A team of researchers at Michigan State University have utilized lightwave-driven terahertz scanning tunneling microscopy and spectroscopy to probe 7-atom-wide armchair graphene nanoribbons (7-AGNRs) at ultralow heights. They have uncovered highly localized wave functions in GNR edges that conventional scanning tunneling microscopes have not revealed.
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Designing advanced optoelectronic devices requires the development of diagnostic tools operating at nanometer scales. Terahertz radiation is becoming an important tool in the development of new technologies.
To achieve nanoscale resolutions, terahertz scanning probe microscopes couple terahertz radiation to subwavelength-sized probes. This opens up new possibilities for the precise characterization of wavefunction dynamics in nanostructures and bodes well for future optoelectronic devices tailored through the modulation of local electronic properties.
A Quick Primer on Scanning Probe Microscopy
Ever since the scanning tunneling microscope was invented by two IBM researchers in 1981, scanning probe microscopes have underpinned innovations in materials science by revealing the morphology, topography and composition of structures at nanometer scales.
The many types of scanning probe microscopes include scanning tunneling microscopes, atomic force microscopes, and scanning electrochemical microscopes. They reveal far greater details than conventional optical microscopes because they use electron waves instead of light waves to “see” into objects. Since electron wavelengths are hundreds of thousands of times smaller than optical wavelengths, electron microscopes can resolve objects hundreds of thousands of times smaller than optical microscopes.
In scanning electron microscopes, a microscopic probe scans a sample with an electron beam so that the scattered electrons can be analyzed to form an image.
The study of graphene nanoribbons (GNRs) is essential to the development of novel nanoelectronic devices. Graphene nanoribbons are strips of graphene less than 100 nm wide. Graphene is one of the many allotropes of carbon - diamond and graphite are two other well-known allotropes of carbon.
At nanoscales, electronic confinement effects and edge structures govern the properties of graphene. In the zigzag edge structures of GNRs, each edge segment is at the opposite angle to the previous. In armchair edge structures, each pair of segments is at a 120-degree rotation angle to the prior pair. Armchair edge structures are either metallic or semiconducting. Zigzag edge structures are always metallic.
Lightwave-driven scanning tunneling microscopy (STM) opens up a new dimension of atomically resolved microscopy. Lightwave-control of extreme tunnel currents (and other fields) engenders ultrafast fields which can operate in regimes inaccessible to conventional static STM fields.
Lightwave-Driven Scanning Tunneling Microscopy
The team at Michigan State University, supported by the Swiss Federal Laboratories for Materials Science and Technology and the University of Bern, grew graphene nanoribbons from molecular precursors on a clean gold (Au) substrate using on-surface synthesis. They selected 7-atom-wide GNRs with armchair edges (7-AGNRs) for their study.
The team showed that the differential conductance on the gold surface substrate was highly sensitive to the lateral movement of the microscope’s nanoprobe. Thus, producing differential conductance maps by combining scanning tunneling microscopy (STM) and spectroscopy (STS) reveals a sample’s local density of electronic states (LDOS) as a function of position and energy.
Terahertz spectroscopy (THz-STS) was performed as a function of three-dimensional positioning above the GNRs. It allowed the team to extract the differential conductance sampled by lightwave-driven tunneling microscopy with ångström (10-10 meters) horizontal and sub-ångström vertical resolution. They also introduced lightwave-driven scanning tunneling tomography, where constant-height THz-STM images showed a transition from tunneling dominated by occupied states in the GNR valence bands to tunneling dominated by unoccupied states in the conduction bands.
The team applied spatially dependent THz-STS to disentangle the intrinsic properties of the 7-AGNR at ultralow probe tip heights from the lightwave-driven tunneling process. They defined ultralow tip heights as distances at which orbitally selective imaging by conventional STM with an s-wave tip becomes unfeasible because the DC current would either damage the tip or the sample.
Energetic positions and widths were constrained by fits to the spectroscopy data. Ultrafast photoemission sampling to detect the profile of the terahertz pulse at the tip of the probe was used. Combining this with the differential conductance data was sufficient to reproduce an image at a given probe position.
Lightwave-driven (terahertz) scanning tunneling microscopy, spectroscopy and tomography of GNRs open up new opportunities for the nanoscale engineering of novel materials.
References and Further Reading
Ammerman, S.E., et al., (2021) Lightwave-driven scanning tunnelling spectroscopy of atomically precise graphene nanoribbons. Nature Communications, [online] Available at: https://doi.org/10.1038/s41467-021-26656-3
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