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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. Here, the authors perform lightwave-driven terahertz scanning tunnelling microscopy and spectroscopy of graphene nanoribbons with atomic resolution in three dimensions, revealing localized wavefunctions that are inaccessible by conventional scanning tunnelling microscopy.

We present a real-time investigation of ultra-fast carrier dynamics in single-wall carbon nanotube bundles using femtosecond time-resolved photoelectron spectroscopy. The experiments allow us to study the processes governing the sub-picosecond and the picosecond dynamics of non-equilibrium charge carriers. On the sub-picosecond time scale the dynamics are dominated by ultra-fast electron–electron scattering processes, which lead to internal thermalization of the laser-excited electron gas. We find that quasiparticle lifetimes decrease strongly as a function of their energy up to 2.38 eV above the Fermi level – the highest energy studied experimentally. The subsequent cooling of the laser-heated electron gas to the lattice temperature by electron–phonon interaction occurs on the picosecond time scale and allows us to determine the electron–phonon mass-enhancement parameter λ. The latter is found to be over an order of magnitude smaller if compared, for example, with that of a good conductor such as copper.

Despite the development of crystal engineering, it remains a great challenge to predict the crystal structure even for the simplest molecules and a clear link between molecular and crystal symmetry is missing in general. Here we demonstrate that the two-dimensional (2D) crystallization of heterocirculenes on a Au(111) surface is greatly affected by the molecular symmetry. By means of ultrahigh vacuum scanning tunneling microscopy, we observe a variety of 2D crystalline structures in the coverage range from submonolayer to monolayer for D 8h -symmetric sulflower (C 16 S 8 ), whereas D 4h -symmetric selenosulflower (C 16 S 4 Se 4 ) forms square and rectangular lattices at submonolayer and monolayer coverages, respectively. No long-range ordered structure is observed for C 1h -symmetric selenosulflower (C 16 S 5 Se 3 ) self-assembling at submonolayer coverage. Such different self-assembly behaviors for the heterocirculenes with reduced molecular symmetries derive from the tendency toward close packing and the molecular symmetry retention in 2D crystallization due to van der Waals interactions.

The concept of chirality dates back to 1848, when Pasteur manually separated left-handed from right-handed sodium ammonium tartrate crystals 1 . Crystallization is still an important means for separating chiral molecules into their two different mirror-image isomers (enantiomers) 2 , yet remains poorly understood 3 . For example, there are no firm rules to predict whether a particular pair of chiral partners will follow the behaviour of the vast majority of chiral molecules and crystallize together as racemic crystals 4 , or as separate enantiomers. A somewhat simpler and more tractable version of this phenomenon is crystallization in two dimensions, such as the formation of surface structures by adsorbed molecules. The relatively simple spatial molecular arrangement of these systems makes it easier to study the effects of specific chiral interactions 5 ; moreover, chiral assembly and recognition processes can be observed directly and with molecular resolution using scanning tunnelling microscopy 6 , 7 , 8 , 9 . The enantioseparation of chiral molecules in two dimensions is expected to occur more readily because planar confinement excludes some bulk crystal symmetry elements and enhances chiral interactions 10 , 11 ; however, many surface structures have been found to be racemic 12 , 13 , 14 , 15 , 16 , 17 , 18 . Here we show that the chiral hydrocarbon heptahelicene on a Cu(111) surface does not undergo two-dimensional spontaneous resolution into enantiomers 19 , but still shows enantiomorphism on a mesoscopic length scale that is readily amplified. That is, we observe formation of racemic heptahelicene domains with non-superimposable mirror-like lattice structures, with a small excess of one of the heptahelicene enantiomers suppressing the formation of one domain type. Similar to the induction of homochirality in achiral enantiomorphous monolayers 20 by a chiral modifier, a small enantiomeric excess suffices to ensure that the entire molecular monolayer consists of domains having only one of two possible, non-superimposable, mirror-like lattice structures.

The mechanisms for the dehydrogenation reaction of cyclohexaphenylene at a copper surface are investigated with the help of density functional theory and metadynamics. Our results represent a showcase for an approach that describes the surface using many-body classical potentials and molecule-surface interactions with a van der Waals model. Starting from the experimental observation that dispersion-assisted mechanisms are at least as important as catalytic processes for the description of the reaction, we fully describe the former, we identify intermediate states and estimate the free energy barriers that characterize the reaction.

We have investigated the room temperature growth of C60 overlayers on the strainrelief dislocation network formed by two monolayers of Ag on Pt(111) by means of scanning tunneling microscopy. Extended domains of highly ordered dislocation networks with a typical superlattice parameter of 6.8 nm have been prepared, serving as templates for subsequent C60 depositions. For low C60 coverages, the molecules decorate the step-edges, where also the first islands nucleate. This indicates that at room temperature the C60 molecules are sufficiently mobile to cross the dislocation lines and to diffuse to the step-edges. For C60 coverages of 0.4 monolayer, besides the islands nucleated at the step-edges, C60 islands also grow in the middle of terraces. The C60 islands typically extend over several unit cells of the dislocation network and show an unusual orientation of the hexagonally close-packed C60 lattice as compared to that found on the bare Ag(111) surface. Whereas C60 grows preferentially in a (2 3 × 2 3) R30° structure on Ag(111), on the Ag/Pt(111) dislocation network the C60 lattice adopts an orientation rotated by 30°, with the close-packed C60 rows aligned along the dislocations which themselves are aligned along the Ag⟨1-10⟩ directions. For higher coverages in the range of 1-2 monolayers, the growth of C60 continues in a layer-by-layer fashion.

The lack of female representation and gender imbalance in the superintendent position nationwide has been a focus of scholars for many years. Researchers have demonstrated that females have the experiences, preparations, and expertise necessary but are not securing the superintendent position at the same rate as their male counterparts, causing a disproportionate representation in the superintendent position nationwide. The purpose of this study was to examine the perspectives of female superintendents on the challenges, preparation, and experiences important for females to succeed in attaining a superintendent position in a large southwestern state. The social cognitive career theory was used to analyze the perspective of the participants. Using a qualitative method, data from 10 currently or previous serving female superintendents in a large southwestern state were collected for this study. Two research questions guided the study. The first question included obtaining information about the preparations and experiences important to prepare females to be successful in attaining the superintendent position. The second was to obtain information about the challenges to attaining the superintendent position for aspiring females. The results of these analyses indicated that activities like networking, group activities, mock interviews, professional learning, and creating a network of supportive advocates were imperative when trying to attain a superintendent position. Aspiring females and districts may develop positive social change from the results of this study by leveling the playing field to the superintendent position, by removing the expectation of the high school principalship, and by addressing gender inequities in the position.

Phenalenyl is a radical nanographene with triangular shape that hosts an unpaired electron with spin S = 1/2. The open-shell nature of phenalenyl is expected to be retained in covalently bonded networks. Here, we study a first step in that direction and report the synthesis of the phenalenyl dimer by combining in-solution synthesis and on-surface activation and its characterization both on Au(111) and on a monolayer of NaCl on top of Au(111) by means of inelastic electron tunneling spectroscopy (IETS). IETS shows inelastic steps that, together with a thorough theoretical analysis, are identified as the singlet-triplet excitation arising from interphenalenyl exchange. Two prominent features of our data permit to shed light on the nature of spin interactions in this system. First, the excitation energies with and without the NaCl decoupling layer are 48 and 41 meV, respectively, indicating a significant renormalization of the spin excitation energies due to exchange with the Au(111) electrons. Second, a position-dependent bias-asymmetry of the height of the inelastic steps is accounted for by an interphenalenyl hybridization of the singly occupied phenalenyl orbitals that is only possible via third neighbor hopping. This hybridization is also essential to activate kinetic interphenalenyl exchange. Our results set the stage for future work on the bottom-up synthesis of spin S = 1/2 spin lattices with large exchange interaction.

The desire to push aging civil, mechanical, and aerospace structures beyond their intended design lives has highlighted the need for structural health monitoring (SHM) strategies that are able to detect, locate, and quantify various forms of damage within them. SHM strategies may also be tailored for newly-deployed structures in an attempt to optimize their performance and maintenance over an entire life cycle so that total ownership costs are reduced. Specifically within the aerospace industry, standard non-destructive evaluation (NDE) techniques have been used for decades for inspection of components and systems. One of the most common and widely-accepted NDE domains is ultrasonic inspection, where components are imaged with the component out of service. Recent advances in sensor technology, distributed networks, and advanced signal processing techniques have begun to be exploited for in situ ultrasonic (and other forms of) SHM systems that are being deployed in a wide variety of real-world structures. In most cases, however, the ultrasonic excitation signals and feature extraction techniques being employed are the same as the standard NDE methods that have been in use for decades and are only applicable to relatively simple component geometries. This dissertation contributes to the body of knowledge in this field by introducing a new class of excitation signals and pattern recognition algorithms that, when paired with novel sensor networks, improve on the ability of standard SHM techniques to locate and identify damage on more complex geometry systems, including bolted joints and composite materials. This dissertation describes a methodology whereby chaotic guided waves are created and optimized (in a detection sense) and used as probes to perform damage assessment by building both time- and state-space domain models (rooted in pattern recognition) and using statistical modeling for performing damage classification under Type I/II error control. Multiple chaotic ultrasonic excitation formats are explored, including short-time chaotic wave packets and long-time chaotic bulk insonification, in which the diffuse, reverberant wave field is examined to identify structural changes. This method of insonification, in addition to enhanced pattern recognition techniques, allows this damage detection scheme to be employed on complex structural geometries with which standard ultrasonic-based SHM methods cannot be used. The outlined SHM method is applied to various test structures with different forms of induced damage including an aluminum plate with corrosion damage, bolted connections on several aluminum test structures (single and multiple-bolt configurations) and several adhesively-bonded composite wing-to-spar structures. Chaotic signal creation parameters are optimized for the composite structures and attempts are made to examine and, where possible, compensate for several sources of variability, such as temperature, within-unit variability and unit-to-unit variability.