Surface Science 651 (2016) 118–122
Contents lists available at ScienceDirect
Surface Science
journal homepage: www.elsevier.com/locate/susc
Energy dependent sticking coefficients of trimethylamine on
Si(001)—Influence of the datively bonded intermediate
state on the adsorption dynamics
M.A. Lipponer a , M. Reutzel a , M. Dürr a, b * , U. Höfer a
a
b
Fachbereich Physik und Zentrum für Materialwissenschaften, Philipps-Universität, D-35032 Marburg, Germany
Institut für Angewandte Physik, Justus-Liebig-Universität Giessen, D-35392 Giessen, Germany
A R T I C L E
I N F O
Article history:
Received 31 January 2016
Received in revised form 12 April 2016
Accepted 15 April 2016
Available online 19 April 2016
Keywords:
Reaction dynamics of gas surface reactions
Organic molecules
Silicon
Si(001)
Molecular beam techniques
TMA
THF
Potential energy surface
A B S T R A C T
The adsorption dynamics of the datively bonded trimethylamine (TMA) on Si(001) was investigated by
means of molecular beam techniques. The initial sticking probability s0 of TMA on Si(001) was measured as
a function of kinetic energy at two different surface temperatures (230 and 550 K). At given surface temperature, s0 was found to decrease with increasing kinetic energy (0.1 to 0.6 eV) indicating a non-activated
reaction channel. At increased surface temperature, s0 is reduced due to the onset of desorption into the gas
phase. The energy dependence of s0 is compared to the results for the adsorption of tetrahydrofuran (THF)
on Si(001), which reacts via a datively bonded intermediate into a covalently bound final state. As s0 follows
the same energy dependence both for TMA and THF, the datively bonded intermediate state is concluded to
dominate the reaction dynamics in the latter case as well.
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
Adsorption of organic molecules on semiconductor surfaces, in
particular on silicon, has attracted much interest based on the perspectives offered by a controlled organic functionalization of silicon
surfaces [1]. Detailed investigations on adsorption sites and chemical binding have been performed for many systems [2–7], however,
only limited experimental information is available on the respective adsorption dynamics and underlying potential energy surfaces
[8–11]. On the other hand, the reaction dynamics are integral part of
the reaction mechanism and an improved understanding is expected
to give access to better control of the reaction.
Adsorption of organic molecules on silicon typically proceeds via
a weakly bound precursor or intermediate state. In the case the
organic molecule contains a heteroatom such as nitrogen or oxygen,
the intermediate is formed by a dative bond between the heteroatom
and the unfilled Ddown -orbital of the silicon dimer involving the
lone pair electrons of the heteroatom [12]. In Fig. 1 (a) the situation is illustrated for trimethylamine (TMA) on Si(001) which serves
* Corresponding author.
E-mail address: [email protected] (M. Dürr).
http://dx.doi.org/10.1016/j.susc.2016.04.005
0039-6028/© 2016 Elsevier B.V. All rights reserved.
as a prototypical system for such a datively bonded configuration
of organic molecules on silicon surfaces [13–18]. In many cases, the
reaction further proceeds into a covalently bonded final state as the
activation energy for such a process is often lower than for desorption of the intact molecule back into the gas phase, i.e., the whole
reaction is non-activated. This situation is exemplary shown in Fig. 1
(b) for the adsorption of tetrahydrofuran (THF) on Si(001), which
forms a dative bond in the intermediate state (I) and further reacts
via O–C dissociation into the final state (F) [19]; the latter includes a
Si–O and Si–C covalent bond [19,20].
In contrast, N–C cleavage and the formation of covalently bound
fragments is not observed for TMA at low coverage [17] since this
process is associated with an activation energy higher than for
desorption of the intact molecule back into the gas phase (Fig. 1
(a), [14,16]). The adsorption of TMA on Si(001) thus allows to directly
probe the reaction dynamics into the datively bonded state without
any possible influence of additional reaction channels. By comparison with systems such as THF/Si(001), for which the datively bonded
configuration represents a true intermediate state and further reaction can occur, the influence of this datively bonded intermediate on
the dynamics of such more complex reactions can be elucidated.
In this letter, we present measurements on the energy dependence of the initial sticking probability of TMA on Si(001). At a given
M. Lipponer, et al. / Surface Science 651 (2016) 118–122
H3C
H3C CH3
CH3
N
H3C N
CH3
CH3
Ddown
CH3
H3C
Covalent
Bond
E
Dup
N
Dative Bond
1.0
0.5
Gas Phase
0.0
–0.5
–1.0
Dative Bond (I)
Covalent Bond (F)
s
(b) Tetrahydrofuran/Si(001)
(F)
H2C H2C
O
CH2 CH2
(I) H C
2
CH2
CH2
H2C O
Fig. 1. (a) Schematic adsorption process and potential energy diagram for the reaction
of TMA on Si(001). TMA adsorbs on the electrophilic Ddown -atom of the dimer, forming
a datively bound product. Since N–C dissociation exhibits a higher activation barrier
than desorption into the gas phase (blue potential energy curve, [14]), reaction into
the covalently bound configuration is not observed. (b) THF also adsorbs via a datively
bonded intermediate (I) but O–C dissociation has a lower activation energy than desorption from this intermediate (red dashed curve in (a), [10,11]) and a covalently
bound final configuration (F) is observed at room temperature. (For interpretation of
the references to color in this figure legend, the reader is referred to the web version
of this article.)
surface temperature, the initial sticking probability s0 decreases with
increasing kinetic energy Ekin , thus indicating a non-activated reaction channel [21]. With increasing surface temperature, the overall
sticking probability decreases as desorption takes place at increased
temperature on the time scale of the experiment. The results of
s0 (Ekin ) for TMA/Si(001) closely resemble the results reported for
THF/Si(001) [10], although in the latter case the reaction further proceeds into a covalently bound final state. We interpret this as a strong
indication that in both cases the gas–surface reaction dynamics are
dominated by the datively bonded intermediate state and propose a
qualitative potential energy surface for the two systems.
2. Methods
The experiments were performed in a four-stage molecular beam
apparatus [22,23] with a base pressure of 3×10 −11 mbar. The molecular beam was produced via supersonic expansion from a continuous
nozzle (diameter 50 lm), which was kept at room temperature for
all experiments. The nozzle was located 30 cm upstream from the
sample and was supplied from a 5-l-container to ensure a constant molecular flux during beam operation. In order to increase the
kinetic beam energy, seeded-beam techniques were used by preparing different TMA/helium mixtures in the 5-l-container. Purity of
TMA was 99.8% and of helium was 99.9999%; the flux of the pure
TMA beam was in the order of 1013 cm −2 s −1 . Time-of-flight measurements with nozzle at room temperature were performed to
determine the velocity distribution and mean kinetic energy; speed
ratio S was found to be in the range between S = 3 for pure TMA
and S = 10 for seeded beams. During beam operation, a typical background pressure of 5 × 10 −11 mbar was measured for pure TMA. For
seeded beams, a pressure increase up to 2 × 10 −8 mbar was observed
due to higher amounts of helium in the beam.
Sample preparation was described in detail elsewhere [24].
In short, the natural oxide of the rectangular Si(001) sample
(11×60 mm2 ) was removed by repetitive resistive heating up to
1300 K. The sample temperature was measured using a thermocouple glued to the center of the sample’s rear side. After sample preparation, a well ordered Si(001)2×1 reconstruction was observed in the
low energy electron diffraction pattern. Between subsequent adsorption experiments, the surface was cleaned by heating the sample to
1150 K. No contamination of the sample was observed by means
of Auger electron spectroscopy when applying this preparation and
cleaning procedure.
In order to determine sticking coefficients, the method of King
and Wells was applied [25]: The clean Si sample was positioned
in the molecular beam in front of a quadrupole mass spectrometer (QMS). When the beam enters the main chamber, it was first
blocked by an inert shutter. The reflected molecular flux leads to a
corresponding pressure rise of TMA as recorded by the QMS (main
fragment signal, m/z = 57 ± 2). Upon retraction of the shutter,
the sample surface is exposed to the molecular beam and a drop
in the background pressure is observed. In a first approximation,
the observed pressure drop directly reflects the sticking coefficient
s as long as the sample is larger than the cross section of the beam.
However, in the case of TMA, the pressure change is superimposed
by a slow chamber response [26] which is observed both when the
beam enters the chamber as well as when the beam is blocked again
outside the chamber (Fig. 2). This chamber response is attributed
to adsorption/desorption phenomena at the chamber walls and the
corresponding chamber response function fc is taken into account
when evaluating the actual sticking probability on the sample surface (compare supplementary material). All measurements were
performed under normal incidence of the molecular beam.
QMS–Signal (arb. units, arb. offset)
(a) Trimethylamine/Si(001)
119
2.0
TMA/Si(001)
1.5
1.0
Ekin = 540 meV
Ekin = 430 meV
Ekin = 300 meV
0.5
0.0
–40
–30
0
Time (s)
10
20
Fig. 2. Background pressure of TMA, measured during molecular beam operation for
three different kinetic energies at Ts = 200 K. At t ≈ −40 s the molecular beam enters
the sample chamber and is reflected from a non-reactive shutter in front of the clean
sample surface. At t = 0 s (arrow), the shutter is removed and a pronounced pressure
drop is observed. By evaluating this pressure drop, the sticking coefficient s can be
determined. For Ekin = 300 meV, the sample surface is saturated at t ≈ 10 s, and
the background pressure is rising again. Depending on gas mixture and stagnation
pressure, TMA flux varies and saturation is observed at different exposure times for
different kinetic energies. The black dashed line indicates the chamber response when
turning off the beam instead of allowing it to impinge on the sample surface.
120
M. Lipponer, et al. / Surface Science 651 (2016) 118–122
Typical King-and-Wells measurements are presented in Fig. 2 for
three different beam energies. For Ekin = 300 meV, an exemplary
measurement cycle is shown. When the beam enters the main chamber at t ≈ −40 s but is still blocked from the reactive surface, a
fast initial increase of the QMS signal and a further slow increase
is observed. We attribute this slow increase to the aforementioned
chamber response function due to adsorption/desorption on and
from the chamber walls [10,26]. When the sample is unblocked at
t = 0 s, the signal drops rapidly due to adsorption on the surface,
saturates during further uptake and increases again to the baseline
level once the surface is fully covered (t ≈ 10 s). The functional form
of both the initial drop at t = 0 s as well as the increase at t ≈ 10 s
are governed by the chamber response as they strongly resemble the
increase at t ≈ −40 s. At higher beam energy (430 and 540 meV in
Fig. 2), the initial drop of the signal, which is correlated to the initial
sticking probability s0 , i.e., the sticking probability at coverage h = 0,
is reduced.
Background-subtraction and inversion of the curves in Fig. 2 leads
to apparent sticking probabilities as a function of time as depicted
in Fig. 3. Also in this representation, the influence of the chamber
response function (black dot-dashed curve) is clearly discernible. To
determine initial sticking probabilities, we fit s(t) curves weighted
by the chamber response function fc (t) to the data (for details see
supplementary information). For the representation of s(t) we chose
s-shaped tanh-curves [10], their value at t = 0 s then indicates s0
(arrows in Fig. 3). With increasing kinetic energy, one observes a
monotone decrease of s0 . In Fig. 4, this energy dependence of s0 is
shown for two different surface temperatures. For Ts = 550 K, the
overall sticking probabilities are lower than for Ts = 230 K, however,
the data show a similar decrease with increasing kinetic energy.
In the following, first we will briefly discuss the coverage and
temperature dependence of the sticking probability. The main focus
of the discussion will then lay on the observed energy dependence
of s0 , especially in comparison with s0 (Ekin ) as reported for THF on
Si(001).
Sticking Coefficient s
1.0
TMA/Si(001)
s0
Ekin = 300 meV
Ekin = 430 meV
Ekin = 540 meV
0.5
0.0
0
5
10
Time(s)
15
20
Fig. 3. Apparent sticking coefficients as a function of time for three different kinetic
energies (blue connected lines). As in Fig. 2, the black dot-dashed line indicates the
chamber response function as obtained when the beam is blocked/unblocked outside
the main chamber. The fitted functional forms of the sticking coefficients are shown as
dashed lines and have been determined by taking the chamber response function into
account. The resulting fits to the data are shown as thin red connected lines. When
the uptake saturates, the decrease in s(t) is again dominated by the chamber response
function which is not taken into account in the fitting procedure in this part of the
adsorption trace. (For interpretation of the references to color in this figure legend, the
reader is referred to the web version of this article.)
Initial Sticking Coeffient s0
3. Results and Discussion
233 K
1.0
TMA/Si(001)
THF/Si(001)
200 K
0.5
595 K
550 K
0.0
0.0
0.1
0.2
0.3
0.4
0.5
Kinetic Energy Ekin (eV)
0.6
Fig. 4. Initial sticking coefficients s0 of TMA on Si(001) as a function of beam energy
Ekin at two different surface temperatures. The results of a comparable experiment
with THF on Si(001) are also shown (open symbols, from Ref. [10]). All lines are guides
to the eye.
Similar to previously studied systems such as ethylene/
Si(001) [8,9], acethylene/Si(001) [8], and ether/Si(001) [10,11], at low
surface temperature the sticking probability does not follow a simple
Langmuir-type adsorption behavior but is constant for an extended
coverage regime (Fig. 3 and supplementary material). Such a behavior is typically interpreted in terms of an extrinsic precursor, i.e., the
incoming TMA molecule is not necessarily reflected from the surface when it hits an already occupied adsorption site, but can move
to an unoccupied site [27]. At higher surface temperature, the effect
of this weakly bound extrinsic precursor is reduced and the sticking
probability drops monotonically with increasing coverage (data not
shown).
The observed decrease of s0 with surface temperature is typically interpreted in terms of an indirect reaction channel, i.e., the
molecules are first trapped in an intermediate or precursor state
(binding energy 4d ) before they adsorb in the final configuration
(binding energy 4f ). This conversion from intermediate to final state
involves breaking chemical bonds and is thus associated with an
energy barrier. For many systems, this energy barrier is lower than
the barrier for desorption out of the intermediate state. The reaction into the final state is thus the predominant reaction channel at
low temperatures but at higher surface temperature, desorption into
the gas phase becomes more efficient; as a consequence s0 decreases
with increasing surface temperature. In the case of TMA on Si(001),
conversion from the datively bonded configuration into a covalently
bonded configuration implies breaking the N–C bond and is associated with an energy barrier substantially larger than the barrier for
desorption ([14,16], Fig. 1). As a consequence, at low surface coverage, no covalently bound TMA is formed on the surface [17]. The
observed decrease of s0 with surface temperature (compare supplementary material) thus directly reflects the onset of desorption out
of the datively bound configuration which is associated with a desorption barrier of approx. 1.1 eV [15]. At increased coverage and
elevated surface temperature, dissociative adsorption is reported in
the case of triethylamine [28], which is most similar to TMA; we thus
restrict our further discussion to results obtained for h → 0, i.e., the
initial sticking probability s0 .
A decrease of s0 with increasing kinetic energy as observed in
Fig. 4 is characteristic of a non-activated reaction channel. The more
kinetic energy the incoming molecules carry, the less likely it is
that they loose enough energy during surface collision to be trapped
M. Lipponer, et al. / Surface Science 651 (2016) 118–122
in a surface bound state [21,29]. Although reaction into the covalently bonded configuration of TMA on silicon is activated [14,16],
we observe such a decrease of s0 with Ekin . Under the given experimental conditions, the datively bonded Si–TMA complex can thus be
seen as the final state and further reaction, including N–C cleavage,
is not accessed in the energy range of our experiment. The observed
energy dependence solely represents the adsorption dynamics into
the datively bonded configuration.
When we compare s0 (Ekin ) for TMA/Si(001) and THF/Si(001), we
find an almost identical behavior (Fig. 4). In the light of a onedimensional energy curve as depicted in Fig. 1, such a similar
behavior of the two systems is surprising on a first view: whereas
the incoming TMA molecules are reflected at the barrier between
datively and covalently bonded states, the THF molecules on such a
one-dimensional potential energy surface (PES) would be reflected
at the repulsive part of the Lennard-Jones-Potential associated with
Potential Energy
(a) Trimethylamine/Si(001)
I
0
F
εf
Ga
0
s-
Su
rfa
ce
εd
CH3
CH3
H3C
N
Co
or
din
te
ina
ord
o
nC
tio
ac
at
e
ac
urf
e
Re
S
Potential Energy
(b) Tetrahydrofuran/Si(001)
I
0
F
εf
0
εd
Ga
H2C
s-
Su
H2C
rfa
ce
Co
O
CH2
CH2
n
or
din
ctio
te
na
rdi
o
Co
ea
at
e
R
ce
rfa
Su
Fig. 5. (a) Schematic potential energy surface for TMA on Si(001) as a function of two
reaction coordinates, the gas–surface and the surface reaction coordinate. Impinging
molecules are either adsorbed in the datively bonded state (I) or are reflected back
into the gas phase. Further reaction from the datively bonded intermediate state via
N–C dissociation of the TMA molecules into a covalently bound state (F) involves an
energy barrier (orange arrow) higher than the barrier for desorption, 4d . (b) Similar
representation of the PES for THF on Si(001). In contrast to TMA/Si(001), the conversion barrier (orange arrow) between datively bonded intermediate (I) and covalently
bound final state (F) is lower than the binding energy in the intermediate state. (For
interpretation of the references to color in this figure legend, the reader is referred to
the web version of this article.)
121
the covalently bound configuration. For these very different parts of
the potential, different dynamics and thus a different dependence
of s0 on Ekin are expected. However, such a one-dimensional representation of the PES most likely oversimplifies the real situation.
For the adsorption into the intermediate state, the molecule–surface
distance will be the most important coordinate. On the other hand,
for O–C or N–C cleavage, the O–C and N–C interatomic distance,
respectively, will be a much more important coordinate, whereas
the molecule–surface distance will not change much during cleavage of the respective bond. Thus, the reactions are better represented
by two independent coordinates as sketched in Fig. 5 for TMA and
THF. In such a representation, the first coordinate (gas–surface coordinate) governs the adsorption into the intermediate state, whereas
the second coordinate (surface reaction coordinate) describes the
conversion from the intermediate into the final state.
The resulting model potentials can be seen in analogy to the
elbow potentials derived for activated dissociation of diatomic
molecules when taking into account the two most relevant molecular coordinates, i.e., the distance from the surface and the intramolecular distance [30,31]. In the entrance channel, these potentials are
often first controlled by the molecule–surface distance; closer to the
surface, dissociation is then indicated by an increasing intramolecular distance with less change of the molecule–surface distance.
The shape of these potentials is characterized by the resulting “late
barrier” in the entrance channel.
In the case of the described surface reactions of TMA and THF on
Si(001), the conversion barrier between intermediate and final state
also appears as such a late barrier in the entrance channel of our
model PES, i.e., it mainly occurs on the surface reaction coordinate.
As a consequence, both for the incoming TMA and THF molecules,
the relevant part of the PES is determined by the datively bonded
complex, although in the case of THF/Si(001), further reaction into
the covalently bound final state can occur. As the datively bonded
Si–TMA and Si–THF complexes are of similar chemical structure, a
similar shape of the PES in this part is expected. Furthermore, the
molecular mass of the two molecules is comparable (TMA: 59 u, THF:
72 u) and the number of atoms in the molecules is identical. Thus
the energy transfer both from the molecules to the surface as well as
from the molecules’ translation into their inner degrees of freedom
during surface collision will be comparable for THF and TMA leading
to a similar dependence of s0 on Ekin as observed. The difference in
the absolute sticking probabilities might then be assigned to further
parameters such as, e. g., the molecular orientation, which might be
more important in the case of TMA with a single, more localized electron lone pair of nitrogen in comparison with the two electron lone
pairs of THF’s oxygen.
From our results, we deduce that the adsorption dynamics of
TMA and THF on Si(001) are dominated by the adsorption into the
datively bonded intermediate state, regardless the details of the further progress of the reaction. We interpret this behavior in terms of
two largely decoupled reaction coordinates of the PES, one associated with the initial gas–surface reaction into the intermediate and a
second one associated with the further reaction of the adsorbate into
its final state on the surface. Qualitatively, such a configuration of
the PES is also expected for other organic molecules which react on
Si(001) via a datively bonded intermediate: reaction from this rather
strongly bound intermediate into the final state typically includes
breaking intramolecular bonds of the adsorbate and formation of
new bonds with the surface. In most cases, this should lead again
to a decoupling of the respective reaction coordinates and thus to a
more general applicability of our findings. Our interpretation should
even hold in the case of intermediates which do not involve dative
bonding via a heteroatom such as oxygen or nitrogen. As an example,
unsaturated hydrocarbons such as ethylene adsorb on Si(001) via an
asymmetric intermediate state forming a three-center p-complex at
the lower atom of the Si dimer [32,33]. The further reaction into the
122
M. Lipponer, et al. / Surface Science 651 (2016) 118–122
final [2+2] cycloaddition product involves the formation of two Si–C
s-bonds and thus a rearrangement from the asymmetric intermediate into the symmetric final configuration, either on top of one dimer
or bridging two dimers of one dimer row [34]; adsorption into the
intermediate state and the further reaction into the final state should
thus be largely decoupled. In consequence, the adsorption dynamics
should be dominated by the intermediate state in this case as well.
4. Conclusions
In conclusion, the adsorption dynamics into the datively bonded
adsorption state of TMA on Si(001) was probed by means of molecular beam techniques. The initial sticking probability was found to
decrease with increasing kinetic energy indicating a non-activated
reaction channel into this state. The dependence of s0 on Ekin was
found to be very similar for TMA/Si(001) and THF/Si(001) despite
the pronounced difference in the reaction of the two systems. This
similarity was discussed on a model PES with two coordinates, the
gas–surface reaction coordinate leading into the datively bonded
intermediate state and the surface reaction coordinate for conversion from the intermediate into the covalently bonded final state.
If the two coordinates are largely decoupled, the entrance channel
is dominated by the adsorption into the datively bonded configuration regardless the further course of the reaction as observed in the
comparison of TMA and THF on Si(001).
Acknowledgments
We thank the Deutsche Forschungsgemeinschaft for financial
support through GRK 1782.
Appendix A. Supplementary data
Supplementary data to this article can be found online at http://
dx.10.1016/j.susc.2016.04.005.
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