Surface Science

Breaking individual chemical bonds via STM-induced excitations

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We present experimental and theoretical results on the STM-induced Si-H bond-breaking on the Si(100)-(2 × 1):H surface. First, we examine the character of the STM-induced excitations. Using density functional theory we show that the strength of chemical bonds and their excitation energies can be decreased or increased depending on the strength and direction of the field. By shifting the excitation energy of an adsorbate below the tip, energy transfer away from this excited site can be suppressed, and localized excited state chemistry can take place. Our experiments show that Si-H bonds can be broken when the STM electrons have an energy >6 eV, i.e. above the onset of the σ→σ* transition of Si-H. The desorption yield is ∼2.4 × 10-6 H-atoms/electron and is independent of the current. We also find that D-atom desorption is much less efficient than H-atom desorption. Using the isotope effect and wavepacket dynamics simulations we deduce that a very fast quenching process, ∼1015 s-1, competes with desorption. Most of the desorbing atoms originate from the "hot" ground state produced by the quenching process. Most interestingly, excitation at energies below the electronic excitation threshold can still lead to H atom desorption, albeit with a much lower yield. The yield in this energy range is a strong function of the tunneling current. We propose that desorption is now the result of the multiple-vibration excitation of the Si-H bond. Such excitation becomes possible because of the very high current densities in the STM, and the long Si-H stretch vibrational lifetime. The most important aspect of this mechanism is that it allows single atom resolution in the bond-breaking process - the ultimate lithographic resolution.