Single-molecule chemistry and analysis: Mode-specific dehydrogenation of adsorbed propene by inelastic electron tunneling

Article abstract of DOI:10.1021/ja200143z

A single propene molecule, located in the junction between the tip of a scanning tunneling microscope (STM) and a Cu(211) surface can be dehydrogenated by inelastic electron tunneling. This reaction requires excitation of the asymmetric C-H stretching vibration of the =CH₂ group. The product is then identified by inelastic electron tunneling action spectroscopy (IETAS).

Full text of DOI:10.1021/ja200143z

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pubs.acs.org/JACS
Single-Molecule Chemistry and Analysis: Mode-Specific
Dehydrogenation of Adsorbed Propene by Inelastic
Electron Tunneling
Manfred Parschau, Karl-Heinz Rieder, Hans J. Hug, and Karl-Heinz Ernst*
Nanoscale Materials Science Laboratory, Empa, Swiss Federal Laboratories for Materials Science and Technology, Ueberlandstrasse 129,
CH-8600 D€ubendorf, Switzerland
S Supporting Information
b
then cause molecular surface dynamics, such as desorption,6
dissociation,7 chemical bond formation,8 conformational
ABSTRACT: A single propene molecule, located in the
changes,9 rotation, and hopping.10 Recently, we reported the
junction between the tip of a scanning tunneling microscope
(STM) and a Cu(211) surface can be dehydrogenated by
inelastic electron tunneling. This reaction requires excita-
tion of the asymmetric CÀH stretching vibration of the
dCH2 group. The product is then identified by inelastic
electron tunneling action spectroscopy (IETAS).
manipulation of single propene molecules adsorbed on the
copper(211) surface by inelastic electron tunneling (IET) as
well as lateral pulling over the surface.11 This induced rotation,
hopping, and enantio-conversion.11 Here, we present the IET-
induced mode-selective dissociation of single propene molecules
on Cu(211) and show that dissociation occurs only when the
asymmetric CÀH stretching mode of the olefinic dCH2 group is
excited. In addition, the reaction product is identified by IETAS,
relating an action with a specific vibrational mode.
urface chemistry is governed by competing energy dissipation
Propene has been adsorbed at 40 K on the intrinsically stepped
Cu(211) surface under ultrahigh-vacuum conditions and cooled
to 7 K in the STM. The Cu(211) surface consists of (111)
terraces separated by (100) steps (Figure 1a). Its STM image is
dominated by dark and bright stripes, the latter located on the
(111) terrace near the step edge.12 Two different types of
propene adsorbates were identified; the methyl group points
either toward the lower terrace or is located at the upper
terrace.11 DFT calculations showed that in both adsorbate
configurations the molecules are bound with the CdC double
bond on top of a step edge copper atom.11 In addition, both
configurations support two enantiomeric states, because the
prochiral propene creates a chiral adsorbate upon adsorption.13
Figure 1b shows two opposite enantiomers of protrusions with
opposite tilt angles relative to the Cu(211) step edges (indicated
by red dashed lines).
The procedure to induce a molecular response by IET has
been described previously.14 After imaging, the STM tip is
positioned over a target molecule and the tunneling current is
recorded as a function of time at a fixed bias voltage. A jump in
current then indicates that an action has occurred, and rescan-
ning of the same area reveals the nature of this action. In addition
to translations and rotations induced at lower voltages and
currents, high currents at sufficiently high bias voltages generate
a new species, appearing in STM as elongated lobe aligned
parallel to the steps (Figure 1c and Supporting Information).
Note that the creation of such species does not change the
appearance of propene molecules at other locations. Hence, the
STM tip remains unchanged by this process. The product can
still be manipulated, that is, a negative voltage pulse transfers the
molecule to the STM tip, and subsequent scanning of the area
S
channels of vibrationally excited molecules.1 On metal sur-
faces, excited molecular vibrations decay quickly via energy
transfer to the substrate electrons by electronÀhole pair forma-
tion. Alternatively, conversion into adsorbate dynamics (in the
following referred to as actions) such as lateral translation,
rotation, or dissociation can occur, but how the vibronic excita-
tion of a molecule couples to these actions and which state-
resolved excitations preferentially support them remains an open
question. Adsorbate translations and rotations are fundamentally
important, because they constitute the rate-limiting steps for
reaction partners to meet. They are in competition with single
molecule decomposition, and it is of interest to know which
excited vibration leads to a certain motion or causes dissociation.
Lifetimes of excited vibrations, determined by the efficiency of
coupling between electronic and vibrational states, is connected
to structural aspects of the adsorbate, like metallicity, adsorption
site, and orientation of the adsorbent. Studying the decay path-
ways of excited molecules in connection with the structure of the
adsorbate and the lifetimes of excited states contributes to a
better understanding of the selectivity of surface chemical
reactions.
Invented 30 years ago,2 the scanning tunneling microscope
(STM) has revolutionized surface science. Moreover, it has been
developed as a tool for single molecule manipulations,3 which go
beyond repositioning of atoms and molecules for building
artificial structures.4 Nowadays, an STM allows time-resolved
studies in the microsecond range by measuring the tunneling
current versus time in the so-called open-feedback loop, that is,
with the tip held stationary over the surface.5 Hence, the STM
actually provides an electron source with ultimate spatial resolu-
tion and current density. With a molecule in the tunnel junction,
most of the electrons tunnel elastically, but a small fraction can
tunnel inelastically by excitation of vibrational modes. This may
Received: January 6, 2011
Published: March 28, 2011
r
2011 American Chemical Society
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dx.doi.org/10.1021/ja200143z J. Am. Chem. Soc. 2011, 133, 5689–5691
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Figure 2. IET action spectrum for propene dissociation (left) and
dependence of the reaction probability on tunneling current (right).
Each data point results from at least 20 consecutive manipulation
experiments. The threshold bias voltage for dissociation clearly corre-
lates with the asymmetric CÀH stretching mode of the olefinic
CH2 group.
higher quantum numbers by a many-electron process (so-called
ladder-climbing) until bond breaking occurs.
It has been shown that the function of reaction probability
versus bias voltage, the so-called action spectrum, is related to the
vibrational density of states.17 Figure 2 presents such action
spectrum (I = 300 nA). At a bias voltage of 380 mV, the reaction
probability increases sharply to 75%. Below 300 mV, no reac-
tion was observed, while between 300 and 380 mV, a reaction
probability of less than 20% was determined. For voltages larger
than 380 mV, there is a 25% probability for other actions, such as
hopping, rotation, and enantiomer inversion. We attribute the
sharp step at 380 mV to the excitation into the first vibrational
level of the asymmetric stretching vibration (νas) at the sp2-CH2
group. IR measurements show this vibration for propene on
Cu(111) at 381.5 meV (3079 cmÀ1).18 From the currents
needed for dissociation (Figure 2), one can also estimate the
lifetime of the excited vibration, that is, it must be longer than 0.5
ps in order to allow multiple electron excitation. Note that νas-
CH2 is the highest-energy mode of adsorbed propene,18 and only
if this vibrational mode becomes excited, dissociation occurs.
In the following, we discuss possible products of the dissocia-
tion reaction. Previously performed IET-induced dissociations
on similar molecules led to dehydrogenation instead of CÀC or
CÀN bond cleavage.19 Examples are the conversion of trans-
2-butene to butadiene,19a CÀH bond cleavage of acetylene to
ethynyl (HCtCÀ) and dicarbon (CC),19b and methylisocya-
nide formation from methylaminocarbyne by selectively break-
ing a NÀH bond.19c Therefore, and because STM line profiles
along the long molecular axis of the product do not suggest a
shorter entity here (Supporting Information), we assume an IET-
induced dehydrogenation as dissociation reaction. Consequently,
products like allene (propadiene, H2CdCdCH2) or propyne
(CH3ÀCtCH) must be considered. Because the product could
still be manipulated laterally and vertically, we exclude a radical like
Figure 1. IET-induced decomposition chemistry of propene on Cu-
(211). (a) Model of an fcc(211) surface with (111) terraces and (100)
steps, indicated by an hexagon and a square, respectively. (b) Tunneling
electrons are injected into a single molecule (red arrow). (c) At 400 mV
bias voltage and current of 300 nA, a new species is formed, which can be
picked up by a negative voltage pulse with the tip. (d) Scanning of the
same area does not show the product, because it is now located at the tip,
and the STM contrast has improved. (e) Applying the opposite voltage
pulse deposits the product back to the surface and the STM image shows
the product again. Scanning parameters: 2.8 nm Â5 nm, U = 70 mV,
I = 320 pA.
does no longer show the molecule in the image (Figure 1d). With
the molecule at the tip, the STM contrast improved (Figure 1d).
An opposite voltage pulse deposits the molecule back to the
surface (Figure 1e). After such vertical manipulation, the product
is always aligned parallel to the steps, while the vertical mani-
pulation of propene leads always to the initially observed
configurations.11 We therefore conclude that the new species
must be a product of a chemical reaction. Hydrogenation or
oxidation of propene from species located at the tip can be
excluded, because we were able to use the same tip with identical
outcome for this manipulation for many times on different
propene molecules. This leaves only dissociation as possible
reaction.
The reaction rate R depends on the tunneling current as RµIN,
with N being the reaction order, that is, the number of electrons
involved in the quantum mechanical excitation. We showed
previously that actions like rotation and hopping of propene
are first-order processes, while the inversion of the enantiomers is
a second-order process.11a Because of insufficient time resolu-
tion, we could not determine the reaction rate here. However, the
high currents needed suggest a multiple electron process. Basi-
cally, three distinct mechanisms must be considered for electron-
induced dissociation, that is, either dissociation after electron
attachment (negative ion resonance), electronic excitation, or
vibrational excitation.15 The possibility of electron attachment or
an electronic excitation can be excluded here, because the energy
levels of the lowest unoccupied molecular orbitals (LUMOs) of
small physisorbed hydrocarbons are located a few electron volts
above the Fermi level.16 The bias voltages used here are too low
(<400 mV) to allow tunneling into such states. Hence, the
mechanism of excitation of propene occurs via vibrational
heating. That means that the vibration is excited to levels of
CH3ÀCHdCH . Such entity would be too strongly bound to the
3
surface, thus, preventing vertical manipulation.
Previously, single-molecule IET dissociation products have
been identified with IET spectroscopy (IETS).19 The change in
tunneling current due to excitation of vibrations via IETS thereby
allows molecular vibrations to be directly monitored.3 However,
our attempts to directly identify vibrations via IETS were not
successful. IETS relies on conductance change, and often the
elastic tunneling channel changes simultaneously with opposite
sign to the inelastic channel, thus, canceling the signal.20 In our
case, however, the reaction product can still be further manipu-
lated by IETAS. Figure 3 shows the IET-induced hopping of the
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This material is available free of charge via the Internet at http://
pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
karl-heinz.ernst@empa.ch
’ ACKNOWLEDGMENT
Figure 3. IET action spectroscopy performed with the dissociation
product. Injection of tunneling electrons (a) causes the molecule to
jump parallel to the step edges (b). The hopping rate increases
substantially when the threshold of 370 mV is exceeded (c). The
STM image in panel c shows propyne for comparison. Scanning
parameters a and b, 2.2 nm Â6.4 nm U = 70 mV, I = 320 pA; c,
3.2 nm Â3.7 nm, U = 70 mV, I = 1.2 nA.
Financial support of the Swiss Secretary for Education and
Research (SER) and the Swiss National Science Foundation
(SNSF) is gratefully acknowledged.
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’ ASSOCIATED CONTENT
S
Supporting Information. STM images and line profiles
b
of propane, propene, propyne and the IET product on Cu(211).
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