Article pubs.acs.org/JPCC
Molecular Motion Induced by Multivibronic Excitation on Semiconductor Surface Tatsuya Momose,† Ken-ichi Shudo,*,‡,§ Hannes Raebiger,*,‡ Shin-ya Ohno,‡ Takeshi Kitajima,∥ Masanobu Uchiyama,§,⊥ Takanori Suzuki,∥ and Masatoshi Tanaka‡ †
Graduate School of Engineering and ‡Faculty of Engineering/Science, Yokohama National University, Tokiwadai 79-5, Hodogaya-ku Yokohama 240-8501, Japan § Elements Chemistry Laboratory, RIKEN, Hirosawa 2-1, Wako-shi Saitama 351-0198, Japan ∥ School of Applied Sciences, National Defense Academy, Hashirimizu 1-10-20, Yokosuka 239-8686, Japan ⊥ Graduate School of Pharmaceutical Sciences, University of Tokyo, Hongo 7-3-1, Bunkyo-ku Tokyo 113-0033, Japan ABSTRACT: A low-temperature observation with a scanning tunneling microscope (STM) of CO adsorbed on the Si(001) surface shows a peculiar change in adsorbate structure. Only after repeated scanning, approximately half of the adsorbed CO become visible as bright spots due to an irreversible lateral motion of the CO molecules, while at first the adsorbed CO is invisible to STM. The reaction rate of this motion shows hyperlinear dependence on the tunneling current. This implies that the adsorbate displacement is caused by multiple excitations of adsorbate vibronic modes, which is a mechanism thus far observed only at metal surfaces. We observe an activation barrier of 0.11 eV for the irreversible motion of CO, in agreement with the adiabatic potential obtained from first-principles calculation. The atomic-scale local heating is site-specific due to highly efficient inelastic scattering by charging of a surface-localized midgap state induced by the STM tip−sample bias voltage.
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INTRODUCTION Techniques of atom-by-atom manipulation, including desorption, hopping, and chemical reaction, to modify surfaces at nanometer scale with scanning tunneling microscopes (STM) are well-established.1−3 High electron density of the tunneling current plays an important role in these surface reactions, which are typically interpreted in terms of one of the following two mechanisms. (i) In the Menzel-Gomer-Redhead (MGR) model, an electronic excitation to a stable antibonding state leads to nuclear motion along an adiabatic potential.4,5 This usually occurs when the applied bias voltage is relatively high,6 and the reaction rate R shows a linear dependence on the tunneling current I.7−9 (ii) In the ladder-climbing model,10 energy quanta of a local vibration are multiply excited by the inelastic scattering of tunneling electrons applied at a relatively low bias voltage, until the accumulated energy exceeds the activation energy. In this case, the reaction is governed by the power-law, R ∼ In, where n is the reaction order. Adsorbate motion induced by multivibronic excitation has only been observed on metal surfaces;11,12 for semiconductor surfaces, only few adsorbate desorptions have been observed.3,13 In fact, this kind of power-law behavior is not necessarily applicable to molecular-adsorbed systems, because accumulation of vibrational energy at molecules on surfaces is competitive against dissipation into equilibrium. For instance, © 2013 American Chemical Society
the lifetime of the C−O stretching vibration on Pt(111) is 2.2 ps,14 whereas that of CO on Si(001) is 1.87 ns.15,16 The latter longer lifetime suggests that the accumulated energy would overcome the reaction barrier, but the observed rate of a vibration-induced reaction is linear on a semiconductor.17 In this work, we observe the motion of an adsorbate with a reaction rate that exhibits neither linear nor power-law behavior. Our detailed STM experiments combined with firstprinciples calculations show that the accumulated energy of CO vibrations on Si(001) during an STM observation is sufficient to induce a change of the adsorption structure of the CO molecules. The power-law, however, breaks down for this system, and we show instead that the vibrational temperature is a good indicator of atomic-scale reactions taking place on the semiconductor surface. The Si(001)-c(4 × 2) surface adsorbs CO molecules below ∼100 K with an adsorption probability close to unity without decomposition.18,19 There are two stable adsorption sites,20−22 where CO either (i) terminates a dangling bond at the down-Si of a dimer (T-CO), or (ii) sits on the metastable bridge site of two Si atoms of the dimer (B-CO). On a clean surface, T-CO is Received: September 2, 2013 Revised: December 19, 2013 Published: December 20, 2013 1554
dx.doi.org/10.1021/jp408775s | J. Phys. Chem. C 2014, 118, 1554−1559
The Journal of Physical Chemistry C
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the ground-state configuration, and the formation of B-CO requires an activation energy of 1.1 eV, which can be achieved, for example, by an accelerated CO molecular beam.23 Regardless of the adsorption probability close to unity, our STM images of the CO-adsorbed surface look like pristine Si(001) surfaces, suggesting that the 0.5 ML saturation coverage (one CO per Si dimer) by T-CO is invisible to STM. However, we show that after multiple scannings bright spots emerge due to the formation of B-CO during the STM experiment. Our density-functional calculations show that TCO is difficult to be discriminated in STM images but that the bright spots due to B-CO are visible. We further show that BCO is stable only when sandwiched between two T-COs and thus at most half of the adsorbed CO is visible to STM. The transformation of the adsorbate structure is then described in terms of an inelastic tunneling mechanism.
Article
RESULTS
Figure 2 shows STM images of successive scans of the same 50 × 50 nm2 area after the CO exposure was stopped. In the first scan, Figure 2a, despite the 0.5 ML coverage of CO only a few bright spots, corresponding to
