Many-body transitions in a single molecule visualized by scanning tunnelling microscopy

Abstract

Many-body e ects arise from the collective behaviour of large numbers of interacting particles, for example, electrons, and the properties of such a system cannot be understood considering only single or non-interacting particles 1-5. Despite the generality of the many-body picture, there are only a few examples of experimentally observing such e ects in molecular systems 6-8. Measurements of the local density of states of single molecules by scanning tunnelling spectroscopy is usually interpreted in terms of single-particle molecular orbitals 9-11. Here, we show that the simple single-particle picture fails qualitatively to account for the resonances in the tunnelling spectra of di erent charge states of cobalt phthalocyanine molecules. Instead, these resonances can be understood as a series of many-body excitations of the di erent ground states of the molecule. Our theoretical approach opens an accessible route beyond the single-particle picture in quantifying many-body states in molecules. Scanning tunnelling microscopy (STM) allows one to investigate the atomic-scale structure and electronic properties of surfaces and adsorbates. It probes the local density of states (LDOS), enabling direct visualization of the energy and symmetry of molecular electronic orbitals. If an ultrathin insulating layer electronically decouples the molecules from the substrate 9,12 , their resonances can be measured with great detail by scanning tunnelling spectroscopy (STS), allowing the identification of, for example, vibronic satellites 8,12-14. Although there are several examples of many-body (MB) phenomena investigated by STM in solid-state systems 15-20 , such effects are usually neglected when interpreting images and tunnelling spectra of single molecules 9-11. A prominent exception is the Kondo effect, a MB singlet ground state formed between an unpaired spin in a molecule and a screening cloud of quasi-free electrons 6,7. However, the Kondo effect arises owing to the interaction with the electron Fermi sea of the substrate and, thus, is not an intrinsic MB state of the molecule. Quantum chemistry calculations predict the mixing and distortion of molecular single-particle (SP) wavefunctions due to electron correlation 21,22 , which would be an intrinsic MB effect. Nevertheless, most of the predicted effects are subtle and, so far, there is no clear experimental verification of MB transitions in single molecules. Probing excited states in single molecules by tunnelling spectroscopy, we demonstrate the existence of MB effects arising purely from electronic states confined in the molecule. We have carried out low-temperature STM experiments on cobalt phthalocyanine molecules (CoPC) adsorbed on monolayer hexagonal boron nitride (h-BN) grown on an Ir(111) substrate.

Unraveling the Molecular Structures of Asphaltenes by Atomic Force Microscopy

“…28,29 By using STM and ultrathin insulating films as a substrate, one can also map molecular orbitals. [30][31][32] Here, we present atomic-resolution low-temperature AFM data of individual molecules of asphaltene, one of the most complex and intriguing natural mixtures existing. Additionally, orbital imaging with the STM is used to access the PAH moieties of asphaltene, their primary site of inter-molecular interaction.…”

Energy Ordering of Molecular Orbitals

“…3 On the other hand, scanning probe techniques applied to larger molecules adsorbed on surfaces have led to fascinating real space images of more complicated orbital structures with submolecular resolution. 4,8−1200 However, only frontier orbitals are accessible, and experiments require inserting an insulating decoupling layer between the metal substrate and the molecule and/or the use of functionalized tips.…”

Apparent Reversal of Molecular Orbitals Reveals Entanglement

“…In this context, quantum effects such as electronic interference have recently shifted into the focus [1][2][3][4][5][6][7]. Most intriguing in this respect are electron correlation effects [8][9][10][11][12][13][14], which are intrinsically strong in molecules due to their small size [15][16][17][18][19].…”

“…In this context, quantum effects such as electronic interference have recently shifted into the focus [1][2][3][4][5][6][7]. Most intriguing in this respect are electron correlation effects [8][9][10][11][12][13][14], which are intrinsically strong in molecules due to their small size [15][16][17][18][19].In general, Coulomb charging energies strongly depend on the localization of electrons and hence, on the spatial extent of the orbitals they occupy. Therefore, the orbital sequence of a given molecule can reverse upon electron attachment or removal if some of the frontier orbitals are strongly localized while others are not, like in, e.g., phthalocyanines [20][21][22][23][24].…”