Nanojunctions from molecules to graphene nanoribbons : optical characterization and transport properties

Abstract

In this thesis, the properties of nanojunctions formed by low-dimensional, bottom-up synthesized materials were investigated. Specifically, we investigated molecules and graphene nanoribbons as representatives of future bottom-up synthesized functional circuit elements, as well as graphene synthesized by chemical vapor deposition for use as an electrode material. We demonstrate the controlled formation of organometallic oligomer chains in mechanically controlled break junction experiments. Additionally, we discuss a new fabrication approach to extend the break junction method with lithographically fabricated samples to oxidizing materials, and we report on the investigation of an electrical pump-probe measurement scheme. The setup for mechanical control over substrate bending was used to introduce strain in lithographically patterned stripes of graphene, which was monitored via the changed vibrational properties with polarized Raman spectroscopy. This technique was then used to investigate a new class of $\text{sp}_2$-carbon materials, atomically precise graphene nanoribbons (GNRs), which have several attractive properties that can be tuned by designing their shape. We describe a substrate for GNR-devices optimized for Raman spectroscopy and discuss considerations for measurement parameters and measurement strategies. We then report on a low-energy Raman mode in armchair GNRs and describe its length-dependence, sensitivity to the underlying substrate, and its behavior as a function of temperature. Finally, we discuss the current state of electrical transport measurements on GNRs with graphene electrodes. In particular, we discuss challenges for device fabrication and reliability and present strategies for optimized design and fabrication of future device generations incorporating GNRs.