Dissociation of Single 2-Chloroanthracene Molecules by STM-Tip

Jan 19, 2012 - TPD spectra of 2-chloroanthracene-exposed TiO2(110) surfaces and ... Antonio M. Echavarren , Daniel Sanchez-Portal , Christian Joachim ...
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Dissociation of Single 2-Chloroanthracene Molecules by STM-Tip Electron Injection Denis V. Potapenko, Zhisheng Li, and Richard M. Osgood* Laboratories for Light-Surface Interactions, Columbia University, New York, New York 10027, United States S Supporting Information *

ABSTRACT: We have studied the adsorption and tipinduced chemistry of 2-chloroanthracene on TiO2(110). STM images show that at 135 K and low coverage, i.e., ∼0.1 ML, these molecules are physisorbed along the fivecoordinated titanium rows on the rutile(110) surface as a result of electrostatic interaction. Applying electric pulses >2.5 V from the STM tip to individual molecules causes either desorption or dissociation of the molecules, as indicated by the changes in the STM images. We have observed dissociative electron capture of a single 2-chloroanthracene molecule, which leaves behind a surface chlorine atom adsorbed in the on-top configuration on a surface Ti atom. The threshold energy required for the dissociation was found to be ∼2.7 eV.

1. INTRODUCTION Since the invention of scanning tunneling microscopy (STM) many research groups have successfully demonstrated the possibility of single-atom or -molecule manipulation with its tip.1−3 The ability to cause chemical reactions on the singlemolecule scale and visualize the reaction products is a more challenging task, although seminal results have been reported for reactions on metal surfaces at cryogenic temperatures4−8 or at room temperature on highly reactive semiconductor surfaces.9−14 Studies of STM tip-induced surface reactions have a high potential for elucidating the dynamics and mechanism of, generally, any charge-driven chemical process, including photodriven reactions, since in many cases photoreactions of adsorbed molecules involve charge transfer between a surface and adsorbate molecule, followed by its chemical transformation.15−17 In this case, excited carriers are generated by the absorption of light in the bulk of the substrate crystal. However, it is possible to investigate the reaction mechanism through the use of electron emission from an STM tip, which is also employed to image the surface. The advantages of such an approach would be that it enables obtaining precise knowledge about (a) the initial adsorption geometry of a specific target molecule, (b) the energy and flux of the exciting electrons, and (c) the reaction products, if they remain bound on the surface. At present, titanium dioxide is considered to be a promising photocatalyst material due to its high stability and relatively low price. This oxide can be used for photodecomposition of organic pollutants, water photolysis by sunlight15−17 and in solar cells.18 This crystal has also been discussed for use in a novel family of new electronic devices including memristors.19 An ample body of STM studies of (110) face of rutile polymorph of TiO220−24 makes this surface a natural choice for © 2012 American Chemical Society

the present work. STM investigations of thermally activated,25,26 light-,27,28 and STM tip-induced29 chemical reactions on a TiO2 surface have generally concentrated on simple chemisorbed molecules such as water, alcohols, and carboxylic acids. However, the chemistry of larger molecules adsorbed on TiO2 is also an area of relevance for surface processes involved in applications such as dye-sensitized solar cells or decomposition of organic pollutants by TiO2 photocatalysis. In our experiments, chlorinated anthracene was selected for the target molecule for several reasons. First, it is a relatively simple aromatic molecule, which is heavy enough to bind to the surface molecularly at room temperature and which can be applied via a simple dosing apparatus. Second, our previous work has shown that, when this species is adsorbed onto rutile(110), it has a reproducible adsorption geometry with the elongated structure of anthracene aligning along the [001] atomic rows.30 Finally, a chlorinated derivative of anthracene is used in this study because earlier studies demonstrated that such halogenated hydrocarbons readily decompose in the gas phase through dissociative electron attachment.31 In a preliminary study, we have shown that 2-chloroanthracene molecules are physisorbed on rutile(110) surface but are mobile on the surface at room temperature, moving readily along the rows of 5-coordinated surface Ti atoms (see the Supporting Information). For this reason, the molecules were immobilized for STM imaging and manipulations by bringing the sample temperature to 135 K in the STM stage. However, to ensure that the molecules are adsorbed in the most stable configuration, chloroanthracene was deposited on a room Received: November 8, 2011 Revised: January 19, 2012 Published: January 19, 2012 4679

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temperature surface, thus preventing adsorption of the molecules in any metastable configurations. In this paper, an STM tip-induced reaction of a physisorbed organic molecule on a TiO2 surface is demonstrated and investigated. Current pulses from the STM tip with electron energies of up to 4 eV above Fermi level of the substrate were used to inject carriers into adsorbed molecules of 2chloroanthracene. Imaging of the sample surface and a single molecule before and after electron injection allowed direct observation of electron-induced dissociation of a single adsorbed chloroanthracene molecule into chlorine, which remains adsorbed on the surface, and an anthracene radical, with an unknown final state.

2. EXPERIMENTAL SECTION Experiments were performed in a customized UHV chamber,32 which was equipped with an Omicron VT-STM, a LEED/ Auger system, a differentially pumped VGQ mass-spectrometer and a sputtering gun. The base pressure in the STM chamber was 4 × 10−11 Torr. All STM images were acquired using W tips; these tips were prepared by the drop-off technique described elsewhere.33 Each newly introduced tip was cleaned through heating for 10 − 20 s by electron field emission current of ∼5 μA achieved by bringing the tip within 20−50 μm from a metal sample inside the STM stage and applying a negative voltage up to 400 V to the tip until the emission current reached the desired value. The TiO2 sample was cut from 1 mm-thick plates of (110)oriented rutile (Princeton Scientific). The sample, with 2.5 × 10 mm dimensions, was mounted in a commercial Omicron “resistive heating” sample holder sandwiched together with a highly doped 0.5 mm-thick Si(111) strip of identical dimensions that served as a heating element. The sample was heated by passing current through the silicon strip. The temperature of the sample was monitored with a K thermocouple attached to a Ta mounting band in direct contact with the silicon-titania assembly. Pristine TiO2(110) surfaces were prepared by cycles of Ar+ sputtering (1 keV) and then annealing at 950−1000 K. The quality of the surface was checked by STM imaging. Typical concentration of bridging oxygen vacancies on the surface was 15%. For deposition of 2-chloroanthracene, a small 50 mg sample of this compound was placed in a gold-foil mini-beaker supported on a transfer manipulator in the evacuated load-lock chamber and was allowed to degas for at least 1 day. For deposition, the mini-beaker with chloroanthracene was brought into the preparation chamber at room temperature in a direct line of sight from the sample and to within ∼1 cm from the surface. The sample was also kept at ∼295 K during deposition. A typical exposure time for one monolayer coverage was 30 s. Calibration of the coverage versus dose was via a series of prior TPD experiments, in which the onset of the multilayer desorption was observed and used as an indicator of onemonolayer coverage.

Figure 1. 30 × 30 nm STM image of rutile(110) surface with 0.07 ML of 2-chloroanthracene. The sample temperature is at 135 K, and the tunneling current is 20 pA. The sketches below show the chemical structure of chloroanthracene and the schematic adsorption geometry of a chloroanthracene molecule. Here the red balls represent oxygen; blue - titanium; gray - carbon; white - hydrogen; and green - chlorine atoms. The added symbols in the image are explained in the text.

bright lines.20,25 The image shows a number of nearly identical elongated bright features, marked with blue ovals, that correspond to chloroanthracene molecules. The majority of these molecules (>95%) are oriented along the atomic rows on rutile(110), such as the four marked with the solid ovals. A small number ( +1.5 V. These observations are fully consistent with the differences in profiles of the adatom in Figure 4. Two more observations concerning the nature of the adatom can be made from Figure 2b. As mentioned above, first, the adatom is centered on the bright line of the substrate, corresponding to the row of the 5-coordinated surface Ti atoms, as shown in the sketch in Figure 6. All other bright

Figure 6. Profiles of an OH group and of the adatom in Figure 2b. The sketch illustrates the adsorption sites of the adatom on TiO2 surface.

circular features that represent bridging oxygen vacancies, OH groups, and H2O molecules adsorbed on oxygen vacancy sites are centered on the dark rows that correspond to the rows of bridging oxygen atoms.25 Second, a dark halo surrounding the adatom was observed in the image. This phenomenon is more obvious in a profile view of the adatom compared to a profile of a typical OH adsorbate on TiO2, as is shown in Figure 6. The depression seen on both sides of the adatom distinguishes it from all other objects of similar size in the STM images of the surface.

4. DISCUSSION In this section we examine our evidence for fragmentation of chloroanthracene molecules into chlorine and hydrocarbon fragments. We then discuss the dynamics of electron-induced chemistry of chloroanthracene and the fate of the hydrocarbon fragment. 4.1. Chlorine Fragments. The difficulty in directly determining the chemical identity of individual atoms even in atomic-resolution images is an unfortunate shortcoming of STM. However, taken as a whole, our data allows us to identify the adatom in Figure 2b as a chlorine atom that appears after tip excitation. Our identification is based on the following observations and reasoning: First, the halo-like depression seen around the adatom in Figure 6 suggests a manifestation of a negatively charged adatom. For example, in a prior study, dark halos were seen in the STM images around negatively charged oxygen molecules and oxygen adatoms adsorbed on Ti(5) rows of a TiO2(110) surface.23 In this case, the circular depression was attributed to the repulsion of electrons tunneling from a STM tip from a negatively charged species on the surface. This repulsion creates an additional energy barrier for tunneling, thus requiring a 4683

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chlorine atom. Conceivably this could be anthracenyl fragment or anthracene molecule, which has been, in one case deposited on the surface or recaptured from the tip. Clearly however, more experimental data must be obtained to explore the actual anthracenyl dynamics.

When this thickness decreased below 5 ML, the dissociation probability of the molecules also decreased due to higher probability of charge transfer to the metal substrate. Thus, in our experiment it is likely that the presence of the conducting surface in immediate contact with the chloroanthracene molecule results in a reduction of the lifetime of the electron on a π* orbital. This reduction decreases the probability of electron transfer to the C−Cl σ* orbital, which is the orbital that can otherwise cause dissociation of the molecule. This would then explain why it is necessary to inject a tunneling electron directly in σ* orbital for dissociation to occur, thus requiring electron energies above 2.5 eV. 4.3. Anthracene Radical Fragments. In our experiments, the chlorine adatom remained on the surface after DEA of chloroanthracene. The surface capture of this product is expected since chlorine is formed in its ionic state and thus strongly interacts via image forces with the conducting substrate. On the other hand, no evidence of anthracene radicals (anthracenyl) was detected via STM imaging despite the fact that an anthracene radical is also a product of the DEA process. In contrast, for example, work by others has shown that both chlorine and phenyl fragments can be imaged in near proximity on a Si(111) surface after tip-induced dissociation of adsorbed chlorobenzene molecules.9,14 However, in this case, the molecule is initially weakly chemisorbed on the surface through a di-σ bond from the benzene ring to a silicon adatom. Such a preexisting bond may redirect energy of the dissociation into the surface, thus allowing both benzyl radical and chlorine atom stay on the surface after the reaction. Regarding the apparent absence of anthracene in our experiments, consider first the expected molecular dynamics of the anthracene radical and its kinetic energy. Assuming the C−Cl dissociation energy of chlorobenzene to be 4.07 eV, based on the known bond strength, and the electron affinity of chlorine to be 3.62 eV,38,43 the total kinetic energy of the fragments can be estimated. For example, a 3.2 eV tunneling electron would yield fragments having a total energy of 2.8 eV after electron transfer. We can then estimate the kinetic energy of the anthracene radical on the basis of simple conservation of energy and momentum, ignoring the interaction of the molecule with the surface and also ignoring excitation of internal degrees of freedom. Under these conditions the anthracene radical would have a kinetic energy of ∼0.5 eV. It is of interest to compare the kinetic energy available to the binding energy of the anthracenyl to the surface. First we assume that the binding energy of an anthracenyl is comparable to that of anthracene. Then, using our ∼0.5 eV estimate of the fragment kinetic energy given above, an anthracene radical fragment most likely would not have enough kinetic energy to overcome the surface potential of about 0.9 eV, an estimate obtained from our earlier anthracene TPD experiments.30 Hence using this reasoning, we should expect to detect, via imaging, an anthracenyl on the surface in the vicinity of the reaction site. The fact that we do not observe an anthracenyl suggests that different dynamics considerations apply. One possible scenario is that in many cases the anthracenyl is transferred to the STM tip instead of being ejected into the vacuum. A similar behavior has been reported recently in STM tip manipulation studies of H atoms on rutile(110).29 In this case, no anthracenyl would be seen on the surface. Note, however, that in one of our images, i.e., Figure 2f there is a feature (marked with a dashed arrow) that appeared on the surface after the voltage pulse and that is located adjacent to the

5. CONCLUSIONS We have shown that it is possible to reproducibly induce a dissociative electron attachment reaction on single physisorbed molecules using electrons that have been injected onto the surface from an STM tip. We have identified both the initial adsorption geometry of a 2-chloroanthracene molecule and the location and chemical nature of reaction product, i.e., a chlorine atom. The other reaction product, an anthracene radical, apparently desorbs into the vacuum or is captured by the STM tip. The results of our experiments suggest the possibility of using chloroanthracene molecules as probes for the presence of excited electrons (E ≥ 2.7 eV) that are injected from the bulk of the TiO2 semiconductor.

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ASSOCIATED CONTENT

S Supporting Information *

TPD spectra of 2-chloroanthracene-exposed TiO2(110) surfaces and STM images of the same surface with 0.3 ML of chloroanthracene at different temperatures in the 210−260 K range. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS R.O., D.P, and Z.L. gratefully acknowledge support of this work by the U.S. Department of Energy, Contract No. DE-FG0290ER14104. We thank Nader Zaki for help with the experimental setup, Nicholas Choi for help with graphics, and George Flynn and Kwang Rim for helpful discussions.

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