Eve Ammerman

Excitons, bound electron–hole pairs, govern the optical response of semiconductors and underpin technologies ranging from photovoltaics to quantum light sources. In nanoscale systems, the spatial extent of excitons approaches atomic dimensions, yet direct imaging of excitonic processes in real space has remained elusive. Two-dimensional semiconductors such as transition metal dichalcogenides provide a platform where reduced screening, high light extraction and atomically defined defect sites enable the exploration of exciton physics at atomic length scales. However, the microscopic origin of defect-related emission and its variability across samples have remained debated. Here we show that scanning tunnelling microscopy-induced luminescence reveals the atomic and optical fingerprints of individual defects in monolayer MoS ₂ . Decoupling the material from its substrate using bilayer hexagonal boron nitride enables the detection of sharp excitonic emission and spatially localized defect emission, which we attribute to free-to-bound recombination ( e ,A ⁰ ), and map its distribution with subnanometre-resolved fluorescence imaging. We identify negatively charged carbon–hydrogen complexes as the defect species responsible for a ubiquitous defect emission band in MoS ₂ . These results provide atomically resolved images of exciton emission in a solid-state material, linking atomic defect structure to its optical fingerprint and establishing a microscopic basis for deterministic quantum emitter engineering in two-dimensional materials.

Scanning tunneling microscopy-induced luminescence (STML) provides access to the optical properties of individual molecules through a cascade of relaxation processes between many-body states. Insufficient charge attachment energies prevent relaxation via optically active states, causing even intrinsically bright molecules to remain dark in STML. Here, we utilize substrate work function control and tip-induced gating of the double-barrier tunnel junction to induce an energy shift of the ionic transition state of a single free-base tetrabenzoporphyrin (H2TBP) to control access to optically active states and bright exciton emission. The experimental observations are validated by a rate equation and a polaron model considering the relaxation energy of the NaCl decoupling layer upon charging of the molecule.

Controlling electron dynamics at optical clock rates is a fundamental challenge in lightwave-driven nanoelectronics and quantum technology. Here, we demonstrate ultrafast charge-state manipulation of individual selenium vacancies in monolayer and bilayer tungsten diselenide using picosecond terahertz source pulses, focused onto the junction of a scanning tunneling microscope. Using pump–probe time-domain sampling of the defect charge population, we capture atomic-scale snapshots of the transient Coulomb blockade, a hallmark of charge transport via quantized defect states. We leverage the Franck–Condon blockade, which restricts accessible vibronic transitions and promotes unidirectional charge transport, to effectively mitigate back tunneling to the tip electrode. Our master equation approach models the non-reciprocal tunneling process due to vibrations and angular momentum multiplicities, accurately reproducing the time-dependent tunneling current across different coupling regimes. Capturing and controlling ultrafast charge dynamics in low-dimensional materials at the atomic scale opens frontiers in lightwave-driven nanoscale science and technology.

On-surface synthesis enables the fabrication of atomically precise graphene nanoribbons (GNRs) with properties defined by their shape and edge topology. While this bottom-up approach provides unmatched control over electronic and structural characteristics, integrating GNRs into functional electronic devices requires their transfer from noble metal growth surfaces to technologically relevant substrates. However, such transfers often induce structural modifications, potentially degrading or eliminating GNRs’ desired functionality - a process that remains poorly understood. In this study, we employ low-temperature scanning tunneling microscopy and spectroscopy (STM/STS) to characterize 9-atom-wide armchair GNRs (9-AGNRs) following polymer-free wet-transfer onto epitaxial graphene (EG) and quasi-freestanding epitaxial graphene (QFEG) substrates. Our results reveal that armchair GNRs maintain their structural integrity post-transfer, while GNRs with extended or modified edge topologies exhibit significant structural changes, including partial disintegration. Additionally, STS measurements reveal differences in the Fermi level alignment between GNRs and the graphene substrates, a key factor in optimizing carrier injection efficiency in electronic transport devices. This study establishes a framework for detecting postprocessing structural modifications in GNRs, which are often hidden in optical ensemble measurements. By addressing the challenges of substrate transfer and providing insights into GNR-substrate interactions, these findings pave the way for the reliable integration of atomically precise GNRs into next-generation nanoelectronic and optoelectronic devices.

Controlling electron dynamics at optical clock rates is a fundamental challenge in lightwave-driven nanoelectronics. Here, we demonstrate ultrafast charge-state manipulation of individual selenium vacancies in monolayer and bilayer tungsten diselenide (WSe$_2$) using picosecond terahertz (THz) source pulses, focused onto the picocavity of a scanning tunneling microscope (STM). Using THz pump--THz probe time-domain sampling of the defect charge population, we capture atomic-scale snapshots of the transient Coulomb blockade, a signature of charge transport via quantized defect states. We identify back tunneling of localized charges to the tip electrode as a key challenge for lightwave-driven STM when probing electronic states with charge-state lifetimes exceeding the pulse duration. However, we show that back tunneling can be mitigated by the Franck-Condon blockade, which limits accessible vibronic transitions and promotes unidirectional charge transport. Our rate equation model accurately reproduces the time-dependent tunneling process across the different coupling regimes. This work builds on recent progress in imaging coherent lattice and quasiparticle dynamics with lightwave-driven STM and opens new avenues for exploring ultrafast charge dynamics in low-dimensional materials, advancing the development of lightwave-driven nanoscale electronics.

THz-pulse driven scanning tunneling microscopy (THz-STM) enables access to the ultrafast quantum dynamics of low-dimensional material systems at simultaneous ultrafast temporal and atomic spatial resolution. State-selective tunneling requires precise amplitude and phase control of the THz pulses combined with quantitative near-field waveform characterization. Here, we employ our state-of-the-art THz-STM with multi-MHz repetition rates, efficient THz generation, and precisely tunable THz waveforms to investigate a single sulfur vacancy in monolayer MoS2. We demonstrate that 2D transition metal dichalcogenides (TMDs) are an ideal platform for near-field waveform sampling by THz cross-correlation. Furthermore, we determine the THz voltage via QEV scans, which measure the THz rectified charge Q as a function of THz field amplitude E and dc bias Vdc. Mapping the complex energy landscape of localized states with a resolution down to 0.01 electrons per pulse facilitates state-selective tunneling to the HOMO and LUMO orbitals of a charged sulfur vacancy.

Terahertz scanning tunneling microscopy (THz-STM) enables ultrafast measurements of surfaces, single molecules, and nanostructures with simultaneous subpicosecond temporal resolution and atomic spatial resolution. In pump-probe THz-STM experiments employing femtosecond optical pump pulses, lightwave-driven tunneling by a time-delayed THz probe pulse accesses the evolving differential conductance of the tunnel junction following photoexcitation. However, a general theoretical approach to extract the time- and voltage-dependent differential conductance from THz-STM measurements is lacking. Here, we introduce an algorithm for pump-probe THz scanning tunneling spectroscopy (THz-STS) analysis. Our approach allows us to reliably reconstruct the tunnel junction's differential conductance in steady-state or time-dependent scenarios from simulated THz-STS data. The algorithm achieves subcycle time resolution, which we demonstrate by retrieving dynamics faster than the bandwidth of the input THz voltage transient. Subcycle THz-STS will make lightwave-driven microscopy yet more powerful as a tool for characterizing ångström-scale ultrafast dynamics in novel materials.

Atomically precise electronics operating at optical frequencies require tools that can characterize them on their intrinsic length and time scales to guide device design. Lightwave-driven scanning tunnelling microscopy is a promising technique towards this purpose. It achieves simultaneous sub-ångström and sub-picosecond spatio-temporal resolution through ultrafast coherent control by single-cycle field transients that are coupled to the scanning probe tip from free space. Here, we utilize lightwave-driven terahertz scanning tunnelling microscopy and spectroscopy to investigate atomically precise seven-atom-wide armchair graphene nanoribbons on a gold surface at ultralow tip heights, unveiling highly localized wavefunctions that are inaccessible by conventional scanning tunnelling microscopy. Tomographic imaging of their electron densities reveals vertical decays that depend sensitively on wavefunction and lateral position. Lightwave-driven scanning tunnelling spectroscopy on the ångström scale paves the way for ultrafast measurements of wavefunction dynamics in atomically precise nanostructures and future optoelectronic devices based on locally tailored electronic properties.