STM contrast of a CO dimer on a Cu(1 1 1) surface: a wave-function analysis

Abstract

We present a method used to intuitively interpret the STM contrast by investigating individual wave functions originating from the substrate and tip side. We use localized basis orbital density functional theory, and propagate the wave functions into the vacuum region at a real-space grid, including averaging over the lateral reciprocal space. Optimization by means of the method of Lagrange multipliers is implemented to perform a unitary transformation of the wave functions in the middle of the vacuum region. The method enables (i) reduction of the number of contributing tip-substrate wave function combinations used in the corresponding transmission matrix, and (ii) to bundle up wave functions with similar symmetry in the lateral plane, so that (iii) an intuitive understanding of the STM contrast can be achieved. The theory is applied to a CO dimer adsorbed on a Cu(111) surface scanned by a single-atom Cu tip, whose STM image is discussed in detail by the outlined method.

Scanning Tunneling Microscopy for Molecules: Effects of Electron Propagation into Vacuum

“…They observed that STM shows spatially expanded images of molecular orbitals (MOs) of a pentacene molecule . Ab initio calculations of STM images have been performed for clean surfaces and for very small molecules adsorbed directly on metal surfaces. ,,− More closely related to the present work are studies of single copper phthalocyanine (CuPc) molecules using a model that includes the highest occupied MO (HOMO), the lowest unoccupied MO (LUMO), and one σ-orbital, located just below the energy of the CuPc HOMO. − CuPc has been widely studied due to its demonstration of negative differential resistance, while the related compound H 2 Pc has garnered significant interest due to observations of up-conversion electroluminescence at tunneling energies in the H 2 Pc transport gap. − These experiments raise important and fundamental questions about electrons tunneling through the transport gap.…”