The reaction dynamics of allyl methyl ether (AME) on Si(001) was studied by means of molecular beam techniques. The reaction of this bifunctional molecule comprising an ether and an alkene group was found to proceed via an intermediate state as deduced from the temperature dependence of the initial sticking probability s0. At constant surface temperature Ts, s0 decreases continuously with increasing kinetic energy Ekin, indicating a non-activated adsorption channel. Qualitatively and quantitatively, the energy dependence is almost identical to the adsorption dynamics of diethyl ether on Si(001). We attribute this to a similar nature of the intermediate state, which largely determines the adsorption dynamics. In consequence, this indicates a predominant role of the ether group and a minor influence of the C=C double bond on the adsorption dynamics of AME on Si(001).
Adsorption of organic molecules on the Si(001) surface typically proceeds via an intermediate state that is characterized by a relatively weak, non-covalent bond between the functional group of the adsorbing molecules and the surface. The type of the functional group thus determines the nature of the binding in the intermediate state,1–4 which, in turn, governs the adsorption dynamics5–9 (Fig. 1). The adsorption dynamics of alcohols and ethers on Si(001)6,9 may serve as a good example: in both cases, the adsorbing molecules form a dative bond with the surface in the intermediate state10–18 (lower right sketch in Fig. 1), and despite major differences in their structure, mass, and the energetics of the further reaction into the final state, they show almost identical adsorption dynamics as determined by the dependence of the initial sticking probability s0 on the kinetic energy Ekin of the incoming molecules.9 On the other hand, molecules with different functional groups such as alkenes feature a considerably stronger dependence of s0 on Ekin and thus a much lower reactivity at high kinetic energy of the incident molecules5 (Fig. 1). In the latter case, the intermediate state involves a weaker three-center bonding between the surface and the carbon–carbon double bond of the alkene molecules19–23 (upper right sketch in Fig. 1).
In the case of molecules with two or more functional groups, the reaction pathway was determined for a couple of systems,3 both theoretically and experimentally. To the best of our knowledge, however, the effect of different functional groups on the adsorption dynamics of such bifunctional molecules has not been investigated so far.
In this paper, we study the adsorption dynamics of the bifunctional molecule allyl methyl ether (AME) on Si(001). Using molecular beam techniques for the adsorption experiments, the kinetic energy of the molecules impinging on the surface can be controlled, in contrast to adsorption from thermal gas. Whereas the latter experiments measure the kinetics of a reaction, e.g., at different surface temperatures, and give valuable information on the energetics of the reaction,24–26 the molecular beam experiments exploit, in addition, the dynamics of the reaction in terms of the influence of the kinetic energy of the incoming molecules on the reactivity.27
The two functional groups in the AME molecule, i.e., the ether group with its oxygen heteroatom and the C=C double bond, react via two very different intermediate states that exhibit different adsorption energetics and dynamics. We compare the results of AME to the adsorption dynamics of diethyl ether (Et2O) and ethylene (C2H4) on Si(001) in order to disentangle the contribution of each functional group to the overall reaction dynamics. Despite the high reactivity of the C=C double bond on Si(001),24,28–32 we show here that the adsorption dynamics is mainly determined by the ether group. The C=C double bond then influences the further reaction from the intermediate into the final state as deduced from the dependence of s0 on surface temperature Ts.
The experiment was described in detail in Refs. 27 and 33. In short, the experimental setup consists of a four stage molecular beam apparatus, and the beam is generated via supersonic expansion through a nozzle of 100 μm in diameter. The sample is located 30 cm downstream of the nozzle. The kinetic energy of the molecules in the beam was varied by heating the nozzle, seeding the beam in noble gas of different atomic mass (i.e., helium), or both seeding and heating of the nozzle. The mean kinetic energy and its distribution were determined by means of time-of-flight measurements. Typical values of the ratio between the mean velocity of the molecules and the width of the velocity distribution, i.e., the speed ratio S, range between S = 2.5 for the unseeded beam to S = 11 for the seeded beam of Ekin = 550 meV.
The sample was prepared by resistive heating up to 1300 K and subsequent cooling with cooling rates of ≈1 K/s. We verified the 2 × 1 reconstruction of the Si(001) surface by means of low energy electron diffraction. During the experiment, the sample was heated up to 1100 K between two successive adsorption measurements in order to desorb the adsorbates of the former measurement from the surface. The temperature of the sample was measured using a thermocouple glued to the rear side of the sample.
The absolute sticking coefficient s was determined by means of the King-and-Wells method:34 when the molecular beam enters the sample chamber and an inert shutter is placed in the front of the sample, the partial pressure related to the investigated molecules is increased due to the molecules being reflected from the inert shutter. The partial pressure is monitored by using a quadrupole mass spectrometer (QMS), which is positioned out of the line of sight of the molecular beam. The QMS was operated at m/z = 45, the most intense signal in the mass spectrum of AME. When the shutter is opened, a drop of the QMS signal is observed due to adsorption on the reactive sample surface. This drop of the partial pressure is proportional to the initial sticking coefficient s0 = s(t = 0) as long as the beam diameter (≈8 mm) is smaller than the sample surface, which is 5 × 1 cm2 in size. From the raw data, the background partial pressure is subtracted. The latter is caused by thermal gas that enters the main chamber effusively via the aperture and can be measured by blocking the beam in the first differential pumping stage; it typically accounts for less than 5% of the total signal. After background subtraction, the absolute value of s0 is determined from the initial signal drop when the data are normalized to the maximum value of the signal.
In Fig. 2, the adsorption measurements, which are represented by the partial pressure measured as a function of time, are shown for three different beam energies Ekin at a given surface temperature Ts = 540 K. The initial signal drop after opening the shutter at t = 0 s as indicated by the black arrows decreases with increasing Ekin. This implies a reduced initial sticking coefficient s0 at higher Ekin. The kinetic energy was varied between 90 meV (unseeded beam) and 550 meV by seeding the beam and heating the nozzle to TN = 1000 K.
The dependence of the initial sticking coefficient on kinetic energy is shown in Fig. 3 for two different surface temperatures. At low kinetic energy, the sticking coefficients exhibit the highest values, indicating a high sticking probability for the adsorption of thermal gas, in qualitative agreement with thermal gas adsorption measurements. With increasing Ekin, the initial sticking coefficient decreases continuously for a given surface temperature. For comparison, we additionally measured s0(Ekin) for diethyl ether and further compare to the data for ethylene on Si(001) taken from Ref. 5. For Et2O/Si(001) (blue lines, data shown in Fig. S1 in the supplementary material), the decrease in s0 with increasing Ekin is very similar to AME/Si(001). For C2H4/Si(001) (ocher dashed-dotted line, from Ref. 5), s0 drops much faster with increasing Ekin when compared to AME and Et2O.
The dependence of the initial sticking coefficient on surface temperature was determined at a constant kinetic energy of Ekin = 90 meV (room temperature nozzle, unseeded beam). The results are shown in Fig. 4 (red data): s0 is constant for Ts < 400 K and drops continuously at surface temperatures Ts > 400 K. This dependence indicates that the adsorption proceeds via an intermediate state: Once the impinging molecule is trapped in the intermediate state, it can either react into the final state or it desorbs back into the gas phase. For a non-activated reaction, the desorption barrier ϵd is higher than the conversion barrier ϵa; thus, at increased surface temperature, desorption is favored over conversion, leading to the drop of s0. With νd and νa being the prefactors for the desorption and conversion rate, respectively, and s0,max being the probability for adsorption in the intermediate state, the temperature dependence of s0 can be expressed according to the Kisliuk model35 as
Fitting Eq. (1) to the data as shown in the inset of Fig. 4, the difference between the two energy barriers ϵd − ϵa can be deduced. For AME/Si(001), we obtain eV. Despite the similarity of the dependence of s0 on Ekin for AME and diethyl ether on Si(001), the s0(Ts) data of Et2O/Si(001), which are shown in Fig. 4 as well (blue data, taken from Ref. 36), significantly deviate from the AME/Si(001) system. For Et2O/Si(001), eV was reported.36
We first discuss the continuous decrease in the initial sticking coefficient s0 with increasing beam energy Ekin , as shown in Fig. 3. Such a dependence is typical for a non-activated adsorption channel. The higher the kinetic energy of the incoming molecules, the less likely it is that the molecules dissipate enough energy that they are trapped in the intermediate state. When comparing the results for AME and diethyl ether, it is obvious that both datasets follow the same functional form, i.e., a linear dependence of s0 on Ekin. Moreover, the slopes are almost identical for comparable values of s0. In contrast, the data for ethylene on Si(001) show a much stronger dependence of s0 on Ekin. In a previous study, we have shown that the nature of the intermediate determines the adsorption dynamics.8 As we observe almost identical adsorption dynamics for AME and diethyl ether, but very different adsorption dynamics in case of ethylene on Si(001), we conclude that in the case of the bifunctional molecule AME, the dynamics are largely determined by the ether group and the corresponding intermediate. The latter comprises a dative bond between the oxygen atom of the ether group and the unfilled Ddown state of a silicon dimer. This bond is relatively strong,36 in particular, when compared to the three-center π complex, which is formed in the intermediate between the C=C double bond of ethylene and the unfilled Ddown state of the silicon surface.19,23 From our results, we conclude that the C=C double bond as a second functional group in the AME molecule does not significantly affect the adsorption dynamics into the intermediate state, which are governed by the ether group.
In our discussion, we made use of the fact that the nature of the intermediate determines the adsorption dynamics,8,9 which enabled us to identify the reaction channel that governs the reaction of AME into the intermediate state. At this point, we would like to note that one can further exploit this observation to determine the reaction channels including the involved intermediate state also for molecules which do not give easy access to the intermediate state, e.g., in case it is too short-lived for the experimental techniques applied.
Although the C=C double bond in the AME molecule does not influence the adsorption dynamics into the intermediate, it has a non-negligible influence on the further reaction from the intermediate into the final state as deduced from the temperature dependence of the sticking probability: The energy difference ϵd − ϵa = 0.31 eV and the prefactor ratio for AME/Si(001) deviate significantly from the values measured for Et2O/Si(001) (ϵd − ϵa = 0.24 eV, 36). This difference is attributed to the presence of the C=C double bond in the AME molecule, which is the major difference between the two molecules.
The further reaction of the ether group into the final state proceeds via the cleavage of the O—C bond (ϵa = 0.4 eV in the case Et2O36), leading to a Si—OCH2CH3 and a Si—CH2CH3 entity. The presence of the C=C functional group in the AME molecule may provide an additional reaction channel for the conversion from the intermediate into the final state via the [2 + 2] cycloaddition of C=C on silicon, which proceeds with a reduced barrier.20 Indeed, the higher value of ϵd − ϵa indicates a lower energy of the transition state between the intermediate and final state, which translates into a lower conversion barrier if we assume the same binding energy in the intermediate state for both molecules. Both the decreased transition state energy and the increased value for the ratio of the prefactors, , are thus in line with an additional reaction channel for the further reaction into the final state, which might be a combination of the final states of the ether and the C=C entities.
The adsorption dynamics of the bifunctional molecule AME was found to be mainly determined by the ether group with a negligible influence of the C=C double bond. On the other hand, the further reaction into the final state deviates from the kinetics deduced for molecules with an ether functionality only, indicating a substantial influence of the C=C double bond in the further course of the reaction. The results thus demonstrate that the initial adsorption dynamics can be controlled by one functional group only even when the further reaction comprises both functional groups of a bifunctional molecule.
The supplementary material includes the data on the initial sticking probability for diethyl ether on Si(001) measured as a function of the kinetic energy of the incoming molecules.
We gratefully acknowledge funding by Deutsche Forschungsgemeinschaft through Grant Nos. DU 1157/4-1, GRK 1782, and SFB 1083 (Project No. 223848855).
The data that support the findings of this study are available from the corresponding author upon reasonable request.