Disentangling the electronic structure of an adsorbed graphene nanoring by scanning tunneling microscopy

Abstract

Graphene nanorings are promising model structures to realize persistent ring currents and Aharonov–Bohm effect at the single molecular level. To investigate such intriguing effects, precise molecular characterization is crucial. Here, we combine low-temperature scanning tunneling imaging and spectroscopy with CO functionalized tips and algorithmic data analysis to investigate the electronic structure of the molecular cycloarene C108 (graphene nanoring) adsorbed on a Au(111) surface. We demonstrate that CO functionalized tips enhance the visibility of molecular resonances, both in differential conductance spectra and in real-space topographic images. Comparing our experimental data with ab-initio density functional theory reveals a remarkably precise agreement of the molecular orbitals and enables us to disentangle close-lying molecular states only separated by 50 meV at an energy of 2 eV below the Fermi level. We propose this combination of techniques as a promising new route for a precise electronic characterization of complex molecules and other physical properties which have electronic resonances in the tip-sample junction.

Generalized kekulenes and clarenes as novel families of cycloarenes: structures, stability, and spectroscopic properties

“…As we can see, the data points show a linear correlation with a high squared correlation coefficient (R 2 = 0.984) between the relative isomer energy and the HMO energy, confirming that p stabilization is indeed the decisive factor in isomer stabilities. Note that two isomers of [18]kekulenes have been successfully synthesized experimentally, namely, [3,3,3,3,3,3] (see Fig. 2(b)) and [2,4,2,4,2,4] (see Fig.…”

“…Cycloarenes are a class of fully fused macrocyclic aromatic hydrocarbons comprising angular and linear annellations of benzene units enclosing a cavity into which C-H bonds point. 1 Due to their remarkable properties and potential applications in optoelectronics, 2,3 synthetic chemistry, [4][5][6] supramolecular chemistry, 7,8 etc., cycloarenes have attracted the scientific community for decades. 5 In addition, the large cavity structure offers cycloarenes some promising applications in host-guest chemistry.…”

Automated Structure Discovery for Scanning Tunneling Microscopy

“…Scanning probe microscopy (SPM) methods are powerful tools for studying nanoscale systems with atomic resolution. As the basis of SPM methods, scanning tunneling microscopy (STM) and atomic force microscopy (AFM) have been widely utilized in the characterization of various systems, such as biological samples, hybrid inorganic–organic interfaces, and individual steps of on-surface reactions. − To enhance the spatial accuracy in the characterization, the probe tip can be functionalized using a chemically inert, flexible apex (often a CO molecule) to allow scanning at a very close tip-sample distance where Pauli repulsion is the dominating interaction. , With respect to the detailed characterization of atomic structures, there exist many demonstrations of the improved spatial resolution of AFM scanning with functionalized tips, ,, but STM is now also being increasingly used in this bond-resolved mode. ,− STM in particular benefits significantly from tip functionalization as sharp submolecular features appear in the image, revealing both the molecular skeleton and the electronic structure in high detail within the same experimentthis is impossible using a bare metal tip or by high-resolution AFM. In addition to atomic structural characterization, STM with a functionalized tip offers interesting approaches for electronic structure characterization via Frontier orbitals of the tip apex by improving the visibility of molecular orbitals and by distinguishing nearby molecular states …”

“…In addition to atomic structural characterization, STM with a functionalized tip offers interesting approaches for electronic structure characterization via Frontier orbitals of the tip apex 20 by improving the visibility of molecular orbitals 21 and by distinguishing nearby molecular states. 17 In terms of structure characterization, the main limitations of STM are its inability to see beyond the closest atom to the tip and the insurmountable problem of chemical identification of the atoms. In fact, research using bond-resolved STM has so far been mostly limited to planar molecules consisting of only a priori known atomic species, and the recognition of an unknown sample can require an extensive search through all possible molecules and configurations.…”

“…In addition to atomic structural characterization, STM with a functionalized tip offers interesting approaches for electronic structure characterization via Frontier orbitals of the tip apex 20 by improving the visibility of molecular orbitals 21 and by distinguishing nearby molecular states. 17 …”