"Ph.D. Thesis Defense: Electronic Transport through Suspended Graphene Nanoribbons Using a Scanning Tunneling Microscope"

Who: Niklas Friedrich, Pre-doctoral Researcher, Nanoimaging Group

Place: "Sala de Actos", Faculty of Chemistry UPV/EHU

Date: Friday, 14 October 2022, 10:00

Electronic Transport through Suspended Graphene Nanoribbons Using a Scanning Tunneling Microscope

Niklas Friedrich

Nanoimaging Group, CIC nanoGUNE

Single molecule spintronics aims to implement information processing techniques using electronic spins with tailored single molecules produced by organic chemistry. Graphene nanostructures are promising molecular platforms for single molecule spintronics because they combine electron mobility through conjugated parts with the possibility of hosting spin states. Industry driven research is capable of the integration of graphene flakes into transport devices for information processing. Recently, collaborations between chemists and physicists produced important advances in fabricating and characterizing spin-hosting graphene nanostructures. However, the study of the electronic transport through spin-hosting graphene nanostructures remains a challenge for testing the potential of single molecule spintronics.

In this Thesis, I investigated the two-terminal electronic transport through individual spin-hosting graphene nanoribbons (GNRs) suspended between the tip and the substrate of a low-temperature scanning tunneling microscope. Three types of GNRs were investigated: a seven and a five-seven-five armchair graphene nanoribbon, both with substitutional boron doping and a hybrid structure of (3,1)-chiral graphene nanoribbons and iron porphyrin. The ribbons were fabricated in situ under ultra-high vacuum conditions using on-surface synthesis strategies and characterized by means of scanning tunneling microscopy and spectroscopy (STM and STS). Bond resolved low bias images using a CO-functionalized tip confirmed the atomic structure of the molecules. Selected ribbons were positioned in a free-standing configuration bridging STM tip and substrate by mechanical manipulation with the STM tip.

The substitutional boron doping induces localized states in the seven armchair GNRs around the dopant sites. The high spatial localization of this states enables us to create a double tunneling barrier configuration, similar to transport experiments through quantum dots, by suspending GNRs with the dopant site spaced from source and drain electrodes. Probing the transport by STS experiments in this configuration resolved resonant electron tunneling through boron state, excitations, and the ribbon?s band states for free-standing segments up to more than 8nm. In all these cases, we found that the transport is bipolar through the occupied boron state.

Density functional theory simulations show that the presence of the boron dopants interrupts the free propagation of electrons in the topologically non-trivial valence band of the seven armchair graphene nanoribbon. Consequently, a spin-polarization of the topologically protected edge state is predicted. We confirmed the magnetic character of the boron induced state experimentally through a Kondo resonance that emerged upon cleaving the boron dopants from the substrate.

Furthermore, we provide a detailed study of the topological origin of the boron induced state combining density functional theory (DFT) and mean field Hubbard (MFH) model calculations. We found both from experiment and calculations that it is possible to selectively shift the spinpolarized state to various positions inside the nanoribbon by controlling the hybridization of boron dopants with the surface.

Since the ribbons are semiconducting materials, electronic transport is essentially governed by electron tunneling through the band gap or by electron injection into frontier bands. We found a family of GNRs where the transport is ballistic while hosting localized spins inside. In agreement with this, two-terminal transport experiments showed ballistic transport with constant conductance (? 0.2G0) for retraction length up to 3nm and a simultaneous Kondo resonance at zero bias. Atomic defects in the ribbon enable inelastic electron scattering resulting in electron-hole pair excitations that are described in an underfilled Hubbard model.

I also studied hybrid nanoribbon structures formed by chiral graphene nanoribbons contacted to spin-hosting iron porphyrin molecules. In twoterminal transport experiments through suspended hybrid ribbons, we were able to excite the iron?s spin by inelastic electrons injected into the chiral GNRs. We found that on the surface, some iron porphyrin molecules undergo a renormalization of their magnetic anisotropy energy due to charge fluctuations in the spin-hosting d orbitals. Upon cleaving it from the substrate

Supervisor: Josť Ignacio Pascual

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