Negative Differential Resistance in a Hybrid Silicon-Molecular System: Resonance between the Intrinsic Surface-States and the Molecular Orbital

It has been a long-term desire to fabricate hybrid silicon-molecular devices by taking advantages of organic molecules and the existing silicon-based technology. However, one of the challenging tasks is to design applicable functions on the basis of the intrinsic properties of the molecules, as well as the silicon substrates. Here we demonstrate a silicon-molecular system that produces negative differential resistance (NDR) by making use of the well-defined intrinsic surface-states of the Si (111)-√3 × √3-Ag (R3-Ag/Si) surface and the molecular orbital of cobalt(II)–phthalocyanine (CoPc) molecules. From our experimental results obtained using scanning tunneling microscopy/spectroscopy, we find that NDR robustly appears at the Co²⁺ ion centers of the CoPc molecules, independent of the adsorption configuration of the CoPc molecules and irrespective of doping type and doping concentration of the silicon substrates. Joint with first principle calculations, we conclude that NDR is originated from the resonance between the intrinsic surface-state band S₁ of the R3-Ag/Si surface and the localized unoccupied Co²⁺ <i>d</i>_<i>z</i>² orbital of the adsorbed CoPc molecules. We expect that such a mechanism can be generally used in other silicon-molecular systems