Excitation of molecular vibrational modes with inelastic scanning tunneling microscopy processes: examination through action spectra of cis-2-butene on Pd(110).

Abstract

Inelastically tunneled electrons from a scanning tunneling microscope (STM) were used to induce vibrationally mediated motion of a single cis-2-butene molecule among four equivalent orientations on Pd(110) at 4.8 K. The action spectrum obtained from the motion clearly detects more vibrational modes than inelastic electron tunneling spectroscopy with a STM. We demonstrate the usefulness of the action spectroscopy as a novel single molecule vibrational spectroscopic method. We also discuss its selection rules in terms of resonance tunneling.

Full text

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University of Tsukuba
Title
Excitation of Molecular Vibrational Modes with Inelastic
Scanning Tunneling Microscopy Processes: Examination
through Action Spectra of cis-2-Butene on Pd(110)
Author(s)
Sainoo, Yasuyuki; Kim, Yousoo; Okawa, Toshiro; Komeda,
Tadahiro; Shigekawa, Hidemi; Kawai, Maki
Citation Physical review letters, 95(24): 246102-246102-4
Issue Date2005-12
Text versionpublisher
URL http://hdl.handle.net/2241/100017
DOI 10.1103/PhysRevLett.95.246102
Right © 2005 The American Physical Society

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Excitation of Molecular Vibrational Modes with Inelastic Scanning Tunneling Microscopy
Processes: Examination through Action Spectra of cis-2-Butene on Pd(110)
Yasuyuki Sainoo,1,*Yousoo Kim,1,†,‡Toshiro Okawa,2Tadahiro Komeda,3,6
Hidemi Shigekawa,4,6and Maki Kawai1,5,6,†,x
1RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
2Department of Physics, Gakushuin University, Mejiro, Toshima-ku, Tokyo 171-8588, Japan
3Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba, Sendai 980-8577, Japan
4Institute of Applied physics, 21th Century COE, NANO project, University of Tsukuba, 305-8573, Japan
5Department of Advanced Materials Science, The University of Tokyo, Kashiwa, Chiba 277-8651, Japan
6CREST, Japan Science and Technology Corporation (JST), Toshima-ku, Tokyo 171-0031, Japan
(Received 8 March 2005; published 5 December 2005)
Inelastically tunneled electrons from a scanning tunneling microscope (STM) were used to induce
vibrationally mediated motion of a single cis-2-butene molecule among four equivalent orientations on
Pd(110) at 4.8 K. The action spectrum obtained from the motion clearly detects more vibrational modes
than inelastic electron tunneling spectroscopy with a STM. We demonstrate the usefulness of the action
spectroscopy as a novel single molecule vibrational spectroscopic method. We also discuss its selection
rules in terms of resonance tunneling.
DOI: 10.1103/PhysRevLett.95.246102PACS numbers: 68.35.Fx, 63.22.+m, 68.37.Ef, 82.65.+r
The excitation of molecular vibration by means of the
inelastically tunneled electrons from the tip of a scanning
tunneling microscope (STM) can lead to various dynami-
cal processes at surfaces [1,2]. In addition, inelastic elec-
tron tunneling spectroscopy with the STM(STM-IETS)[3]
is now applicable to the vibrational spectroscopy of the
individual molecules. The vibrational spectrum of a single
molecule provides useful information not only for the
chemical identification of the molecule [4–6], but also
for investigating how molecular vibration can couple
with the relevant dynamical processes [7,8].
However, many experimental results [1,3–8] have
shown that not all the vibrational modes are observed in
STM-IET spectra, which indicates that the establishment
of proper selection rules is necessary. In order to establish
the selection rules, it is of crucial importance to clarify
experimental observation of the molecular vibration in-
duced by inelastically tunneled electrons. STM-IETS de-
tects the vibrational modes of a single molecule by mea-
suring the total conductance change resulting from the
inelastic electron tunneling. However, the total conduc-
tance change is not always detectable, since the elastic
component can be adversely reduced giving rise to a de-
crease in IET signals for some vibrational modes [9]. The
response of vibrationally mediated molecular motion to
applied bias voltage, namely, an ‘‘action spectrum,’’ can
reveal vibrational modes that are not visible in STM-IETS,
because the molecular motion is induced via only inelastic
tunneling. Thus, the action spectrum would be a candidate
for detecting which vibrational mode is actually excited
and associated with molecular motions. Here, we show the
usefulness of action spectroscopy as an alternative vibra-
tional spectroscopic method for the STM-IETS through
the study of vibrationally mediated reversible motion of a
cis-2-butene molecule among four equivalent adsorption
orientations on Pd(110) at 4.8 K. The STM-IET spectrum
shows only two peaks corresponding to the metal-carbon
stretch mode, ??M-C?, and the CH stretch mode in the CH3
group, ??CH3?. However, the action spectrum reveals that
not only ??M-C? and ??CH3? but also the C-C stretch
mode, ??C-C?, and the bending mode in CH3, ??CH3?,
are actually excited via inelastic tunneling leading to the
motions.
All experiments were performed using a commercially
availablelow-temperature
GmbH) equipped in an ultrahigh-vacuum chamber (<3 ?
10?9Pa) [8,10]. The Pd(110) surface was cleaned by
cycles of Ar ion sputtering and annealing cycles, and
exposed to cis-2-butene molecules below 50 K. All images
and spectroscopic data were acquired at 4.8 K; experimen-
tal details for spectroscopy have been reported elsewhere
[8,11,12].
Acis-2-butene molecule adsorbed onPd(110)appears as
a gourd shape in the STM image with a large bright region
(head) and a small less bright region in Fig. 1(a). There are
four equivalent orientations, labeled CUR, CUL, CDR, and
CDL. An isolated cis-2-butene is ?-bonded to off-centered
position of the Pd atom, where the molecule is slightly
shifted towards the hollow site [11–13]. The adsorption
geometry is proposed as shown in Fig. 1(b).
The procedure for inducing molecular motion is de-
scribed previously [12]. In brief, after imaging the target
molecule at initial orientation, the STM tip is positioned
over the center of the head part of the molecule. Then, the
bias voltage is increased to a certain value and the tunnel-
ing current is recorded as a function of time. A rescanning
of the same area shows that the molecule has changed its
orientations one to the other. The repeated sequences show
STM (LT-STM, Omicron
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that the trace of tunneling current reflects residence time of
the molecule at a certain orientation. Figure 1(c) shows a
typical change in tunneling current while the sample bias
voltage is kept constant at 170 mV.
The motions of cis-2-butene can be categorized into two
types [12]; they are extracted from the difference in current
changes and simplified by defining the pairs of (CUR, CUL)
and of (CDR, CDL); the pairs are indicated by arrows in
Figs. 1(a) and 1(b). CURand CUL(CDRand CDL) are mirror
images with respect to a plane parallel to ?11?0?, while CUR
and CDR(CULandCDL)are alsomirror images withrespect
to a plane parallel to [001]. The motion with small changes
in current, such as either between I1and I2or between I3
and I4in Fig. 1(c), corresponds to the change between two
orientations with the mirror plane parallel to [110]. This
motion is named ‘‘low barrier (LB)’’ motion. The other
motion is named ‘‘high barrier (HB)’’ motion with larger
current change between (I1, I2) and (I3, I4) in Fig. 1(c),
corresponding to the orientation change between two pairs
for LB motion. The potential barrier for HB motion is
higher than that of LB motion, which is clearly seen in
action spectra.
Figure 2 shows the action spectra of cis-2-butene, where
the motion yield (number of molecular motions per in-
jected electron) is indicated as a function of applied sample
bias voltage at a chosen tunneling current. There are clear
thresholds for both LB and HB motions of C4H8in the
upper spectra of Fig. 2(a). For LB motion, the motion yield
markedly increased at ?37 mV followed by a slight in-
crease at ?115 mV. For HB motion, on the other hand,
clear increases were observed at ?115 and ?360 mV. A
comparison between C4H8 and fully deuterated cis-2-
butene (C4D8) helps to assign the active vibrational modes
to the above-mentioned motion. In the action spectrum of
C4D8,the increases ofthe motionyield forLBmotion were
observed at ?31 mV and at ?95 mV. For HB motion, the
increases of the motion yield appeared at ?110 mV and at
?270 mV. The assignment for those modes is described
below with combing to STM-IETS, which gives us the
vibrational signature of an individual molecule.
Figure 3 shows the STM-IET spectra of C4H8[Fig. 3(a)]
and C4D8[Fig. 3(b)]. The significant features appear at
(a)
(b)
Sample bias (mV)
n
o r t c
e l e / s
n
o i t o
M
10-12
10-8
10-12
10-8
0 100 200 300 400
C4H8
C4D8
LBHB
10-10
10-10
LB HB
40160 100
C4H8
C4D8
Sample bias (mV)
n
o r t c
e l e / s
n
o i t o
M
LB
FIG. 2.
C4H8(upper) and of C4D8(lower). Data were taken under fixed
tunneling current of 3 nA for C4H8and of 2 nA for C4D8.
(b) Magnification of the action spectrum for low barrier motion
at around the threshold energy. Slight increase in the yield was
observed around 115 mV for C4H8and 95 mV for C4D8, as
indicated by arrows, respectively.
(a) Action spectra for described motions both of
CUL
(a)
CUR
CDL
CUL
CDR
3
2
1
0.64 0.600.56
Time [sec]
0.52
current [nA]
I3
I4
I1
I2
3
1
2.01.00
(b)
(c)
CUR
T
CDR
CDL
T
TT
LB motion
HB motion
[001]
[110]
FIG. 1.
cis-2-butene molecule labeled CUR, CDR, CUL, and CDL, which
are obtained by moving the molecule by injecting tunneling
electrons. The suffixes UR, UL, DR, and DL correspond to the
relative location of the head part, i.e., up-right, up-left, down-
right, and down-left, with respect to the center of the molecule in
the STM images, respectively. A trans-2-butene molecule (la-
beled T) was coadsorbed as a marker. The superimposed grid
indicates the position of Pd atoms, which is determined from the
STM image obtained with a molecular tip. The additional four
dotted lines forming a rhombus correspond to the molecular long
axes at each orientation. An asterisk represents the center of the
large bright region of the molecule in each orientation, where the
tunneling electrons were injected. (b) Proposed structure of cis-
2-butene on Pd(110) at four equivalent adsorption orientations.
(c) Current changes measured at sample bias voltage of 170 mV
with the tip fixed over the center of the head of a cis-2-butene
molecule. The inset shows the wide time range of the same data.
(a) STM images of four equivalent orientations of a
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?358 mV for C4H8and at ?268 mV for C4D8. Identical
signals were observed in the STM-IET spectra of trans-2-
butene on Pd(110), and they were assigned to the vibra-
tional peaks for ??CH3? and ??CD3?, respectively [8]. An
additional feature with a very low intensity at about 36
(32) mV for C4H8(C4D8) was observed repeatedly for dif-
ferent sets of molecule and tip. Novibrational feature other
than the twopeakswasobservable in the STM-IETspectra.
The high-resolution electron energy loss spectroscopy
(HREELS) spectrum of cis-2-butene on Pd(110) [13]
presents four groups of vibrational energies observed in
different regions: (1) metal-carbon stretching mode at 20–
40 meV; (2) CH bending and C-C stretching modes at
approximately 100 meV; (3) C—
??C—
at approximately 360 meV.
Comparing with the HREELS spectrum, it turns out that
both the first threshold in the action spectrum for LB
motion of C4H8(C4D8) observed at 37 (31) mV and the
small peak observed in STM-IET spectrum at 36 (32) mV
correspond to the vibrational energy of ??M-C?. Thus, we
conclude that the excitation of ??M-C? directly couples
with LB motion. For region (2), while the lower threshold
for HB motion was found at ?110 mV both for C4H8and
for C4D8[Fig. 2(a)], a clear isotopic shift was observed
from 115 (C4H8) to 95 mV (C4D8) for LB motion in
Fig. 2(b). The latter case can be explained with the isotope
shift of ??CH3? and ??CD3?. On the other hand, ??C-C?
should be expected to show a weak isotope shift. Thus,
these two modes overlap in energy for C4H8but are
separated for C4D8, and both modes contribute to inducing
molecular motion. For region (4), it is obvious that ??CH3?
corresponds to both vibrational signals, observed at ?358
(268) mV in the STM-IETS spectrum and at ?360
??270? mV in the action spectrum of HB motion of
C4H8(C4D8). However, ??C—
ible in both the STM-IETS and the action spectrum. A
—C stretching mode,
—C?, at 160–180 meV; and (4) CH stretching mode
—C? in region (3) was invis-
similar behavior was observed in the hopping motion of
C2H4on Pd(110), where no response was observed in the
C—
—C stretching mode region [14].
From the above-mentioned analysis of the action spec-
tra, it is clear that the vibrational modes ??M-C?, ??CH3?,
??C-C? and ??CH3? are excited via the inelastic electron
tunneling. Since the ??M-C? mode is directly related to the
motion of the molecule, the excitation of all the higher vi-
brational modes can induce the same motion in their re-
laxation processes. The activation barrier for the motion of
the molecule discussed here is pretty small, and the result
of motion rate measurement as a function of applied cur-
rent indicates that the excitation oftwoquanta of??C-M?is
sufficient to overcome the barrier [15]. However, although
the stretching mode and the bending mode of sp3CH in
CH3groups were clearly observed in the action spectra,
those for sp2CH at the C—
—C bond were not observed.
Note that ??—
regions of different energies from those of sp3CH’s, typi-
cally at 380 meVand at 80 meV, respectively, in HREELS
spectrum [13,16]. In order to explain this issue, we discuss
the mechanism of the excitation of molecular vibration
modes in STM via the inelastic electron tunneling.
Within an STM junction, electrons from the tip first
encounter the molecular orbital at the lowest unoccupied
molecular orbital (LUMO) level forming a temporary
negative-ion state, inducing slight distortion of the mole-
cule [17]. Since the lifetime of the temporary negative-ion
state should be extremely short, the distorted molecule in
the electronic ground state remains. The actual LUMO
state of the adsorbate is the hybridized state, consisting
of the highest occupied molecular orbital (HOMO) state,
the LUMOstate of the isolated molecule, and the d orbitals
of Pd. Even so, the distortion in the molecule should re-
flect the shapes of HOMOand LUMOof the isolated mole-
cule unless the molecule is significantly distorted upon
adsorption.
The shapes of HOMO and LUMO of a cis-2-butene
molecule are depicted in Fig. 4. Both HOMO and LUMO
have significant parts at the ? bonding and antibonding
orbitals, and at the sp3hydrogen atoms, but not at sp2
hydrogen atoms. Indeed, the action spectra revealed that
the motion of cis-2-butene was actually enhanced for
??CH3? but was inactive for ??CH?, corresponding to the
fact that the molecular orbitals of cis-2-butene have a
distribution at sp3CH in methyl groups but not at sp2
CH at the C—
—C bond. Because of the distribution of the
electronic state, ??CH? was not directly excited via inelas-
tic electron tunneling process even though the energy of
the electron was sufficient for the excitation.
Although the C—
—C bond is strongly associated with the
adsorption states through both HOMO and LUMO, there
were no signs of the excitation of ??C—
spectrum. Response to the molecular motion is only avail-
able if the vibrational mode can couple with the mode of
reaction coordinate for the motion via anharmonic cou-
pling. Since cis-2-butene is ? bonded to the Pd atom,
—CH? and ??—
—CH? for sp2CH appear in
—C? in the action
Sample Bias (mV)
(a)
0
200 400
d2
V
d
/ I
2
V
/
A
n
(
2)
0
0
-200-400
358
0
-3
3
C4H8
36
(b)
C4D8
0
200400
d2
V
d
/ I
2
V
/
A
n
(
2)
00
-200-4000
-3
3
Sample Bias (mV)
32
268
FIG. 3.
Solid lines and dashed lines represent vibrational spectra of the
positive and negative bias range, respectively. The background
spectrum taken over bare Pd(110) surface has been subtracted
from each spectrum. All spectra were obtained at a tunneling gap
set point of 0.25 nA and 20 mV, and Vrms?15mV ac modulation
at 797 Hz. Each spectrum is the average of 16 bias voltage
ramps.
STM-IETS spectra taken over (a) C4H8and (b) C4D8.
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??C—
the Fermi level. Thus the vibrational damping through
electron-phonon pair excitation could be quite efficient
[18]. Despite that the energy is sufficient to overcome the
barrier for the molecular motion, ??C—
the action spectra of cis-2-butene due to the insufficient
coupling between vibrational modes similar to the case of
CO on Cu(110) [10].
We have discussed the plausible mechanism of vibra-
tional excitation via inelastic electron tunneling in the
STM junction by the use of STM-IETS and action spec-
troscopy for the vibrationally mediated molecular motion
of cis-2-butene. Taking the action spectrum into account,
we have shown here that such an excitation is basically
through resonant tunneling [2,17,19,20]. In other words,
the coupling of the vibrational excitation to molecular
motion reflects the feature of the LUMO of the adsorption
state. For cis-2-butene on Pd(110), ??M-C?, ??CH3?,
??C-C?, and ??CH3? are found to be excited, while only
??M-C? and ??CH3? are observed in STM-IETS. In order
to understand the reason why the other modes are not
observed in STM-IETS, a precise theoretical calculation
would be necessary; even though it has been claimed that
the elastic component contribute to diminish the inelastic
component when the density of states is very high at the
Fermi level [9,19].
The determination of which vibrational mode is respon-
sible for and how they are associated with the various
surface dynamics is essential for achieving mode-selective
control of chemical reactions. A combination of a dynamic
spectroscopic technique such as an action spectroscopy
and a static technique as an STM-IETS would give an
important information to establish the actual dynamics
related to the electron-vibration coupling in a single mole-
cule on the surface.
The authors thank Dr. Hirokazu Fukidome and
Dr. Satoshi Katano for valuable discussion on the assign-
ment of vibrational modes. The present work was sup-
ported, in part, by the Grant-in-Aid for Scientific Re-
search on Priority Areas ‘‘Electron transport through a
linked molecule in nano-scale’’ (Grant No. 17069006)
from the Ministry of Education, Culture, Sports, Science
and Technology and International Joint Research Grant
—C? is strongly coupled with the electronic states near
—C? is invisible in
‘‘Molecular wire’’ project (03BR1) from the New Energy
Development Organization (NEDO) of Japan.
*Present address: Institute of Multidisciplinary Research
for AdvancedMaterials,
Katahira, Aoba, Sendai 980-8577, Japan.
†The authors towhom correspondence should be addressed.
‡Email address: ykim@riken.jp
xEmail address: maki@riken.jp
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FIG. 4.
(center) and LUMO (right) of a cis-2-butene molecule. The
molecular orbitals (MO) were calculated using a semiempirical
quantum-mechanical calculation package MOPAC.
Molecular structure (left), and the shapes of HOMO
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