Periodic Reporting for period 1 - BioGraphING (Biomolecule Sensing with Graphene-Integrated Nanogaps)
Reporting period: 2018-04-01 to 2020-03-31
Graphene combines many of the requisites for an electrical sensor material: high conductivity, atomic thinness, flexibility, chemical inertness in air and liquid and mechanical strength, as well as compatibility with standard lithographic patterning techniques. In BioGraphING, the researcher developed a novel graphene-based technology that utilises the mechanically controlled break junction (MCBJs) technique. The MCBJ is conceptually simple: a suspended graphene bridge is positioned on a flexible metal substrate. Bending the substrate in a 3-point geometry causes the graphene to stretch and eventually break. The conductance at a fixed bias voltage is measured throughout the bending/breaking/remaking processes, giving detailed information of the structure-electronic property relationship. One- and two-dimensional histograms of conductance versus electrode displacement crucially allow mapping of the breaking dynamics in real-time and with statistical significance.
The project was structured to achieve three objectives:
O1. Demonstrating controlled nanogap formation in graphene MCBJs in air, vacuum and liquid.
O2. Identifying molecular signatures from ‘reference’ molecules e.g. anthracene- or pyrene-funtionalized curcuminoids or oligo-phenylene-ethynylene (OPE3) molecules (i.e. π-π interactions with graphene), porphyrins w/o amine anchors (i.e. C-C and amine bonding with graphene respectively).
O3. Demonstrating proof-of-concept peptide and protein fingerprinting using tunnelling currents.
The major advances that were made throughout the MC project are:
• Mechanical and electrical robustness and stability of the device compared to its bulk metal counterpart (i.e. Au break junctions used in the host group).
• Reliability and reproducibility of the fabrication using standard cleanroom processing.
• Statistical significance of the results (> 10,000 cycles at room temperature in air).
• Sub-nanometer tuning of the electrode spacing using a simple mechanical tuning knob to control the inter-electrode distance.
Furthermore, the new platform allowed the discovery of new fundamental quantum physical process, including quantum interference phenomena, that had only been reported from a theoretical standpoint but not achieved in practice:
• Observation of rare room temperature quantum interference effects during sliding of two graphene sheets across each other
• Low temperature (4 K) measurements of mechanically-tunable Coulomb blockade in a single graphene quantum dot inside the junction
The first paper published during the project (Nature Nanotechnology, 13, 1126–1131(2018)) demonstrates the first room-temperature, periodic conductance oscillations as a function of atomic-scale displacements in quantum coherent graphene nanoconstrictions. Our approach, based on monitoring the electrical conductance of graphene during uniaxial deformation, is entirely novel and overcomes a longstanding challenge for controllably tuning quantum interference effects in graphene by using a mechanical tuning knob with subnanometer resolution.
Given that the graphene MCBJ device demonstrates excellent electronic and mechanical properties, as well as a host of intriguing fundamental physical effects, the focus of the project shifted towards understanding the origin of the latter phenomena.
During the MC fellowship, massively parallel fabrication of crack-defined gold break junctions featuring sub-3 nm gaps was demonstrated, in a collaborative project with KTH University. The current-voltage characteristics were studied in detail and proved that single molecules can be individually trapped and measured at room temperature and at liquid nitrogen temperature. The results were published in Nature Communications (2018).
During the two-year period, the researcher disseminated the results of the project at the following international conferences as contributed speaker:
- From Solid state to Biophysics- From Basic to Life Sciences, Cavtat, Croatia, 2018
- Graphene Week 2018 - San Sebastian, Spain, 2018
- International Conference on Materials for Advanced Technologies - Singapore, 2019
- NT19: International Conference on the Science and Application of Nanotubes and Low-Dimensional Materials - Würzburg, Germany, 2019
The researcher was also an invited speaker at the 1st Kavli NanoLab Cleanroom User Meeting 2018.
The results produced during the fellowship opened up the route to several collaborations, including with the Norte lab and the Steeneken lab at TU Delft, as well as with Dr Pascal Gehring at IMEC (Leuven).