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Adsorption of Methanol on Si(001) – Reaction Channels and Energetics Christian Länger, Tamam Bohamud, Julian Heep, Timo Glaser, Marcel Reutzel, Ulrich Hoefer, and Michael Dürr J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b04583 • Publication Date (Web): 06 Jun 2018 Downloaded from http://pubs.acs.org on June 7, 2018

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The Journal of Physical Chemistry

Adsorption of Methanol on Si(001) – Reaction Channels and Energetics Christian Länger,† Tamam Bohamud,‡ Julian Heep,† Timo Glaser,† Marcel Reutzel,‡,¶ Ulrich Höfer,‡ and Michael Dürr∗,† Institut f¨ ur Angewandte Physik, Justus-Liebig-Universität Giessen, Heinrich-Buff-Ring 16, D-35392 Giessen, Germany, and Fachbereich Physik und Zentrum f¨ ur Materialwissenschaften, Philipps-Universität, D-35032 Marburg, Germany E-mail: [email protected]

June 6, 2018

∗

To whom correspondence should be addressed Institut f¨ ur Angewandte Physik, Justus-Liebig-Universität Giessen, Heinrich-Buff-Ring 16, D-35392 Giessen, Germany ‡ Fachbereich Physik und Zentrum f¨ ur Materialwissenschaften, Philipps-Universität, D-35032 Marburg, Germany ¶ Present Address: Department of Physics and Astronomy, University of Pittsburgh, Pennsylvania 15260, USA †

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Abstract Adsorption of methanol on Si(001) was studied by means of X-ray photoelectron spectroscopy (XPS), molecular beam techniques, and scanning tunneling microscopy (STM) at surface temperatures between 50 and 800 K. Even at lowest temperatures, only the final reaction products, i.e., a silicon bound methoxy group and a Si-H entity, were observed in the XPS and STM experiments. However, the initial sticking probability drops with increasing surface temperature indicating that the reaction proceeds via an intermediate state. Two final configurations with the dissociation products adsorbed either on one or two silicon dimers were observed; their branching ratio does not change with temperature indicating very similar, low conversion barriers for the two pathways.

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INTRODUCTION Proton transfer reactions are ubiquitous in various fields of chemistry and typically proceed with low activation energy. 1–4 For the adsorption of organic molecules on semiconductor surfaces, in particular on silicon, reactions which proceed analogously to proton transfer reactions play an important role as well: Most organic molecules with a heteroatom such as nitrogen or oxygen, first adsorb on the silicon surface via a datively bonded intermediate state, i.e., the heteroatom transfers electronic density to the substrate. 5–7 For primary and secondary amines, further reaction via O-H dissociation was observed, 5,8–10 whereas the cleavage of the N-C bond is overall activated and was not observed in the low coverage regime. 8,11 This observation was explained in terms of a reaction which formally involves a proton transfer 9 from the nitrogen with formal charge +1 to the electron-rich, nucleophilic silicon atom of the Si-dimer with formal charge -1. For alcohols, adsorption via O-H dissociation was also observed experimentally, 12–14 however, the datively bonded intermediate state (Fig. 1) could not be isolated, although it was predicted theoretically. 14–20 Thus the question remains if alcohol dissociation on Si(001) proceeds via an intermediate state and which final configurations can be reached. Here we use X-ray photoelectron spectroscopy (XPS), molecular beam techniques, and scanning tunneling microscopy (STM) to investigate the reaction pathways of methanol on Si(001) as a prototypical system for alcohol adsorption on silicon. We find clear evidence that the reaction proceeds via an intermediate state but the conversion barrier between intermediate and final state is very low and the datively bonded intermediate cannot be isolated at 50 K. The distribution of the two final configurations after O-H dissociation, i.e., with the methoxy group and the hydrogen atom either on the same dimer or on two neighbored dimers (Fig. 1), stays the same between 50 K and 300 K indicating similar conversion barriers into the two configurations.

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Gas Phase

Ea

Potential Energy

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Ed

H

H3C

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Datively bonded intermediate

O

Intermediate State Final State Reaction Coordinate

HO

O+

H3C

CH3

O

H

H

-

Proton transfer

O

CH3

H

One dimer C-O cleavage

Two dimer

O-H cleavage

Figure 1: Schematic depiction of possible adsorption pathways for methanol on Si(001). The reaction via an intermediate state involving a dative bond between the oxygen atom of methanol and the Ddown state (unfilled dangling bond) associated with the lower atom of the silicon dimer is shown with three possible final adsorption configurations: (i) adsorption on one dimer via C-O cleavage, which is associated with a high conversion barrier 15 (compare schematic potential energy curve, dashed grey line). (ii) O-H dissociation on one dimer. (iii) O-H dissociation involving two neighbored dimers. The latter two reaction channels are associated with a low conversion barrier (blue and orange line in the potential energy curve shown in the inset). Legend: yellow circles – silicon dimer atoms; white, light grey, and shaded ellipses – unfilled, partly occupied, and doubly occupied dangling bonds, respectively. Dark grey ellipses – lone pair orbitals.

EXPERIMENTAL SECTION XPS and STM experiments were performed in the same UHV chamber with a base pressure < 1 × 10−10 mbar. Si(001) samples were prepared by degassing at 700 K and repeated direct current heating cycles to above 1450 K. 21,22 Cooling rates of about 1 K/s lead to a wellordered 2×1 reconstruction. Distilled methanol was dosed via a leak valve from the vapour phase in a test tube. STM measurements were performed with a variable temperature STM (Omicron VT-STM). XPS measurements were performed using a Al Kα X-ray source with monochromator (Omicron XM1000) and a hemispherical energy analyzer (Omicron EA125). All XPS spectra were referenced to the Si peak at 99.4 eV. 23 Absolute sticking probabilities were measured by means of the King-and-Wells method 4


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in a molecular beam apparatus. 24,25 Base pressure was < 1 × 10−10 mbar, sample preparation was analogous to the procedure in the STM/XPS chamber. 26 Surface reconstruction and surface cleanliness were checked by low energy electron diffraction and Auger electron spectroscopy, respectively.

RESULTS In Fig. 2, XPS measurements after adsorption of a submonolayer of methanol on Si(001) at 90 and 300 K are compared. Both, for the spectra in the region of the O 1s and C 1s energy, one single peak is observed; they are at the same position both for adsorption at 90 K and 300 K. When taking into account the different relative sensitivity factors for the O 1s and C 1s peaks, intensity ratios for I(O):I(C) of 1.07 : 1 at 90 K and 1.06 : 1 at 300 K were deduced, in accordance with the 1:1 ratio of the abundance of oxygen and carbon in the methanol molecule. The observed binding energy of the O 1s peaks indicates a covalent

O-C-H3

Si-O-C O 1s

Intensity [arb. units]

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C 1s

90 K

300 K

534 533 532 531

288 287 286 285

Binding Energy [eV]

Figure 2: O 1s and C 1s core level spectra after adsorption of methanol on Si(001) at 90 K (top) and 300 K (bottom). Adsorption and measurement temperature were identical, the intensity of the spectra were scaled using the respective relative sensitivity factors (C: 1; O: 2.93). 27 The O 1s peak indicates a covalent bond to the Si surface, the single C 1s peak indicates a C-O bond. Literature values for binding energies of O and C in the respective chemical environment are indicated as grey dotted lines. 23,28 5



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bond between oxygen and silicon, 23,28 which is further backed by an additional shoulder at higher binding energies observed for the Si 2p peak (Fig. S1). The relatively high binding energy of the C 1s peak indicates a covalent bond between carbon and oxygen; 23,29 a C-Si bond can be excluded. As this observation is valid both for adsorption at 300 K and 90 K, we conclude on dissociative adsorption being operative on the time scale of the XPS experiment down to 90 K in agreement with literature. 12–14 In Fig. 3, filled and empty state STM images of Si(001) with a coverage of 0.04 ML methanol adsorbed at room temperature (RT) are shown. Two different adsorption configurations are observed. The predominant two-dimer configuration appears dark at one end of two neighbored dimers in one dimer row in the filled state images. In empty state images, this configuration appears dark at the same end of the two dimers as in filled state images, but bright at the other end. It is associated with methanol dissociation over two neighbored

(a)

(c)

(b)

- 2.0 V

(e)

one dimer two dimer

(d)

- 2.0 V

+ 0.8 V

+ 0.8 V

Figure 3: (a) Filled state (Ut = −2.0 V, It = 0.5 nA) and (b) empty state (Ut = +0.8 V, It = 0.5 nA) STM images of the same area (10×15 nm2 ) of a Si(001) surface after adsorption of 0.04 ML methanol. Adsorption and measurement temperature was 300 K. (c) and (d): close-ups of the areas indicated in (a) and (b), respectively. The one dimer configuration shows dark ellipses on-top of one dimer in both negative and positive bias images. The two dimer configuration appears as an elongated dark feature at the end of two dimers in the negative bias images; in the positive bias image, the dark feature is accompanied by a bright elongated feature on the opposite side of the two dimers due to the isolated unsaturated dangling bonds (compare sketches in (e)).

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Table 1: Distribution of configurations for the three different adsorption temperatures. The ratio between one-dimer, two-dimer and ”double“ configurations is constant over this temperature range. At 50 K, additional features were observed which amount to the remaining 1 % of the distribution of configurations. For each temperature, between 1000 and 2000 configurations have been evaluated. 50 K 90 K 300 K

One dimer 19 % 20 % 19 %

Two dimer 77 % 75 % 76 %

Double 3% 5% 5%

dimers with a Si-O-CH3 entity on one and a Si-H entity on the other dimer (Fig. 1). In filled state images, this leads to a suppression at the reacted dangling bonds, in the empty state images, a suppression is also observed at the reacted silicon atoms but bright signatures opposite to the reacted dimers appear due to the isolated, unreacted dangling bonds. 24,30 As a second configuration, suppression of one dimer both in the negative and positive bias images is observed, which is assigned to methanol dissociation with the final products restricted to one dimer (one-dimer configuration, Fig. 1). Room temperature STM measurements which were taken after adsorption at 90 K surface temperature show the same two adsorption configurations in a similar distribution of configurations (Tab. 1). For both, adsorption at RT and 90 K, dark configurations which cover two neighbored dimers were observed for both sample polarities. These configurations are attributed to dissociation of two methanol molecules next to each other, either both in the one-dimer or both in the two-dimer configuration. As these two possibilities cannot be distinguished, they are listed as ”double“ configurations in Tab. 1. Their abundance is in accordance with a statistical formation of such neighbored configurations. Fig. 4 shows an empty state image of 0.05 ML of methanol adsorbed and measured on Si(001) at TS = 50 K. Both, the one- and two-dimer adsorption configurations are clearly observed. A small number of additional signatures which appear as a dark cross over two neighbored dimers were identified. The origin of the latter configurations was not addressed in this investigation, their abundance is low (< 1.5 % of the total number of configuations)

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+ 1.2 V Figure 4: Empty state STM image (10 × 10 nm2 ) of 0.05 ML methanol adsorbed and measured at 50 K (Ut = +1.2 V, It = 0.5 nA). The same adsorption configurations as after adsorption at RT and 90 K are observed (one-dimer configuration: orange ellipse, two-dimer configuration: blue ellipse). Some configurations consisting of two neighbored methanol configurations as well as a few additional configurations (grey dashed ellipse) are observed. and did not scale with dose. It thus might be attributed to some contamination which adsorbs at low temperature only. The distributions of configurations which were attributed to methanol dissociation are compared for the three different temperatures in Tab. 1. In all three cases, the two-dimer configuration is dominant and the ratio between one- and two-dimer configurations is constant. From our STM and XPS measurements, no indication for an intermediate was observed. We therefore performed measurements of the initial sticking probability s0 as a function of surface temperature as shown in Fig. 5. At temperatures below 400 K, s0 is close to unity and constant. For temperatures higher than 400 K, s0 decreases with increasing surface temperature. This behavior is a clear indication for adsorption via an intermediate state: once trapped in the intermediate, the methanol molecule can either dissociate into the covalently bound final configurations or desorb back into the gas phase. For an overall nonactivated process (Fig. 1), the relative weight of the latter process increases with increasing surface temperature leading to the observed drop in sticking probability. In contrast, a direct reaction channel would result in a constant value of s0 over the whole temperature range. 29

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1

CH3OH/Si(001)

0.8 0.6

ln ((s0,max/s0)–1)

Initial Sticking Coefficient s0

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0.4 0.2

2 0 –2 1.0

1.5 2.0 1000/TS (1/K)

2.5

0 0

200 400 600 800 Surface Temperature Ts (K)

Figure 5: Absolute initial sticking probabilities s0 of methanol on Si(001) measured as a function of surface temperature. Inset: Kisliuk plot of the data with linear fit (solid line). The solid line in the main panel is identical to this fit. Kinetic energy of the molecular beam was 90 meV (super sonic expansion at room temperature). Quantitative evaluation of the data, taking into account the thermally activated conversion and desorption processes (Kisliuk model, 25,31 inset in Fig. 5) reveals the difference between the conversion barrier a , and the desorption barrier d , d − a = 0.37 ± 0.05 eV.

DISCUSSION From the XPS measurements, O-H dissociation leading to Si-OCH3 and Si-H entities on the surface was deduced, in accordance with former experimental results. 12–14 The dependence of s0 on surface temperature clearly indicates that this reaction proceeds via an intermediate state; this intermediate is likely to include an O-Si dative bond, as experimentally observed for ethers 23,28 and theoretically predicted for alcohols as well. 14–20 As this intermediate state could not be isolated in our experiments down to a temperature of 50 K, a low conversion barrier a between the intermediate and the final states is expected. From the molecular beam experiments, the value of a can be further quantified as follows: the difference between a and d was measured to be d −a = 0.37 eV. For diethyl ether on Si(001), the binding energy, 9



which is equivalent to d in the case of an overall non-activated reaction, was determined to be 0.6 eV. On the other hand, in the case of water adsorption on Si(001), d − a = 0.29 eV has been measured 32 and, taking into account a low conversion barrier,, 33,34 d ≈ 0.3 eV was deduced. 35 With a significantly lower positive inductive effect in the case of methanol when compared to diethyl ether (one methyl group versus two ethyl groups), but a higher positive inductive effect when compared to water, a binding energy of the datively bound intermediate in the range of 0.4 to 0.5 eV is expected. Thus the upper limit of the conversion barrier can be estimated to be a < 0.1 eV. This is substantially lower than the conversion barrier which has been experimentally determined for ether cleavage in the case of diethyl ether on Si(001), a = 0.38 eV 35 and points towards a different reaction mechanism, i.e., the proposed protontransfer reaction 14,15,17 in contrast to homolytic ether cleavage. Unfortunately, a rather wide range of barriers has been calculated for O-H dissociation via proton transfer from the datively bonded alcohol molecule (0.02 to 0.5 eV 14–17 ) which limits a direct comparison between theory and experiment. For the discussion of the final configurations as such, their distributions, as well as their dependence on temperature, the comparison with the reaction of water and ether molecules on Si(001) is again most instructive: in the case of ether cleavage, only one single final configuration with the covalent Si-O and Si-C bonds opposite to each other on two neighbored dimer rows was observed. 23,28 This final configuration originates from strong restrictions on the transition state: only the dangling bond of the Si-dimer of the neighbored dimer row can efficiently hybridize with the antibonding C-O orbital as it is necessary to induce the ether cleavage reaction. 23,28,36 For a proton transfer reaction, the orientation of the dangling bonds is expected to be of lower importance. Thus the two closest dangling bond states are involved in the the O-H dissociation, i.e., on the same dimer as well as on the neighbored dimer of the same dimer row as observed in our experiments, but not the more distant dangling bond on the neighbored dimer row. As the measured distribution of the two configurations (Tab. 1) deviates from a purely statistical distribution for the ratio between two-dimer and

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one-dimer configurations, i.e. 2:1, the two pathways have to be slightly different; although the two-dimer pathway involves the larger distance between the reacting dangling bond and the adsorption site of the methanol molecule in the intermediate, it shows a higher reaction probability. This might be attributed to the more unfavourable direction of the dangling bond involved in the one-dimer pathway. Most strikingly, the distribution does not change with surface temperature, indicating that the reaction barriers for the two processes are very similar. For water dissociation on Si(001), the same two configurations are observed, 37–39 indicating a similar mechanism for O-H cleavage in both cases.

CONCLUSION In conclusion, methanol adsorption on Si(001) proceeds via an intermediate with low conversion barrier into the covalently bound final states. The latter process is interpreted in terms of a proton transfer which leads to -Si-O-CH3 and -Si-H entities localized either on one Si dimer or on two neighbored Si dimers of the same dimer row. The branching ratio into these final states stays constant between 50 and 300 K indicating similar conversion barriers for both processes.

ACKNOWLEDGMENTS The authors acknowledge funding by the Deutsche Forschungsgemeinschaft through DU 1157/41, GRK 1782, and SFB 1083. The authors thank AG Prof. Koert (Marburg) for providing purified methanol. MD thanks Sony Materials Science Laboratory Stuttgart under the supervision of Dr. Gabriele Nelles for generous support.

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Associated Content Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: (to be inserted). It includes XPS spectra of the O 1s, C 1s, and Si 2p peaks of a Si(001) surface with a submonolayer of methanol adsorbed at 300 K in comparison to the clean Si(001) surface. Author information Corresponding author E-mail: [email protected]; phone +0049 (0)641 9933490 (M.D.). ORCID Michael D¨ urr: 0000-0002-4676-8715 Notes The authors declare no competing financial interest.

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(38) Warschkow, O.; Schofield, S. R.; Marks, N. A.; Radny, M. W.; Smith, P. V.; McKenzie, D. R. Water on Silicon (001): C Defects and Initial Steps of Surface Oxidation. Phys. Rev. B 2008, 77, 201305–1 – 201305–4. (39) Pierucci, D.; Gallet, J.-J.; Bournel, F.; Sirotti, F.; Silly, M. G.; Tissot, H.; Naitabdi, A.; Rochet, F. Real-Time X-ray Photoemission Spectroscopy Study of Si(001)-2x1 Exposed to Water Vapor: Adsorption Kinetics, Fermi Level Positioning, and Electron Affinity Variations. J. Phys. Chem. C 2016, 120, 21631 – 21641.

17



Page 18 of 18

TOC Graphic:

O

- 2.0 V

CH3

one dimer O

CH3

H

+ 0.8 V

two dimer

18


H

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