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Adsorption of CO Molecules on Si(001) at Room Temperature Eonmi Seo,†,‡ Daejin Eom,‡ Hanchul Kim,*,§ and Ja-Yong Koo*,†,‡ †

Korea University of Science and Technology, 217 Gajeong, Yuseong, Daejeon 305-333, Korea Korea Research Institute of Standards and Science, Yuseong, Daejeon 305-340, Korea § Department of Physics, Sookmyung Women’s University, Seoul 140-742, Korea ‡

ABSTRACT: Initial adsorption of CO molecules on Si(001) is investigated by using room-temperature (RT) scanning tunneling microscopy (STM) and density functional theory calculations. Theoretical calculations show that only one adsorption configuration of terminal-bound CO (T-CO) is stable and that the bridge-bound CO is unstable. All the abundantly observed STM features due to CO adsorption can be identified as differently configured T-COs. The initial sticking probability of CO molecules on Si(001) at RT is estimated to be as small as ∼1 × 10−4 monolayer/ Langmuir, which is significantly increased at high-temperature adsorption experiments implying a finite activation barrier for adsorption. Thermal annealing at 900 K for 5 min results in the dissociation of the adsorbed CO molecules with the probability of 60−70% instead of desorption, indicating both a strong chemisorption state and an activated dissociation process. The unique adsorption state with a large binding energy, a tiny sticking probability, and a finite adsorption barrier is in stark contrast with the previous low-temperature (below 100 K) observations of a weak binding, a high sticking probability, and a barrierless adsorption. We speculate that the low-temperature results might be a signature of a physisorption state in the condensed phase.

I. INTRODUCTION The adsorption of a CO molecule on the semiconductor surface is an interesting topic for elucidating basic chemical reactions such as electron-stimulated desorption or photonstimulated desorption, since CO is a prototypical diatomic molecule and its gas-phase spectroscopy is well understood. Compared to the vast amount of studies on CO reaction with metal surfaces, only limited studies have been reported on the adsorption of CO on the Si(001) surface, and they seem to provide contrasting adsorption behavior of CO molecules. An early study by high-resolution electron energy loss spectroscopy (HREELS), UV photoelectron spectroscopy (UPS), and temperature-programmed desorption (TPD) reported that CO molecules adsorbed almost perpendicularly on the Si(001)-(2 × 1) surface at 100 K and desorbed at 180 K.1 The study by X-ray photoelectron spectroscopy (XPS), UPS, and TPD at room temperature (RT) reported that CO adsorbed molecularly on the Si(001)-(2 × 1) surface with an activation energy of ∼0.48 eV and with a very small sticking probability.2 Hu et al. exposed the Si(001) surface to an energetic molecular beam of CO at 85 K and found two possible stable adsorption configurations of the CO molecules in the terminal-bound CO (T-CO) with no energy barrier and in the bridge-bound CO (B-CO) with an energy barrier of 0.9 eV, where T-CO was more stable with the C atom bonding to the down-buckled Si atom of the Si dimer.3,4 A study by scanning tunneling microscopy (STM) at 70 K and valenceband photoelectron spectroscopy (PES) reported that there existed only one adsorbed state T-CO forming islands near surface defects with no isolated adsorbed CO molecules.5 Early © 2014 American Chemical Society

theoretical studies reported that both T-CO and B-CO were stable,6,7 which is in contrast to a recent theoretical result that the B-CO is unstable.8 A model of CO−CO adsorbate interaction on the Si(001)-(2 × 1) surface was also calculated by using cluster models of the surface.9 Thus, previous studies on the CO adsorption on Si(001) suggest scattered adsorption behaviors lacking a consensus. Some reported a unique adsorption configuration, while others reported two different configurations. The results from lowtemperature (LT) and RT experiments seem to be contradicting to each other, too. For instance, a high sticking probability and a barrierless adsorption process were suggested by the LT experiments, in contrast with a low sticking probability and an activated adsorption at RT. As such, the adsorption of CO molecules on Si(001) needs more detailed investigation for a clear understanding. In this Article, we investigate the adsorption of CO molecule on the Si(001)-(2 × 1) surface by using STM at RT and density functional theory (DFT) calculations. We find by STM three adsorption features (zigzag, ondimer, and interdimer) that increase linearly with the CO dosage. Our theoretical calculations confirm that the T-CO adsorption configuration is solely stable while the B-CO is unstable. We deduce that the zigzag pattern is the footprint of one CO molecule in the T-CO configuration and that the remaining two adsorption features (ondimer and interdimer) are made up of two adsorbed CO Received: June 16, 2014 Revised: August 25, 2014 Published: August 27, 2014 21463

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molecules. The initial sticking probability is estimated to be ∼1 × 10−4 monolayer/Langmuir (ML/L) up to the CO dosage of 50 L, whereas the initial sticking probability is significantly increased for the high-temperature adsorption around 450 K. The dissociation probability of the adsorbed CO molecules is 60−70% for the thermal annealing at temperature as high as 900 K, showing a strong chemisorption at RT in contrast to the previously reported weakly bound adsorption state at LT.

by a circle is characterized by a strong local zigzag pattern in the filled-state image, where the central dimer is strongly buckled. In the empty-state image, this feature appears as a bright spot at the site of the up-buckled Si atom of the central dimer. (ii) The second one enclosed by a diamond looks darker and more localized than the neighboring clean Si dimers in the filled-state image. It also appears darker than clean Si dimers in the emptystate image. (iii) The third one enclosed by a square looks similar to the C defect, showing the dark depression at one side of two successive Si dimers in the filled-state and the bright protrusion at the other side of the two dimers in the emptystate.16,17 Figure 1d shows the initial coverages of the three features. Since the third feature is indiscernible from the C defect caused by residual H2O molecules,17 we subtracted the number of background C defects (0.03%) from the counted number of the third feature. The three STM image features increase in proportion with the CO dosage, which confirms that these are indeed the footprints of the adsorbed CO molecules on the Si(001)-(2 × 1) surface. To compare with the experimental data, we have calculated the ab initio total energies for four types of single-molecule adsorption configurations shown in Figure 2a−d: the T-CO where a CO molecule bound almost vertically to a downbuckled Si, the B-CO where a CO molecule bound to two Si atoms of a Si dimer,4 an on-dimer configuration where a CO molecule bridge bound to both Si atoms of a dimer (OD-CO), and an end-bridge configuration where a CO molecule bridge bound to two dimers along the dimer row (EB-CO). We found that the B-CO is unstable in contrast to previous theoretical calculations4,6,7 but in accordance with a recent theoretical study.8 Both OD-CO and EB-CO were also found to be unstable and relaxed to the T-CO configuration. Resultantly, our calculations for the single-molecule adsorption show that there exists only one stable adsorption configuration, T-CO, shown in Figure 2a. In this unique adsorption configuration, the C atom forms a chemical bond with the down-buckled Si atom of a dimer. We also considered variants of T-CO and B-CO, where the O atom of CO is heading toward Si atoms, and it turned out that the O atom would not bind to a Si atom in any case. Next, we have examined three types of two-molecule adsorption configurations as shown in Figure 2e−g: two CO molecules terminal bound to two Si atoms in an ondimer (2CO-OD), an interdimer (2CO-ID), and a cross-dimer (2COXD) configurations, respectively. The calculated adsorption energies of the stable one- and two-molecule adsorption configurations are listed in Table 1. The adsorption energy of T-CO is as large as 0.92 eV, which is too large for the adsorbed CO molecule to desorb at ∼180 K,1 suggesting that the LT adsorption state should be inherently different from the T-CO. All the two-molecule structures have smaller adsorption energies per molecule compared with the single-molecule TCO structure, suggesting an effective repulsion between adsorbed CO molecules that is in contrast with a LT observation of the adsorbate island formation.5 It is interesting to notice that there is a weakly bound locally stable configuration (P-CO in Table 1) with an adsorption energy as small as 0.024 eV. In the P-CO configuration, the CO molecule is oriented almost vertically with the O atom heading toward the down-buckled Si atom of a Si dimer as shown in Figure 2h, and the C−O bond is slightly dilated from the theoretical molecular bond length of 1.143 Å to 1.144 Å. Thus, this P-CO structure can be regarded as a physisorption state.

II. EXPERIMENTAL AND THEORETICAL METHODS Experiments were performed using a homemade STM in an ultrahigh vacuum (UHV) chamber with a base pressure less than 1 × 10−10 Torr. The samples were phosphorus-doped Si(001) wafers with a resistivity of 1−10 Ω cm. Atomically flat Si(001)-(2 × 1) surfaces were obtained by repeated flashings at temperatures up to 1450 K.10 The CO molecules were introduced through a leak valve and backfilled the STM chamber for a predetermined time interval under the pressure of 5 × 10−8 Torr at RT. The structure of CO/Si(001) was investigated by STM at RT. The DFT calculations were performed within the generalized gradient approximation,11 using the Vienna ab initio simulation package.12 We used a plane-wave cutoff of 400 eV, the theoretical lattice constant of 5.46 Å, the Monkhorst−Pack kpoint sampling equivalent to a 16 × 16 k-mesh in the 1 × 1 surface Brillouin-zone,13 and the projector-augmented-wave potentials.14,15 The Si(001) surface was modeled by a six-layer slab separated by 19 Å vacuum with a (4 × 8) surface supercell. The CO molecules were adsorbed on the c(4 × 2)reconstructed surface, and the opposing surface was passivated by hydrogens. All adsorbate atoms and the top four Si layers were relaxed until the Hellman−Feynman forces were smaller than 0.01 eV/Å. III. RESULTS AND DISCUSSION Figure 1 shows an STM image of the Si(001)-(2 × 1) surface dosed to 20 L CO molecules. There are three types of features created by CO molecules at RT. (i) The first feature enclosed

Figure 1. (a) Filled-state STM image (Vs = −2.0 V) of the Si(001)-(2 × 1) surface exposed to 20 L CO molecules at RT. Three types of features appear as the zigzag (○), ondimer (◊), and interdimer (□). (b, c) Three adsorption CO configurations in the enlarged filled-state (Vs = −2.0 V) and empty-state (Vs = 1.5 V) STM images, respectively. (d) Initial coverages of the three adsorption configurations at RT. 21464

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Figure 2. Possible adsorption configurations of CO molecules on the Si(001)-(2 × 1) surface. Yellow, blue, and red circles represent the surface Si, C, and O atoms, respectively. (a) One CO molecule bound to the down-buckled Si atom of a dimer (T-CO), (b) one CO molecule bridging two Si atoms of a dimer perpendicularly (B-CO), (c) one CO molecule adsorbed on a Si dimer (OD-CO), (d) one CO molecule bridging two Si dimers (EB-CO), (e) two T-COs in an ondimer configuration (2CO-OD), (f) two T-COs in an interdimer configuration (2CO-ID), (g) two T-COs in a cross-dimer configuration (2CO-XD), and (h) one CO molecule in a physisorbed state (P-CO). The three nearest-neighbor adsorption sites of the second CO molecule to a pre-existing T-CO are designated as A, B, and C in (a).

molecules. Since one CO molecule bridging two dimers (e.g., EB-CO) is found to be unstable from our calculations, this feature should be assigned to the 2CO-ID configuration in Figure 2f. (iv) Two CO molecules, adsorbed diagonally on two Si dimers forming the 2CO-XD shown in Figure 2g, are more stable than those in either the 2CO-OD or the 2CO-ID as in Table 1. However, the STM image feature corresponding to 2CO-XD is barely observed, being not dominant to show appreciable proportionality with the CO dosage. The rarely observable 2CO-XD suggests that the adsorption of CO molecules is predominantly governed by kinetics rather than by energetics. We speculate that the impinging CO molecules initially adopt a mobile precursor state (e.g., P-CO found in our calculations). The CO molecules in the precursor state either desorb from the Si(001) surface or wander around on the Si(001) surface until a chemisorption occurs. The chemisorption from the precursor state can be achieved in either an isolated T-CO or the two-molecule adsorption configurations (2CO-OD and 2CO-ID) by encountering a preadsorbed T-CO. As designated in Figure 2a, there are three nearest-neighbor adsorption sites, A, B, and C, of a preadsorbed T-CO. If the A site is taken by a CO molecule, the resulting adsorption configuration becomes 2CO-OD. If the second adsorbing CO molecule occupies either B or C sites, the 2COID configuration is formed. Thus, the probability to form 2COID is doubled than that to form 2CO-OD, which agrees with the measured coverages of different STM image features in Figure 1d. To form a 2CO-XD configuration, however, a second CO molecule should take the second nearest-neighbor site to a preadsorbed T-CO. The experimental observation of the rareness of 2CO-XD implies that the kinetically attractive basin is of very short ranged only to encompass the first nearest-neighbor sites (A, B, and C). The weak interaction

Table 1. Calculated Adsorption Energies (Eads) of the Oneand Two-Molecule Adsorption Configurations configuration

Eads (eV/CO)

T-CO P-CO 2CO-OD 2CO-ID 2CO-XD

0.918 0.024 0.473 0.720 0.901

On the basis of these theoretical findings, we can identify the three types of CO-induced STM image features in Figure 1 as described below: (i) The central Si dimer of the local zigzag feature in the filled-state image is strongly buckled in one direction. The up-buckled Si site of the central dimer is also bright in the empty-state image as shown in Figure 1c, implying that the dangling bond of the up-buckled Si atom is intact. This can be realized when the down-buckled Si atom is reacted by a CO molecule and the π bond over the central dimer is broken. Hence, we assign the local zigzag pattern to the single-molecule T-CO configuration in Figure 2a. (ii) The ondimer feature appears darker than the neighboring Si dimers in both filled- and empty-state images. This suggests that both Si atoms of a dimer are reacted and their dangling bonds are saturated. This can only be achieved by two CO molecules adsorbing in the 2COOD configuration shown in Figure 2e, since the B-CO or OD-CO configurations with one CO molecule on a Si dimer are unstable.8 (iii) The interdimer feature with two bright successive Si sites at one side along the dimer row in the empty-state image implies that two dangling bonds are intact and two Si atoms on the other side are reacted by one or two CO 21465

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Figure 3. (a) Filled-state STM image of the Si(001) surface exposed to 20 L CO followed by thermal annealing at 900 K for 5 min. The carboninduced DV41 defects begin to form line segments perpendicular to the dimer rows. (b−d) Si(001) surfaces exposed to 500, 800, and 1000 L CO followed by thermal annealing, respectively. (b) Small patches of c(4 × 4) begin to form after the carbon concentration exceeds the saturation value (∼0.05 ML) of 2 × n. (c) Much of the Si(001) is covered with c(4 × 4) reconstruction structure. (d) Nearly all the surface is covered with c(4 × 4), where the carbon concentration is ∼1/8 ML.

whole surface was covered by the c(4 × 4) phase as shown in Figure 3d. In our previous publication,19 we were able to identify the atomic structure of the c(4 × 4) phase and found that each c(4 × 4) unit cell contains a single C atom. Since C atoms do not evaporate nor diffuse into the bulk but make the DV41 defects by thermal annealing, we can estimate the number of dissociated CO molecules by counting the DV41 defects on the Si(001)-(2 × 1) surface or the number of c(4 × 4) unit cells. Most of the dissociated O atoms seem to be evaporated as SiO by thermal treatment at 900 K for 5 min. Examining Figure 3a−d, the dissociation probability of adsorbed CO molecule was estimated to be 60−70%, with the rest 30−40% of adsorbed CO molecules being desorbed as molecules. We also tested the high-temperature adsorption of CO molecules. At the sample temperature of 450 K, the sticking probability increased significantly.2 This high-temperature experimental data together with the very small sticking probability at RT imply that the CO molecules experience an appreciable activation energy barrier to adsorb on the Si(001)(2 × 1) surface. Previous experimental studies at LT reported that CO molecules readily adsorbed on Si(001) with no activation barrier below 100 K and that most of the adsorbed CO molecules desorbed above 180 K.1,4 In contrast, Chamberlain et al. reported a very low sticking probability of CO on Si(001) at

between two T-COs in the 2CO-XD configuration is inferred from the similar adsorption energies per molecule of T-CO and 2CO-XD in Table 1. Thus, the formation of 2CO-XD is a result of occasional coadsorption of two CO molecules at second nearest-neighbor sites rather than being driven by intermolecular interaction. Since the ondimer and interdimer features in Figure 1 consist of two CO molecules, we can estimate the initial sticking probability of CO molecules on the Si(001)-(2 × 1) surface by counting the number of three CO adsorption features in many STM images. The resultant initial sticking probability of CO molecules is ∼1 × 10−4 ML/L at RT.2 To investigate the dissociation probability of the adsorbed CO molecules, we exposed the Si(001) surface to CO molecules at RT and subsequently annealed the CO/Si(001) surface at 900 K for 5 min. For the small CO-dosed samples, as shown in Figure 3a, we observed the development of short dimer-vacancy (DV) chains across the dimer rows. This DVlike feature was identified to be the so-called DV41 defect generated by a single C atom incorporated at the fourth subsurface layer in our previous study.18 As we increase the CO dosage, the short chains of DV41 become elongated to form a superstructure, the so-called 2 × n phase, with the carbon saturation concentration of 0.05 ML as in Figure 3b. When the CO dosage was further increased, small patches of the c(4 × 4) phase started to appear as in Figure 3b, c, and eventually the 21466

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RT and an activation energy barrier of ∼0.48 eV.2 Our study shows that an extremely small number of CO molecules adsorb on Si(001) with a finite activation energy barrier and that most of the adsorbed CO molecules are bound strongly enough to be dissociated by thermal annealing instead of the desorption as molecules, supporting the results by Chamberlain et al. A previous STM study at 70 K reported that CO molecules adsorbed as islands with no signature of isolated adsorption.5 However, our experiments at RT evidently showed that CO molecules adsorbed as one- and two-molecule states at low coverage without any preference to form islands. Hence, the underlying mechanism of CO adsorption at low temperatures below 100 K seems to be different from the RT adsorption behavior. We speculate that the LT adsorption of CO occurs to form a kind of physisorption state (possibly in a condensed or liquid phase partly judging from the observation of island formation in LT-STM and partly considering the boiling point 81.6 K of CO) rather than to form a chemisorption state, considering the desorption temperature as low as 180 K. By exposing the Si(001) surface to an energetic CO beam at 85 K, Hu et al. found a new CO adsorption at 211 meV EELS peak and assigned it to B-CO configuration, in addition to one at 261 meV EELS peak. We deduce that the CO adsorption configuration at 261 meV is a physisorbed state (for instance, the P-CO shown in Figure 2h) and that the state at 211 meV EELS peak with the activation energy barrier of 0.9 eV is our TCO at RT. Most of previous studies on this system have been performed at LT where physisorbed CO molecules already cover the entire Si(001) surface and prevent additional CO molecules from approaching. In such an environment, the chemisorption of CO molecules on Si(001) cannot be addressed properly. To study the chemisorption of CO, it would be better to expose the Si(001) surface to a large amount of CO molecules at RT, considering the extremely small sticking probability. The presently proposed adsorption models need to be further confirmed by spectroscopic experiments employing EELS, IR spectroscopy, TPD, XPS, and UPS.

as the mono-oxygen source to investigate the microscopic oxidation mechanism of the Si(001) surface, whereas the number of C atoms (which is countable by examining STM images) can be used to estimate the number of O atoms.

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AUTHOR INFORMATION

Corresponding Authors

*(H.K.) E-mail: [email protected]. *(J.-Y.K.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported partly by Future-based Technology Development Program (Nano Fields) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (no. 20120006200) and partly by the Supercomputing Center/Korea Institute of Science and Technology Information with supercomputing resources including technical support (no. KSC-2012-C2-17).

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REFERENCES

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IV. CONCLUSIONS We have investigated the adsorption of CO molecules on Si(001) by RT-STM and DFT calculations. The previously proposed bridge-bound configuration (B-CO) is calculated to be unstable, and the only stable single-molecule adsorption configuration is confirmed to be the terminal-bound configuration (T-CO). We also found a marginally stable physisorption configuration P-CO. The three frequently observed STM image features are identified to be the singlemolecule T-CO and the two-molecule 2CO-OD and 2CO-ID configurations. The initial stickling probability at RT is determined to be ∼1 × 10−4 ML/L. The adsorbed CO is strongly bound on the Si(001) surface, and 60−70% of the adsorbed molecules are dissociated by thermal annealing. This is in contradiction with the previously reported LT adsorption state that desorb easily above 180 K. Our results imply that the LT adsorption channel of CO on Si(001) may be a physisorption state in the condensed phase. In contrast, it is evident that the RT adsorption results in the chemisorption on the Si(001)-(2 × 1) surface. For the study of chemisorbed CO molecules, the Si(001) surface should be exposed to a large amount of CO molecules at temperatures far higher than 180 K to avoid the physisorbed CO molecules. We envision that the dissociated O atoms at moderate thermal annealing can be used 21467

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(15) Kresse, G.; Joubert, D. From Ultrasoft Pseudopotentials to the Projector Augmented-Wave Method. Phys. Rev. B 1999, 59, 1758− 1775. (16) Hamers, R. J.; Köhler, U. K. Determination of the Local Electronic Structure of Atomic-Sized Defects on Si(001) by Tunneling Spectroscopy. J. Vac. Sci. Technol., A 1989, 7, 2854−2859. (17) Yu, S.-Y.; Kim, H.; Koo, J.-Y. Extrinsic Nature of Point Defects on the Si(001) Surface: Dissociated Water Molecules. Phys. Rev. Lett. 2008, 100, 036107. (18) Kim, W.; Kim, H.; Lee, G.; Koo, J.-Y. Initial Stage of Carbon Incorporation into Si(001) and One-Dimensional Ordering of Embedded Carbon. Phys. Rev. Lett. 2002, 89, 106102. (19) Kim, H.; Kim, W.; Lee, G.; Koo, J.-Y. Two-Dimensional Carbon Incorporation into Si(001): C Amount and Structure of Si(001)-c(4 × 4). Phys. Rev. Lett. 2005, 94, 076102.

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