in the diagonal direction, STM imagesof cis-2-butene molecules appear elongated either
vertically or horizontally; i.e., the symmetry lineof the molecules is either vertical or horizontal.
In addition, the head is located either in the upper side or in the lower side of the molecule invertical molecules (right or left side for the horizontal molecules). Thus we can identify totallyfour types of images for isolated cis-2-butene
molecules which are marked in Fig. 1 as CU , CD ,Fig. 1. (a) An STM image of cis-2-butene molecules adsorbedon the Pd(1 1 0) observed at 4.7 K. The marks CR , CL , CU , andCD correspond to diﬀerent bonding conﬁgurations (see text).
Molecules of trans-2-butene are also dosed for comparison,
which are marked by T. Imaging conditions are Vsample ¼ 20mV and Itunnel ¼ 1:0 nA. (b) Schematic models for the moleculesof trans-2-butene and cis-2-butene, where the circles with gridpattern correspond to the bright area in the STM images.

CR , CL where the suﬃx corresponds to the relative location of the head; up, down, right andleft, respectively. Since the substrate Pd(1 1 0) has
the C2v symmetry, the existence of four types ofSTM images corresponds to a single bonding
conﬁguration of cis-2-butene with oﬀ-symmetricmanner.
As is marked by T in Fig. 1, the STM image ofthe trans-2-butene molecule, which is an isomer ofcis-2-butene, appears as a paired protrusion. We
have analyzed the bonding conﬁguration of trans-SUNIESC RSCE EA TT
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2-butene molecule on Pd(1 1 0) in previous papers[6,7], and concluded that the bright spots correspond to the positions of CH3 . This is considerably
derived from the bent conﬁguration of the C@C–C, in addition to the distribution of the electronÕs
density of state at the position of CH3 in thehighest occupied molecular orbital (HOMO) of atrans-2-butene molecule. In analogy we consider
that the head of the cis-2-butene molecule corresponds to the area marked by a large shaded circle
in Fig. 1(b), to which side both CH3 species arelocated. Consequently the symmetry line of theimage of the cis-2-butene molecule is perpendicular to the carbon double bond.
Here we try to identify the bonding site of thecis-2-butene molecule by inducing the rotation of
the molecule with the injection of the tunneling
electrons into a molecule. It has been demonstrated that the dosing of tunneling electrons on anadsorbed molecule can cause surface phenomena
such as hopping [5], rotation [3], and chemical
reaction [4,6]. The dosing of tunneling electrons in
the present experiment was carried out by positioning the STM tip above the top of the head ofthe molecule followed by turning-oﬀ the feedbackloop and changing the bias voltage from the one
for the observation.
The rotation of the molecule manifests itself inthe variation of the tunneling current, which isshown in Fig. 2(a). Similar results have been reported for the rotation of the C2 H2 molecule on
Cu(1 0 0) [3,8]. Fig. 2(a) corresponds to the casewhere the energy of the tunneling electrons is 60
meV, and the tunneling electrons are dosed in theisolated molecule of CR in Fig. 2(b). The tunnelingcurrent shuttles the high state (I1 ) and the low state(I2 ). The current is in the high state at the beginning of the electron injection, and is in the lowstate at the end of the dosing. The correspondingSTM image of the molecule is observed as CR at
the initial state and as CD at the ﬁnal state, whichsuggests that high state corresponds to CR and low
state corresponds to CD when the tip is positioned
at the marked position in the ﬁgure.
By repeating the dosing of tunneling electrons
to the CR molecule and the observation of themoleculeÕs azimuth rotation, we have found that,with the bias voltage of 60 mV, the CR molecule

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Fig. 2. (a) The variation of the tunneling current when thetunneling electrons with the energy of 60 mV are injected on an
isolated cis-2-butene molecule. The conﬁguration of the molecule corresponding to this current change is shown in (b); westarted the dosing of the tunneling electrons on the molecule of
the CR conﬁguration at the position marked by a star, which
ended with the CD conﬁguration. (c) The variation of the tunneling current when the bias voltage was 170 mV. The change ofthe moleculeÕs conﬁguration is shown in (d); the dosing wasstarted with the CR conﬁguration and ended at the CU conﬁguration.

rotates only to CD . The rotation of CR molecule to
CL or CU state occurs when the energy of thetunneling electrons is 170 meV. The current variation for this case is shown in Fig. 2(c).

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The tunneling current has four stable current
states; I1 , I2 , I3 and I4 in Fig. 2(c). The states of I1
and I2 are identical with the ones in Fig. 2(a). A big
drop of the current can be seen at 330 ms. Thecurrent shuttles the state of I3 and I4 after thatmoment. The ﬁnal STM image of the molecule
corresponds to CU . After repeating the experiments, we conclude that the I3 corresponds to CL
and I4 to CU .
The rotation of the cis-2-molecule with thedosing of the tunneling current can be simpliﬁed if
we assume the pairs of (CR , CD ) and (CL , CU ). The
intra-pair rotation can be induced by dosing tunneling electrons with the energy of 60 meV, but theinter-pair rotation is only observed with a higher
bias voltage of 170 meV. We speculate that thepotential barrier for the inter-pair rotation ishigher than that of intra-pair rotation.
With the control of the bias voltage and monitoring of the tunneling current, the cis-2-butene
molecule can be rotated to one of the four stablebonding conﬁgurations. Now we make the registration of the cis-2-butene molecules on thePd(1 1 0) surface using this technique.
Fig. 3(a)–(d) show the high-resolution images ofa single cis-2-butene molecule at the four stable
conﬁgurations obtained by rotating a single molecule. The lattice grid of the substrate is obtainedby imaging the same surface with a molecule on
the apex of the STM tip. It has been reported thathigh-resolution images can be obtained with thismethod [9]. In addition to the lattice grid, thesymmetry lines of the molecules at the four stable
positions are superimposed.
It is interesting to notice that the four imagesof the cis-2-butene molecules rotate around theterminal site of a single Pd atom. In addition,
four symmetry lines form rhombus instead ofintersecting at the terminal site or at the shortbridging site. In previous works, we have studied
the bonding conﬁgurations of ethylene and trans2-butene molecules on the Pd(1 1 0) surface [6,7,10,11]. In both cases, the molecules show p-bondcharacter and occupy the terminal site. Considering the existence of C@C in the cis-2-butene
molecule similar to the cases of the ethylene andthe trans-2-butene molecules, we are tempted toassume a similar bonding conﬁguration for theFig. 3. (a)–(d) The STM images of the cis-2-butene molecules
at the four stable conﬁgurations, which are obtained by rotatinga single molecule by the injection of the tunneling electrons. The
superimposed grid is the registration of the Pd(1 1 0) substrate,
which is obtained with a molecule-coated tip (see text). Fourlines which form a rhombus correspond to the symmetry line of
the molecule at the four stable azimuth rotations. (e) Schematic
models for the bonding conﬁguration of the cis-2-butene molecule on Pd(1 1 0) at the four stable conﬁgurations. For eachconﬁguration, the positions of the double bond and CH3 areschematically shown.

cis-2-butene molecule and the double bond at theterminal site. However, if the carbon doublebond is located on a terminal site, the symmetrylines of the molecules at the equivalent bonding
conﬁgurations should intersect at the terminalsite [12].
Instead we would propose a model for thebonding conﬁguration, which is schematically

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drawn in Fig. 3(e) where the double bond andthe CH3 are illustrated by bars. The bonding siteof the double bond is not on the terminal site of
the Pd atom; instead the bonding position of thedouble bond is shifted from the terminal site of thePd atom while keeping the p-bonding character
between the molecule and the substrate. The reason why the cis-2-butene occupies such unsymmetrical position on the Pd(1 1 0) is unclear at thismoment, and further theoretical calculation isnecessary. However, we believe the technique ofrotating molecule by the tunneling current, whichis shown in this paper, can be a powerful tool todetermine the bonding conﬁgurations of largemolecules.
In summary we have investigated the bonding
conﬁguration of the cis-2-butene molecule on thePd(1 1 0) surface with a novel method of rotating
the molecule by the injection of tunneling electrons
into the molecule. It is detected that the molecule
rotates around the terminal site of a single Pd
atom with hopping between four equivalent bonding sites, when tunneling electrons with the energyof 170 meV are dosed. It is concluded that thebonding site of carbon double bond of the cis2-butene molecule is shifted from the terminal siteof the Pd atoms, which shows a diﬀerence from theL407

cases of ethylene and trans-2-butene moleculesthat also have carbon double bonds in the middleof the molecule.

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