Extending light wave scanning tunnelling microscopy to ultrafast transport phenomena

Project Information

ELISTRA

Grant agreement ID: 101153258

DOI

10.3030/101153258

Project closed

EC signature date 8 March 2024

Start date 15 May 2024

End date 14 May 2026

Funded under

• Marie Skłodowska-Curie Actions (MSCA)

Total cost

No data

EU contribution

€ 189 687,36

Coordinated by

UNIVERSITAET REGENSBURG
Germany

Periodic Reporting for period 1 - ELISTRA (Extending light wave scanning tunnelling microscopy to ultrafast transport phenomena)

Reporting period: 2024-05-15 to 2026-05-14

Summary of the context and overall objectives of the project

The quest to develop electronic or quantum devices downscaled to atomic dimensions crucially depends on our ability to manipulate and resolve electron motion on its intrinsic scales requiring femtosecond temporal and picometre spatial precision. Additionally, the degree of control is determined by our fundamental understanding of the intricate interactions and quantum phenomena governing the motion of electrons within molecules.

In model systems, such as atoms or semiconductors, electron motion is already controllable on femtosecond scales by driving currents with ultrashort terahertz (THz) laser pulses, an approach commonly referred to as lightwave-driven electronics. Ideally, this paves the way to build electronic devices operating at THz clock rates in the future, outperforming state-of-the-art computing chips by orders of magnitude, increasing power efficiency and reducing their material footprint.

One promising material class for lightwave-driven electronics is graphene due to its high electron mobility. Intriguingly, graphene nanoribbons (GNRs) – one-dimensional “cutouts” of the two-dimensional graphene – and other graphene-based nanostructures allow a wide range tunability of decisive electronic properties like their band gap or spin texture by adjustments of their atomic structure.

Despite the huge technological potential of building metal-free computing units operating at THz clock rates, fundamental research combining atomic-scale precision with ultrafast transport experiments is still at its infancy.
ELISTRA gains unprecedented access to the ultrafast physics of ballistic electronic transport through individual free-standing graphene nanoribbons. Atomic-scale control over the GNR’s structure is reached by fabricating them in-situ by means of on-surface synthesis under ultrahigh-vacuum conditions. Low-temperature scanning tunnelling microscopy (STM)-based transport experiments through individual ribbons are performed. Currents are driven by sub-cycle THz-laser pulses funnelled into the STM junction (lightwave STM) acting as ultrafast bias modulations. These proof-of-principle experiments test the potential of atomically-precise GNRs for lightwave-driven electronics and explore the responsible fundamental processes within the GNRs at ultrafast timescales.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

Precise control of the atomic structure of the ribbons as well as of the transport junction is the key component of ELISTRA. This allows to unambiguously disentangle the fundamental physics governing the conductance of graphene nanoribbons on ultrashort timescales.

Preparing graphene nanostructures containing hundreds of atoms with structural perfection requires an interdisciplinary approach. Small precursor molecules were prepared via in-solution synthesis within a dedicated collaboration with synthetic chemists. Sub-monolayers of the precursor molecules were deposited under ultra-high vacuum conditions onto atomically clean gold substrates. This initial step of the project required the implementation of the growth process at the University Regensburg, and integrating it into the highly specialized low-temperature lightwave-driven scanning tunnelling microscope (STM).

For the subsequent interpretation of signatures from ultrafast currents, an essential prerequisite is the precise knowledge about the sub-cycle shape of the ultrashort terahertz (THz) laser pulses in the near field created by the nanocavity formed between tip and substrate. For pulse characterization, white light pulses confined to only few femtoseconds are created synchronized with the phase-stable terahertz pulses lasting several hundred femtoseconds. Focusing both pulses simultaneously onto the apex of the STM tip allows to characterize the near-field waveform of the THz pulses by slowly varying the delay between white light and THz pulses in a stroboscopic fashion.

Individual graphene nanoribbons are identified and characterized on the substrate by scanning tunnelling microscopy and spectroscopy. Custom software interfacing the microscope’s control electronics is employed in order to create the delicate transport junction. The junction is composed of tip and substrate acting as electrodes and stabilised in a distance of few nanometres from each other with picometre precision. An individual GNR bridges tip and substrate in a freely “hanging” unsupported configuration. The creation of the junction is achieved by adopting protocols for partially lifting individual molecules from the underlying substrate. To facilitate the data analysis, DC biased conductance spectra of the suspended GNRs are acquired prior to starting the ultrashort transport experiments for different tip-substrate separations for each individual GNR that was later probed with THz-driven currents.
The conductance characteristics exhibited by GNRs on ultrashort timescales are explored systematically after the detailed characterization as described above. We investigated the dependency of the ultrafast conductance on influences from various parameters, including the far-field THz pulse amplitude, the specific shape of the pulse waveform, DC-bias applied simultaneously to the junction and the size of the nanocavity formed between tip and substrate.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

ELISTRA has delivered results beyond the state of the art. In particular, we have observed ballistic electron transport on femtosecond timescales, directly addressing the central scientific objectives of the proposal. For a maximal impact of ELISTRA, the experiments were performed while exploring the extremes of parameter spaces and pushing the limits of experimental precision, namely measuring femtoampere currents over nanometre-sized junctions stable at a picometre level and THz waveforms controlled on the femtosecond and millivolt/nanometre scale. This was achieved by rigorous pre-characterisation and preparation followed by extended and still ongoing data acquisition campaigns to capture even minute physical responses at ultrafast timescales.

Given the complexity and novelty of the data, thorough analysis and consolidation with theoretical models naturally extends beyond the funding period, which is typical for this class of experiments combining several pioneering and demanding experimental aspects. Manuscripts are currently under preparation to be submitted to peer-reviewed journals.

The preliminary results confirm the feasibility to drive current on femtosecond timescales through individual graphene nanoribbons. This key finding of ELISTRA provides an experimental basis to push the idea of lightwave-driven electronics operating at THz frequencies to the transport regime beyond conventional tunnelling with individual molecules acting as the functional unit in an atomically controlled device geometry. Lightwave-driven electronics hold the potential to increase computing speeds by orders of magnitude. However, ample research and development remains necessary to move the fundamental findings obtained in ELISTRA to being technologically available in commercial devices. Such technology holds the added benefit to potentially strengthen the EU’s technological sovereignty by focusing on GNRs composed of abundant materials (Carbon, Boron, Hydrogen, Oxygen…) for lightwave-driven electronics - as shown feasible in ELISTRA.