Exchange Interactions and Intermolecular Hybridization in a Spin-1/2 Nanographene Dimer.

Phenalenyl is a radical nanographene with a triangular shape hosting an unpaired electron with spin S = 1/2. The open-shell nature of the phenalenyl is expected to be retained in covalently bonded networks. As a first step, we report synthesis of the phenalenyl dimer by combining in-solution synthesis and on-surface activation and its characterization on Au(111) and on a NaCl decoupling layer by means of inelastic electron tunneling spectroscopy (IETS). IETS shows inelastic steps that are identified as singlet-triplet excitation arising from interphenalenyl exchange. Spin excitation energies with and without the NaCl decoupling layer are 48 and 41 meV, respectively, indicating significant renormalization due to exchange with Au(111) electrons. Furthermore, third-neighbor hopping-induced interphenalenyl hybridization is fundamental to explaining the position-dependent bias asymmetry of the inelastic steps and activation of kinetic interphenalenyl exchange. Our results pave the way for bottom-up synthesis of S = 1/2 spin-lattices with large exchange interactions.

Lightwave-driven scanning tunnelling spectroscopy of atomically precise graphene nanoribbons.

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.

Electronic characterization of silicon intercalated chevron graphene nanoribbons on Au(111).

Electronic and thermal properties of chevron-type graphene nanoribbons can be widely tuned, making them interesting candidates for electronic and thermoelectric applications. Here, we use post-growth silicon intercalation to unambiguously access nanoribbons' energy position of their electronic frontier states. These are otherwise obscured by substrate effects when investigated directly on the growth substrate. In agreement with first-principles calculations we find a band gap of 2.4 eV.

Impact of heterocirculene molecular symmetry upon two-dimensional crystallization.

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 (C16S8), whereas D(4h)-symmetric selenosulflower (C16S4Se4) forms square and rectangular lattices at submonolayer and monolayer coverages, respectively. No long-range ordered structure is observed for C(1h)-symmetric selenosulflower (C16S5Se3) 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.

Mapping the electronic surface potential of nanostructured surfaces.

We present a method for the quantitative determination of the surface potential landscape of nanostructured surfaces based on the local analysis of the lowest field emission resonances by scanning tunneling spectroscopy. The method has a lateral resolution of approximately 1 nm and is applied to elucidate the site-specific adsorption properties of the strain relief pattern formed by two monolayers of Ag on Pt(111). For the example of C60 fullerenes, we show that the surface potential difference of up to 0.35 eV is responsible for the site-selective immobilization on the strain relief pattern.

Fabrication of a well-ordered nanohole array stable at room temperature.

We report on the fabrication of a new type of nanotemplate surface consisting of a hexagonally well-ordered array of one monolayer deep holes with a tunable size of about 4 nm (2) and a fixed spacing of 7 nm. The nanohole array fabrication is based on the strain-relief trigonal network formed in the 2 monolayer Ag on Pt(111) system. Removing about 0.1 ML of the Ag top layer of this surface structure, for example, by He- or Ar-ion sputtering, leads to the formation of nanoholes at specific domains of the trigonal network, which are stable at room temperature.

Coexistence of one- and two-dimensional supramolecular assemblies of terephthalic acid on Pd(111) due to self-limiting deprotonation.

The adsorption of terephthalic acid [C(6)H(4)(COOH)(2), TPA] on a Pd(111) surface has been investigated by means of scanning tunneling microscopy (STM), x-ray photoelectron spectroscopy, and near-edge x-ray absorption fine structure spectroscopy under ultrahigh vacuum conditions at room temperature. We find the coexistence of one- (1D) and two-dimensional (2D) molecular ordering. Our analysis indicates that the 1D phase consists of intact TPA chains stabilized by a dimerization of the self-complementary carboxyl groups, whereas in the 2D phase, consisting of deprotonated entities, the molecules form lateral ionic hydrogen bonds. The supramolecular growth dynamics and the resulting structures are explained by a self-limiting deprotonation process mediated by the catalytic activity of the Pd surface. Our models for the molecular ordering are supported by molecular mechanics calculations and a simulation of high resolution STM images.

Self-assembly and conformation of tetrapyridyl-porphyrin molecules on Ag(111).

We present a low-temperature scanning tunneling microscopy (STM) study on the supramolecular ordering of tetrapyridyl-porphyrin (TPyP) molecules on Ag(111). Vapor deposition in a wide substrate temperature range reveals that TPyP molecules easily diffuse and self-assemble into large, highly ordered chiral domains. We identify two mirror-symmetric unit cells, each containing two differently oriented molecules. From an analysis of the respective arrangement it is concluded that lateral intermolecular interactions control the packing of the layer, while its orientation is induced by the coupling to the substrate. This finding is corroborated by molecular mechanics calculations. High-resolution STM images recorded at 15 K allow a direct identification of intramolecular features. This makes it possible to determine the molecular conformation of TPyP on Ag(111). The pyridyl groups are alternately rotated out of the porphyrin plane by an angle of 60 degrees.

Two-dimensional separation of [7]helicene enantiomers on Cu(111).

The adsorption of heptahelicene, a helically shaped polyaromatic hydrocarbon (C(30)H(18)), on a Cu(111) surface was studied by means of thermal desorption mass spectrometry (TDMS) and low energy electron diffraction (LEED) at temperatures between 130-1,000 K under ultrahigh vacuum (UHV) conditions. The molecule in the monolayer remains intact up to 400 K. Above that temperature it decomposes in several steps into carbon and hydrogen, desorbing subsequently as H(2). In the saturated monolayer of the racemate the enantiomers are separated into two different domains on the surface which are mirror images of each other. After adsorption of one enantiomer only, no mirror domains were observed.

Anisotropy of quasiparticle lifetimes and the role of disorder in graphite from ultrafast time-resolved photoemission spectroscopy.

Femtosecond time-resolved photoemission of photoexcited electrons in highly oriented pyrolytic graphite (HOPG) provides strong evidence for anisotropies of quasiparticle (QP) lifetimes. Indicative of such anisotropies is a pronounced anomaly in the energy dependence of QP lifetimes between 1.1 and 1.5 eV--the vicinity of a saddle point in the graphite band structure. This is supported by recent ab initio calculations and a comparison with experiments on defect-enriched HOPG which reveal that disorder, e.g., defects or phonons, increases electron energy relaxation rates.