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Scattering-type scanning near-field optical microscopy (s-SNOM) allows for the observation of the optical response of material surfaces with a resolution far below the diffraction limit. A spatial resolution of 10-100 nm is routinely achieved in s-SNOM based on amplitude-modulation atomic force microscopy (AM-AFM) with tapping amplitudes of tens of nanometers. However, optical imaging and spectroscopy of structures that are localized to the atomic scale remain a significant challenge. This can be overcome by combining the field enhancement localized at the atomic-scale structure of the tip apex, namely a plasmonic picocavity, with frequency-modulationAFM (FM-AFM), namely non-contact AFM, in a stable cryogenic ultrahigh vacuum environment. Here, we developed picocavityenhanced SNOM (PE-SNOM) under visible laser illumination based on the integration of a quartz tuning fork sensor with small-amplitude oscillations of 1 nm or less. In addition, the use of a focused ion beam-polished silver tip mounted on the sensor leads to strong field enhancement in the picocavity and ensures minimal background scattering from the tip shaft. PE-SNOM allows us to obtain a material-contrast image of silicon islands on a silver surface with 1-nm lateral resolution, which surpasses the conventional limits of s-SNOM. PE-SNOM paves the way for the acquisition of optical information from atomic-scale targets, such as single photo-active defects and molecules.

Spintronic terahertz emitters (STEs) are powerful sources of ultra-broadband single-cycle terahertz (THz) field transients. They work with any pump wavelength, and their polarity and polarization direction are easily adjustable. However, at high pump powers and high repetition rates, STE operation is hampered by a significant increase in the local temperature. Here, we resolve this issue by rotating the STE at a few 100 Hz, thereby distributing the absorbed pump power over a larger area. Our approach permits stable STE operation at a fluence of ~1 mJ/cm$^2$ with up to 18 W pump power at megahertz repetition rates, corresponding to pump-pulse energies of a few 10 $\mu$J and a power density far above the melting threshold of metallic films. The rotating STE is of interest for all ultra-broadband high-power THz applications requiring high repetition rates. As an example, we show that THz pulses with peak fields of 10 kV/cm can be coupled to a THz-lightwave-driven scanning tunneling microscope at 1 MHz repetition rate, demonstrating that the rotating STE can compete with standard THz sources such as LiNbO$_3$., Comment: 15 pages, 4 figures, supplementary material included

Efficient operation of electronic nanodevices at ultrafast speeds requires understanding and control of the currents generated by femtosecond bursts of light. Ultrafast laser-induced currents in metallic nanojunctions can originate from photo-assisted hot electron tunneling or lightwave-induced tunneling. Both processes can drive localized photocurrents inside a scanning tunneling microscope (STM) on femto- to attosecond time scales, enabling ultrafast STM with atomic spatial resolution. Femtosecond laser excitation of a metallic nanojunction, however, also leads to the formation of a transient thermalized electron distribution, but the tunneling of thermalized hot electrons on time scales faster than electron-lattice equilibration is not well understood. Here, we investigate ultrafast electronic heating and transient thermionic tunneling inside a metallic photoexcited tunnel junction and its role in the generation of ultrafast photocurrents in STM. Phase-resolved sampling of broadband THz pulses via the THz-field-induced modulation of ultrafast photocurrents allows us to probe the electronic temperature evolution inside the STM tip, and to observe the competition between instantaneous and delayed tunneling due to nonthermal and thermal hot electron distributions in real time. Our results reveal the pronounced nonthermal character of photo-induced hot electron tunneling, and provide a detailed microscopic understanding of hot electron dynamics inside a laser-excited tunnel junction., Comment: 32 pages, 5 figures, supporting information

The advent of on-surface chemistry under vacuum has vastly increased our capabilities to synthesize carbon-nanomaterials with atomic precision. Among the types of target structures that have been synthesized by these means, graphene nanoribbons (GNRs) have probably attracted the most attention. In this context, the vast majority of GNRs have been synthesized from the same chemical reaction: Ullmann coupling followed by cyclodehydrogenation. Here, we provide a detailed study of the growth process of 5-atom-wide armchair GNRs starting from dibromoperylene. Combining scanning probe microscopy with temperature-dependent XPS measurements and theoretical calculations, we show that the GNR growth departs from the conventional reaction scenario. Instead, precursor molecules couple by means of a concerted mechanism whereby two covalent bonds are formed simultaneously, along with a concomitant dehydrogenation. Indeed, this novel reaction path is responsible for the straight GNR growth, something remarkable considering the initial mixture of reactant isomers with irregular metal-organic intermediates that we find. The provided insight will not only help understanding the reaction mechanisms of other reactants, but also serve as a guide for the design of new precursor molecules.

Glaser-like coupling of terminal alkynes by thermal activation is extensively used in on-surface chemistry. Here we demonstrate an intramolecular version of this reaction performed by atom manipulation. We used voltage pulses from the tip to trigger a Glaser-like coupling between terminal alkyne carbons within a custom synthesized precursor molecule adsorbed on bilayer NaCl on Cu(111). Different conformations of the precursor molecule and the product were characterized by molecular structure elucidation with atomic force microscopy and orbital density mapping with scanning tunneling microscopy, accompanied by density functional theory calculations. We revealed partially dehydrogenated intermediates providing insight into the reaction pathway.

Carbon allotropes built from rings of two-coordinate atoms, known as cyclo[n]carbons, have fascinated chemists for many years, but until now they could not be isolated or structurally characterized, due to their high reactivity. We generated cyclo[18]carbon (C18) using atom manipulation on bilayer NaCl on Cu(111) at 5 Kelvin by eliminating carbon monoxide from a cyclocarbon oxide molecule C24O6. Characterization of cyclo[18]carbon by high-resolution atomic force microscopy revealed a polyynic structure with defined positions of alternating triple and single bonds. The high reactivity of cyclocarbon and cyclocarbon oxides allows covalent coupling between molecules to be induced by atom manipulation, opening an avenue for the synthesis of other carbon allotropes and carbon-rich materials from the coalescence of cyclocarbon molecules.

Atomic force microscopy (AFM) with molecule-functionalized tips has emerged as the primary experimental technique for probing the atomic structure of organic molecules on surfaces. Most experiments have been limited to nearly planar aromatic molecules, due to difficulties with interpretation of highly distorted AFM images originating from non-planar molecules. Here we develop a deep learning infrastructure that matches a set of AFM images with a unique descriptor characterizing the molecular configuration, allowing us to predict the molecular structure directly. We apply this methodology to resolve several distinct adsorption configurations of 1S-camphor on Cu(111) based on low-temperature AFM measurements. This approach will open the door to apply high-resolution AFM to a large variety of systems for which routine atomic and chemical structural resolution on the level of individual objects/molecules would be a major breakthrough.

There is enormous recent interest in weak, van der Waals-type (vdW) interactions due to their fundamental relevance for two-dimensional materials and the so-called vdW heterostructures. Tackling this problem using computer simulation is very challenging due to the non-trivial, non-local nature of these interactions. We benchmark different treatments of London dispersion forces within the density functional theory (DFT) framework on hexagonal boron nitride or graphene monolayers on Ir(111) by comparing the calculated geometries to a comprehensive set of experimental data. The geometry of these systems crucially depends on the interplay between vdW interactions and wave function hybridisation, making them excellent test cases for vdW-treated DFT. Our results show strong variations in the calculated atomic geometry. While some of the approximations reproduce the experimental structure, this is rather based on \textit{a posteriori} comparison with the ``target results''. General predictive power in vdW-treated DFT is not achieved yet and might require new approaches., Comment: More or less published version, with Supporting Material

Scanning tunneling spectroscopy measures how a single electron with definite energy propagates between a sample surface and the tip of a scanning tunneling microscope. In the simplest description, the differential conductance measured is interpreted as the local density of states of the sample at the tip position. This picture, however, is insufficient in some cases, since especially smaller molecules weakly coupled with the substrate tend to have strong Coulomb interactions when an electron is inserted or removed at the molecule. We present theoretical approaches to go from the non-interacting and single-particle picture to the correlated many-body regime. The methodology is used to understand recent experiments on finite armchair graphene nanoribbons and phthalocyanines. We also theoretically discuss the strongly-correlated model system of fractional quantum Hall droplets., Comment: Submitted to Journal of Electron Spectroscopy and Related Phenomena

Hexagonal boron nitride (h-BN) is a prominent member in the growing family of two-dimensional materials with potential applications ranging from being an atomically smooth support for other 2D materials to templating growth of molecular layers. We have studied the structure of monolayer h-BN grown by chemical vapour deposition on Ir(111) by low-temperature scanning tunneling microscopy (STM) and spectroscopy (STS) experiments and state-of-the-art density functional theory (DFT) calculations. The lattice-mismatch between the h-BN and Ir(111) surface results in the formation of a moir\'e superstructure with a periodicity of $\sim$29 \AA\ and a corrugation of $\sim$0.4 \AA. By measuring the field emission resonances above the h-BN layer, we find a modulation of the work function within the moir\'e unit cell of $\sim$0.5 eV. DFT simulations for a 13-on-12 h-BN/Ir(111) unit cell confirm our experimental findings and allow us to relate the change in the work function to the subtle changes in the interaction between boron and nitrogen atoms and the underlying substrate atoms within the moir\'e unit cell. Hexagonal boron nitride on Ir(111) combines weak topographic corrugation with a strong work function modulation over the moir\'e unit cell. This makes h-BN/Ir(111) a potential substrate for electronically modulated thin film and hetero-sandwich structures., Comment: 8 pages, 5 figures