PHYSICAL REVIEW B 71, 193410 共2005兲

Tunneling spectroscopy and manipulation of a single C2H2 molecule on a Cu(111) surface
Yasuaki Konishi, Yasuyuki Sainoo,* Ken Kanazawa, Shoji Yoshida, Atsushi Taninaka,
Osamu Takeuchi, and Hidemi Shigekawa†
Institute of Applied Physics, 21st century COE, CREST-JST, University of Tsukuba, Tsukuba, 305-8573, Japan
共Received 1 June 2004; revised manuscript received 17 August 2004; published 31 May 2005兲
The adsorption structure of a single C2H2 molecule on a Cu共111兲 surface was analyzed by scanning tunneling microscopy 共STM兲 and inelastic tunneling spectroscopy 共IETS兲. The STM result showed a twofold rotational symmetry, in agreement with the structural model previously proposed. The IETS spectrum measured
near a monatomic step exhibited peaks related to the C - H stretch mode, which were as strong as what had beenreported for C2H2 on a Cu共100兲 substrate, where the amplitude of the electronic standing wave that causes
background fluctuation of spectra could be considerably reduced by the reduction of scattering at the step.
Chemical reactions induced by electron injection were also observed by IETS analysis near steps.
DOI: 10.1103/PhysRevB.71.193410

PACS number共s兲: 68.37.Ef

The control and analysis of chemistry at the singlemolecule level is a key to future nanotechnology and related
applications in various fields, and its recent progress has
been considerable.1–12 Inelastic tunneling spectroscopy
共IETS兲 is a promising technique for these purposes, becauseof its high potential for the identification and manipulation of
selected single-vibration modes.3–12 Due to the basic importance and broad catalytic applications in various fields, extensive studies have been performed on the chemical reactions of hydrocarbons on transition metals. Among them,
C2H2 is an important molecule from fundamental and practical points of view. Pioneering works have been performed
on Cu共100兲, Ni共100兲, Ni共110兲, and Pd共111兲 surfaces.3,9,10,13
An interesting characteristic is that the adsorption structure
and molecular conformation vary depending on the surface,resulting in the differences in the STM image and in theIETS spectrum of single molecules. However, the obtained
results are rather complicated. For example, on a Ni共110兲
surface, from the observation of two different sizes of circular C2H2 STM images and two associated C - H stretch
modes in IETS spectra, at 368 and 357 meV, two different
conformations of the C2H2 molecule 共sp2-like and sp3-like兲
were found to exist.10 These characteristics are different form
those on a Ni共100兲 surface, where a single dumbbell-like
STM image and a 360 meV IETS peak were observed.9 On
Cu共100兲 surfaces, a single C - H stretch mode and dumbbelllike STM image were confirmed, similar to those on aNi共100兲 surface, but the stretch mode was 357 meV.9 The
molecular plane was tilted on a Pd共111兲 surface and an asymmetric STM image was observed.13 On a Cu共111兲 surface,
the C2H2 molecule is considered to be adsorbed with its
plane perpendicular to the surface, similar to that on aCu共100兲 surface, but different from that on a Pd共111兲 surface.
Since the molecular conformation strongly influences chemical reactions, a detailed understanding of the adsorption
structures and reaction processes at the single-molecule levelis extremely important.
To advance the research, we investigated the C2H2 molecule on a Cu共111兲 surface by STM and IETS. As mentionedabove, despite the necessity of analysis at the singlemolecule level, tunneling spectroscopy of adsorbates on aCu共111兲 surface has not yet been performed adequately. We
1098-0121/2005/71共19兲/193410共4兲/$23.00

speculate that this is mainly because of the difficulty in thespectroscopy measurement due to the strong influence of thestanding wave of the surface state on the spectra. Namely, asshown by Crommie et al.,14 a surface-state band, whoseparabolic dispersion relation has its minimum at ⬃0.5 eV
below the Fermi level, contributes to the strong electronic
standing wave at the energy levels near the Fermi level.
Since the steps and defects on the surface act as scatterers of
two-dimensional electrons, the standing wave has its fixedend at such local structures and gradually decays with distance from them. Since its decay length is very long, thestanding wave often extends over almost the entire surface
area. Since the wavelength of the standing wave is a functionof the energy level, when a spectroscopic measurement isperformed by STM, the phase of the standing wave at themeasurement location changes as the measured energy levelchanges. This also causes the density of states to fluctuatelargely there and, as a result, large background fluctuationarises in STS or IETS measurement. Consequently, no successful IETS measurement on Cu共111兲 substrate has beenreported to date.
According to the analysis presented above, we derived astrategy for avoiding the strong effect of the standing wave.
The key issue is that the phase of the standing wave at thesurface step is almost fixed and the density of states for thesurface state is also very low. Thus, the measured spectra
near the step edge should be much less influenced by thestanding wave. By this strategy, we succeeded in measuring
the C - H stretch mode of a single C2H2 molecule adsorbed
on a Cu共111兲 surface. The intensity of the C - H stretch modein the spectra was comparable to that measured on Cu共100兲
surfaces, making it possible to analyze the tip-inducedchemical reaction of a single C2H2 molecule on the surface.
In this paper, we show our results for the tunneling spectroscopy and manipulation of a single C2H2 molecule on aCu共111兲 surface.
The Cu共111兲 surface was prepared by several cycles of Ar
ion sputtering and annealing at 820 K. The base pressure was4.0⫻ 10−9 Pa. After confirming that the Cu共111兲 surface wasclean, the STM tip was retracted from the sample and acetylene gas of ⬃99% purity was introduced through the variableleak valve below 10 K.

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PHYSICAL REVIEW B 71, 193410 共2005兲

BRIEF REPORTS

FIG. 1. Typical STM images of isolated C2H2 molecules on aCu共111兲 surface 共10⫻ 10 nm2 , + 0.1 V , 1.0 nA兲 共a兲 and its highresolution image obtained by a molecule-modified STM tip 共10⫻ 10 nm2 , + 0.5 V , 1.0 nA兲 共b兲. 共c兲 Predicted model of the adsorption site. Green, silver, red, and purple circles indicate Cu, Pd, C,
and H atoms, respectively.

Figure 1共a兲 shows a typical STM image of isolated C2H2molecules on a Cu共111兲 surface. In general, C2H2 molecules
are imaged as a dip with a simple round shape on the surface,
however, they are sometimes imaged with higher resolution
as a dumbbell-like shape, as shown in Fig. 1共b兲. This is probably caused by the modification of the tip apex by another
C2H2 molecule, as previously reported in the case of STM
observation of a CO molecule on a Cu共111兲 surface.15 In thehigh-resolution image, molecules in three orientations areobserved, each of which rotated 120 deg relative to the others, reflecting the threefold symmetry of the Cu共111兲 substrate.
A magnification of the high-resolution image and theschematic model of the corresponding molecular conformation proposed in the previous experimental and theoreticalstudies are shown in Fig. 2共a兲. Since the molecular plane isperpendicular to the surface, the adsorbate has twofold rotational symmetry The corrugation between small white dots in
the STM image is attributed to the backbond between thecarbon and copper substrate, while two dips suggest thatsome charge transfer occurs from the Cu atom at these positions to the C2H2 molecule, the same as in the case of C2H2adsorption on Cu共100兲.3 Indeed, as shown in Fig. 2共b兲, thesituation is very similar in these two systems, except for theslight difference in the distance between the two dips in theSTM image. This difference between the STM images ofFigs. 2共a兲 and 2共b兲 reflects the symmetry of the substrate.
When the molecule is tilted, the situation is very different, asin the case of the C2H2 adsorption on Pd共111兲 shown in Fig.
2共c兲.13 The asymmetry in the molecular conformation isFIG. 2. 共a兲 Magnification of the high-resolution image in Fig.
1共b兲. For comparison, STM images of a C2H2 molecule on Cu共100兲
and Pd共111兲 surfaces and their schematic models are also shown inFigs. 2共b兲 and 2共c兲, respectively. The imaging conditions of 共a兲,共b兲, and 共c兲 were 共+1.0 V , 1.0 nA兲, 共+100 mV, 10 nA兲, and共+40 mV, 400 pA兲, respectively. All images area is 25⫻ 25 Å2.
Green, silver, red, and purple circles indicate Cu, Pd, C, and H
atoms, respectively.

strongly affects the symmetry of the STM image. Overall,our STM observation strongly supports the previously proposed structural model for C2H2 on Cu共100兲.
Next, we present IETS spectra obtained on the bare
Cu共111兲 surface. As mentioned earlier, on the Cu共111兲 surface, the standing wave of the surface state strongly influences the tunneling spectra due to the change in the wavelength with the change in the bias voltage during themeasurement. Figure 3 shows typical IETS spectra obtainednear a step 共夡兲 and at a terrace, ⬃20 nm from a step edge
共夝兲. As expected from the dispersion relation of the surfacestate,14 the signal in the spectrum begins to oscillate significantly with bias voltage Vt above ⬃−0.5 V which is thebottom of the band, and is smooth for Vt Ȿ −0.5 V. The influence is stronger at a terrace 共夝兲.
The spectra presented in Fig. 3 exhibit many peaks anddips although the measurement was carried out on the baresubstrate. We emphasize that these structures in the spectra
are highly reproducible. For example, two IETS spectra obtained for the same position under slightly different tipsample separation distances are shown in Fig. 3共c兲 by blue
and red lines. The spectra are almost identical, except for theslight difference in intensity. The random noise component,
which is given by the difference between the two lines, has
as low an intensity as the fluctuation of the baseline of an
IETS spectrum measured on a substrate without a strong
standing wave, such as the Cu共100兲 substrate 共shown bygreen line兲. Since the interference effect is strong around thesignal position of the C - H stretch mode measured by electron energy loss spectroscopy 共EELS兲 共362 mV兲,16 it is difficult to identify the signal in the spectrum at a terrace.
Solid curves in Fig. 4共a兲 show typical IETS spectra obtained for the C2H2 molecules in Fig. 4共b兲 共T and S兲. Dottedcurves are spectra obtained for the bare surface near T and S.

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PHYSICAL REVIEW B 71, 193410 共2005兲

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FIG. 4. 共a兲 IETS spectra obtained over C2H2 molecules 关T and Sin 共b兲, scale, 8.7⫻ 8.7 nm2 near a step edge. 共c兲 Schematic for thecalculation of IETS intensity. 共Vt = + 80 mV, It = 1.0 nA, ⌬V= 7 mV at 336 Hz, ␶ = 100 ms, 16 times integrated兴. Red marks in共a兲 indicate the positions of signals at ±364 mV. Dotted curves are
the spectra obtained for the bare Cu surfaces near T and S in 共a兲.

The difference between the solid and dotted curves represents the molecule-specific feature, in addition to the small
difference in the standing-wave corrugation due to the slightdifference in the measurement position and the scattering
effect of the molecule itself. There exist peaks at ±364 mVfrom the C2H2 molecule in both cases, as indicated by redmarks the −364 mV peak is not yet clear in the spectrum
obtained for T, although ±364 mV peaks are clearly observed in the spectrum obtained just near the step edge 共S兲,as expected.

FIG. 3. 共a兲 STM image 共27⫻ 15.5 nm2 , + 80 mV, 1.0 nA兲 and共b兲 IETS spectra of Cu共111兲 surface obtained near 共夡兲 and ⬃20 nm
from 共夝兲 a step edge. Set point conditions are Vt = + 80 mV, It= 1.0 nA. Modulation and time constant during the lock-in measurement are ⌬V = 7 mV at 794.7 Hz and ␶ = 100 ms, respectively. Signals were integrated 16 times. 共c兲 Comparison of IETS spectra obtained on 共100兲 共green, Vt = + 80 mV, It = 1.0 nA, ⌬V = 7 mV at
766.6 Hz, ␶ = 100 ms, eight times integrated兲 and 共111兲 surfaces
共blue, Vt ⬍⬎= + 60 mV, It = 1.3 nA, ⌬V = 10 mV at 333 Hz, ␶= 100 ms, eight times integrated; red, Vt = + 80 mV, It = 1.5 nA,⌬V = 12 mV at 333 Hz, ␶ = 100 ms, eight times integrated兲.

FIG. 5. STM images before 共a兲 and after 共b兲 applying a voltageof +3.0 V for 10 sec. 共c兲 Cross sectional STM images of the C2H2molecules in 共a兲 and 共b兲, 共d兲 IETS spectra of the molecules shownin 共a兲 and 共b兲. 共Vt = + 80 mV, It = 1.0 nA, ⌬V = 10 mV at 327 Hz,
␶ = 100 ms, 16 times integrated兲. Dotted curves are the spectra obtained for the bare Cu surfaces near the molecule in 共a兲 and 共b兲.

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PHYSICAL REVIEW B 71, 193410 共2005兲

BRIEF REPORTSNext, let us compare the peak energies and intensities ofthe C - H stretch mode measured by IETS for the C2H2 molecules on the Cu共111兲 and Cu共100兲 substrates. The peak energy of 364 mV obtained by our measurement is slightly
different from that on Cu共100兲, 358 meV. According to theresults of previous studies,17,18 the bond angles of C - C - Hfor C2H2 on the Cu共111兲 and Cu共100兲 substrates are 116 and150 deg, respectively. A conformational change from thesp2- to the sp3-like structure is assumed to cause the observed shift in energy level. The intensity of peaks in IETS
spectra can be compared on the basis of the relative change
in the differential conductance: ⌬共dI / dV兲 / 共dI / dV兲, as suggested in Ref. 11. Here, ⌬共dI / dV兲 can be obtained by integrating the difference between two IETS spectra obtained on
共solid line兲 and off 共dotted line, considered as a background兲the molecule, as shown by the hatched area in Fig. 4共c兲. The
values were 6.6% and 12.7% for Cu共111兲 and Cu共100兲 surfaces, respectively. To clarify the difference in detail, further
study including the selection rule, which has not yet been
established, is necessary.
The peak intensity is sufficiently strong for the Cu共111兲
surface, which enables us to analyze chemical reactions on
this surface. Since adsorption structures may influence thechemical reactions, comparison of required conditions on
different surfaces is important. Figures 5共a兲 and 5共b兲 show
the STM images before and after the reaction. Here, the reaction was induced by tunnel electron injection into the molecule with a bias voltage of +3.0 V for 10 sec. The circular
dip corresponding to the molecule became slightly wider anddeeper, as shown by the line profiles in Fig. 5共c兲, which weremeasured along the broken lines in the STM images. We
confirmed that bias voltage higher than 2.8 V was required
for the reaction. A similar experiment has been performed for
the C2H2 adsorbate on Cu共100兲,12 the experimental result
which was almost the same as what we described here. The
authors of the paper interpreted the reaction on Cu共100兲 surface as sequential conversions of the C2H2 molecule into
CCH and CC, on the basis of the results of their IETS analysis. Therefore, we performed IETS measurement to clarify
whether the same process was occurring in our case, andshow the results in Fig. 5共d兲. The intensity of the C - H
stretch vibrational mode is greatly reduced by the reduction,indicating H desorption from the molecule. We concluded
that the reaction that occurred here was the desorption of H
atoms from the C2H2 molecule.
In conclusion despite its importance, tunneling spectroscopy and manipulation of adsorbates on the Cu共111兲 surface
have not been analyzed in detail due to the strong influenceof the standing wave on the spectra. We have shown that theinelastic tunneling spectra of a single C2H2 molecule adsorbed on a Cu共111兲 surface can be obtained clearly near thestep edge, where the influence of the standing wave is reduced. The intensity of the C - H stretch mode on the spectra
was comparable to that on Cu共100兲 surfaces, resulting insatisfactory manipulation and analysis of a single C2H2 molecule on the Cu共111兲 surface.
This work was supported by a Grant-in-Aid for Scientific
Research from the Ministry of Education, Culture, Sports,
Science and Technology, Japan709 共1999兲.

*Present address: Tohoku University.
†http://dora.ims.tsukuba.ac.jp

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