Periodic Reporting for period 1 - GNR CONDUCTANCE (Two-tip STM conductance characteristics of individual planar graphene nanoribbon)
Okres sprawozdawczy: 2021-01-01 do 2022-12-31
Electronic conductance measurements through a single one-atom thick, few-atom wide planar graphene nanoribbon (GNR) have a lasting interest as GNR hold inherent tunable band gaps relevant to numerous technological applications. For future single-molecule based electronic circuits, the complete electronic components are supposed to be embedded inside one single molecule. Here, atomically precise metallic interconnects connected at both ends of an unperturbed GNR/molecular wire structure with a picometer precision is a difficult experimental and technical problem. To address this issue, our Marie Curie action has explored the low-temperature ultrahigh vacuum (LT-UHV) two-tip scanning tunneling microscopy and spectroscopy (STM and STS) conductance characterization approach. Within the scope of this action, atomically clean conductance characterization of a single GNR in its strictly planar configuration was proposed in a way to override (1) the unclean standard nanolithography-based device fabrication and (2) the actual curved conformation conductance measurement of a single GNR using the STM tip lifting approach. Indeed, a clear understanding of the on-surface GNR synthesis protocol, a detail characterization of the GNR electronic structure stabilization and the characterization of metallic back surface support induced electronic effects have also been achieved through this action. Along the path of this action, we have invented an original and totally UHV compatible way for the measurement of the conductance of a single GNR molecular wire in an exact planar and atomically precise surface configuration, the results of which will be published soon.
The project proposal consisted of three scientific work packages: (1) Atomic-scale probing of electronic properties stabilization of on-surface synthesis optimized GNRs using low-temperature one STM tip characterization. (2) Focus Ion Beam (FIB) milling at the apex of electrochemically etched ‘W’ or ‘Pt/Ir’ STM tips to be suitable for two-STM tip conductance characterization of a single individual GNR. (3) Investigation of two-tip STM conductance characteristics of a GNR in its planar non-curved conformation. In this action, our new on-surface synthesis protocol for producing long GNRs (>50 nm) and the probing their length-dependent electronic stabilization behavior were successful. The next step was the contact of two sharp STM tip apices at both ends of the GNR, now identified electronic channels energetic position for conductance. For this purpose, precursor monomer was varied to elongate the GNR length to a desired value (>70 nm) in a way to match up our minimum STM tip-to-tip distance on the LT-UHV 4-STM. FIB milling and reshaping of STM tip apes to narrow down its effective diameter was also successful and a systematic protocol was designed and implemented to ultimately produce the STM tip apex bulk diameter less than 60 nm reproducibly measured. For the in-plane GNR conductance measurement, an electronically decoupling of the GNR from the surface was originally proposed by manipulating the GNR from its initial metallic surface onto an insulating NaCl spacer layer. However, this approach was later modified for a new surface configuration by customizing the back-surface support to be able to maintain an exact atomic scale planar configuration of the GNR in a two-tip STM configuration. The attached image (GNR CONDUCTANCE_1.jpg) shows the (a) STM image revealing the on-surface GNR synthesis optimized for longer GNRs, (b) STM images showing the GNR adopting the curved conformation on a very basic one atom high Au(111) surface trench, which crucially modified the intrinsic electronic delocalization along the complete ribbon and (c) dI/dV mapping of a well characterized first electronic channels of a GNR located close to the Au(111) surface Fermi level.
Beyond the state of the art, we have invented a new planar configuration process which can lead to a truly atomic scale configuration for a UHV clean environment interconnection technology awaited for mono-molecular electronics to become valuable.