Skip to main content
European Commission logo print header

Nanoscale-enhanced Spectroscopies in Electrochemically-Gated Single-Molecule Devices

Periodic Reporting for period 1 - TECh-MoDE (Nanoscale-enhanced Spectroscopies in Electrochemically-Gated Single-Molecule Devices)

Reporting period: 2019-08-01 to 2021-07-31

The molecular electronics field aims at miniaturising electronic devices employing molecules to surpass the space limitation of conventional silicon circuitry integration. Thus, a complete understanding of the physicochemical properties of any molecule targeted as an electronic component is a must since these properties define its functionalities. Gaining access to the characteristic length scale of a target molecule is crucial since it permits us to explore such properties that are inaccessible in ensemble experiments. Today, this is possible thanks to multiple platforms that allow the detection of individual molecules wired between two electrodes in fixed nanogaps. These platforms rely on the electrical detection of molecular junctions.

Unfortunately, electrical current based detection methods do not carry any chemical information of the target molecule’s structural properties and for the interfacial interplay between the wired molecule and the way it binds to the electrodes. Molecule-electrode contact structural and chemical information is thus critical to tailor future molecular-based devices electrical properties. It is then essential to develop complementary and non-destructive single-molecule spectroscopic detection techniques to fill such a crucial gap in the molecular electronics field. Tip-Enhanced Raman Spectroscopy(TERS), have been developed to access the molecular fingerprint on a few scatterers in nanoconfined spaces and adsorbate (sub)monolayers. To overcome the described issues, the ground-breaking nature of this proposal was the development of a beyond the state-of-the-art platform working under ambient conditions, which allows the recording of electrical current and TER spectra under EC-control.
At the MPIP, I have successfully carried a very demanding project combining two extremely challenging nanoscience fields, like single-molecule current detection and molecular spectroscopy. I have developed a unique molecular platform to gather unprecedented single-molecule information about conductive, vibrational, structural, and electrochemical properties under realistic environments, such as physiological ones. The platform relied on two capabilities. First, the STM (Scanning Tunnelling Microscopy) microscope in its Break-Junction approach that allows capturing and electrically characterising individual molecules. The second capability is the TERS (Tip Enhanced Raman Spectroscopy), an ultra-sensitive and non-destructive spectroscopic method able of providing local vibrational fingerprints of the target molecule. The two following papers attest (see links below) that with such platform I have been able to access, in a combined way to (i) the conductance of molecular wires based on organic and biomolecular conductors, along with (ii) the structural changes in the (bio)molecule and/or the molecule|electrode interface. The spectroscopical (TERS) molecular spectroscopy sections presented in both papers are good examples of the results of the developed spectroscopic new platform, and thus of my MSCA-IF.

In addition to my ambitious hybrid molecular platform, I have developed my own optical trapping electrical molecular platform called plasmon-supported break-junction platform (PBJ). It is a novel approach based on my knowledge of electrical molecular detection and optics, acquired at host institution (MPIP) via lab-work (training-through-research), In this new platform, the electromagnetic field enhancement (nearfield) in a plasmonic cavity defined by STM’s interelectrode gap enlarges the molecular trapping timescales for its current detection. The exerted (stabilising) force of the nearfield gradient is exploited to provide additional endurance to junctions, therefore increasing junctions’ lifetime from hundreds of milliseconds to the order of seconds. The observed effect was impressive, a junction lifetime’s increase of one order of magnitude compared with laser-OFF conditions, even employing moderate optical powers. I have published the first PBJ’s results employing a small aromatic molecule (benzenedithiol) as open access (link#1). Like the followed strategy for benzenedithiol experiments, I have also made use of the possibilities to tune to the fermi level of the molecule|electrode contact via the electrochemical capabilities of our PBJ platform. Doing that, we reported a double phenomenology. On one hand, we promoted the resonant excitation conditions of a specific redox state of the target metalloprotein Azurin molecule (Azu) via keeping oxidised its coper centre. As such, when Azu is oxidised presents resonant conditions, and therefore its electric polarizability, and thus, the optical trapping are enhanced. On the other hand, we steered the nanogap’s localised surface plasmon resonance correlated with the electric field enhancement which also enhances the molecular plasmonic trapping. The combined effects, even under optical moderate powers, resulted in a junction’s lifetime increment of factor 40 with respect to the off-conditions. This project has been selected for the 2021 RCS’s Emerging Investigators issue of the Journal of Materials Chemistry – C, and it was reported as open access (see link #2) too. Moreover, the last-mentioned paper was chosen as "back cover" for its publication issue. Additionally, all the above-mentioned results from my MSCA-IF have been the main discussion topic for my two "invited oral contributions" during 2021.

Paper #1:
Paper #2:
The spectroscopical work derived from my MCSA-IF provides fundamental insights of utmost importance for nanoelectronics, molecular spectroscopy and bioelectrochemistry. Vibrational fingerprints provide entirely new insights into the interplay between chemical bonding, molecular geometry, and charge transfer of molecules at electrified interfaces. All the new structural information that the designed platform provides such as conformational changes in the junctions and/or electrode|molecule contact geometries, are well-known to largely tune the conductance of the molecular wires. Likewise, the extension of the electrode-molecule interactions not only defines the electrical response, electron transport mechanism or photonic features, but also the stability of the junctions, critical for the future applicability of single-molecule in electronics, sensors, energy storage and harvesting (needs encompassed in the Horizon Europe 2021-2027 agenda’s cluster #5: Climate, Energy & Mobility). On the other hand, the plasmonic trapping effects are extremely relevant for the molecular electronics field because achieving longer junction timescales implies the improvement of the molecular characterisation and acquired data. An optically promoted junction stabilization yields wider applicability since the increased capturing timescales are effective without the need for chemical modification of the target molecule and/or electrode. It clearly expands the horizons of molecular electronics’ fundamental research.

Both of my newly developed techniques are novel analytical instruments, but it is worth mentioning that beyond the fundamental research potential, the platform also holds immense potential in an economic framework. A spin-off company selling the tool, instrumentation service or derived technology (hardware and software) could be easily envisioned.
Single-molecule trapping and current detection under laser illumination