Enhanced conductance of molecular states at interstitial sites

Abstract

Arrays of phthalocyanine molecules on Pb(100) are investigated with scanning tunneling microsopy. Maps of the differential conductance exhibit drastic changes as the sample voltage is being varied. Maximal conductances are observed at positions between the molecules mimicking bonding states. However, the maxima are shown to result from a superposition of non-interacting states. We expect that this effect may be observed from many other molecules.

Deceptive orbital confinement at edges and pores of carbon-based 1D and 2D nanoarchitectures

“…For the occupied region, the intensity is mainly localized at the lateral phenyl rings, appearing as two bright dots at −1.1 V and winding along the edges at −1.3 V. In the unoccupied region (at 1.9 V and 2.1 V), a highly localized conductance feature is present at the gulfs, notably protruding laterally outside the ribbon. Recently, similar conductance features have been observed in chevron GNRs 52 , holey nanographenes 3 , 32 , 33 , molecular nanoporous networks 53 , molecular arrays 54 , and NPGs 35 . However, as mentioned above, the origin attributed for such unoccupied states is very diverse, ranging from surface state confinement 52 , IPS 35 , LUMO states 3 , 5 , 32 , NIR 31 , SAMO states 33 or a superposition of non-interacting states 54 .…”

“…Recently, similar conductance features have been observed in chevron GNRs 52 , holey nanographenes 3 , 32 , 33 , molecular nanoporous networks 53 , molecular arrays 54 , and NPGs 35 . However, as mentioned above, the origin attributed for such unoccupied states is very diverse, ranging from surface state confinement 52 , IPS 35 , LUMO states 3 , 5 , 32 , NIR 31 , SAMO states 33 or a superposition of non-interacting states 54 .…”

“…Assuming that sample states are typically modeled using two-dimensional Bloch waves, their decay into the vacuum ( z -direction) can be interpreted as a filtered Fourier expansion that, for increasing tip-sample distances, keeps only the components with vanishing lateral wave vector. This well-known filtering effect 54 , 62 – 65 can have a significant influence on the LDOS maps obtained at realistic distances from the sample, as previously demonstrated for 7-AGNRs 11 .…”

“…We can conclude that the strongly localized LDOS and conductance features revealed by DFT simulations and STM experiments, respectively, arise from the combination of two factors: (1) the decay of the sample’s orbitals in the vertical direction, and (2) the confinement of the orbitals into 1D and 0D regions (e.g., edges, gulfs and nanopores), as determined by the exact edge structure of the system under consideration. While the first is a general and well-known effect reported in many surfaces and molecular assemblies 54 , 62 – 65 , the second is expected to be particularly relevant in graphene-based 1D and 2D nanostructures, for many of their edge and nanopore morphologies. Note that the deceptive orbital confinement at the edges and pores explored here focuses mainly on delocalized frontier orbitals (with their wave functions stemming from the carbon backbone) and does not apply to electronic states inherently localized at the edges of graphene-based nanostructures 18 , 19 , although the latter are also subject to a similar decay effect.…”