Bottom-up formation of robust gold carbide

A new phenomenon of structural reorganization is discovered and characterized for a gold-carbon system by in-situ atomic-resolution imaging at temperatures up to 1300 K. Here, a graphene sheet serves in three ways, as a quasi transparent substrate for aberration-corrected high-resolution transmission electron microscopy, as an in-situ heater, and as carbon supplier. The sheet has been decorated with gold nanoislands beforehand. During electron irradiation at 80 kV and at elevated temperatures, the accumulation of gold atoms has been observed on defective graphene sites or edges as well as at the facets of gold nanocrystals. Both resulted in clustering, forming unusual crystalline structures. Their lattice parameters and surface termination differ significantly from standard gold nanocrystals. The experimental data, supported by electron energy loss spectroscopy and density-functional theory calculations, suggests that isolated gold and carbon atoms form – under conditions of heat and electron irradiation – a novel type of compound crystal, Au-C in zincblende structure. The novel material is metastable, but surprisingly robust, even under annealing condition

Electron beam controlled covalent attachment of small organic molecules to graphene

Markevich A, Kurasch S, Lehtinen O, et al. Electron beam controlled covalent attachment of small organic molecules to graphene. NANOSCALE. 2016;8(5):2711-2719.The electron beam induced functionalization of graphene through the formation of covalent bonds between free radicals of polyaromatic molecules and C=C bonds of pristine graphene surface has been explored using first principles calculations and high-resolution transmission electron microscopy. We show that the energetically strongest attachment of the radicals occurs along the armchair direction in graphene to carbon atoms residing in different graphene sub-lattices. The radicals tend to assume vertical position on graphene substrate irrespective of direction of the bonding and the initial configuration. The "standing up" molecules, covalently anchored to graphene, exhibit two types of oscillatory motion bending and twisting - caused by the presence of acoustic phonons in graphene and dispersion attraction to the substrate. The theoretically derived mechanisms are confirmed by near atomic resolution imaging of individual perchlorocoronene (C24Cl12) molecules on graphene. Our results facilitate the understanding of controlled functionalization of graphene employing electron irradiation as well as mechanisms of attachment of impurities via the processing of graphene nanoelectronic devices by electron beam lithography

Defects in bilayer silica and graphene: common trends in diverse hexagonal two-dimensional systems

Peer reviewe

Stop-frame filming and discovery of reactions at the single-molecule level by transmission electron microscopy

We report an approach – named chemTEM – to follow chemical transformations at the single-molecule level with the electron beam of a transmission electron microscope (TEM) applied as both a tuneable source of energy and a sub-Angstrom imaging probe. Deposited on graphene, disk-shaped perchlorocoronene molecules are precluded from intermolecular interactions. This allows monomolecular transformations to be studied at the single-molecule level in real time and reveals chlorine elimination and reactive aryne formation as a key initial stage of multi-step reactions initiated by the 80 keV e-beam. Under the same conditions, perchlorocoronene confined within a nanotube cavity, where the molecules are situated in very close proximity to each other, enables imaging of intermolecular reactions, starting with the Diels-Alder cycloaddition of a generated aryne, followed by rearrangement of the angular adduct to a planar polyaromatic structure and the formation of a perchlorinated zigzag nanoribbon of graphene as the final product. ChemTEM enables the entire process of polycondensation, including the formation of metastable intermediates, to be captured in a one-shot ‘movie’. A molecule with a similar size and shape but with a different chemical composition, octathio[8]circulene, under the same conditions undergoes another type of polycondensation via thiyl biradical generation and subsequent reaction leading to polythiophene nanoribbons with irregular edges incorporating bridging sulphur atoms. Graphene or carbon nanotubes supporting the individual molecules during chemTEM studies ensure that the elastic interactions of the molecules with the e-beam are the dominant forces that initiate and drive the reactions we image. Our ab initio DFT calculations explicitly incorporating the e-beam in the theoretical model correlate with the chemTEM observations and give a mechanism for direct control not only of the type of the reaction but also of the reaction rate. Selection of the appropriate e-beam energy and control of the dose rate in chemTEM enabled imaging of reactions on a timeframe commensurate with TEM image capture rates, revealing atomistic mechanisms of previously unknown processes

Atom-by-atom observations on defect formation and dynamics in 2D materials studied by HRTEM

In this work I perform high resolution transmission electron microscopy studies on 2D materials such as graphene, single layer boron nitride and MoS2 as well as on atomically thin SiO2 glass. This combination turns out to be extremely fruitful because for the first time it is possible to see single atoms instead of atomic columns in transmission mode. This offers experimental excess to fundamental phenomena in materials sciences that so far have been reserved to computer simulations or analytical theory as capturing the dynamics of individual atoms in a bulk material is beyond the spatial and temporal resolution limit of current characterization techniques. Examples are the atomic structure of amorphous materials or the migration of point defects, grain boundaries and dislocations. On the other hand, this setup is an excellent lab-bench to study electron-specimen interaction such as the effect of chemical bonding on the elastic scattering, changes in the crystal structure due to knock-on (bond rotations or sputtering) and surface modifications caused by beam induced chemical reactions (chemical etching). In addition, the well-defined and simple geometry (especially the precisely defined thickness) and high stability of these new 2D materials allows measurements at a level of accuracy that has not been possible before inspiring researchers all over the world to push the limit of traditional microscopy techniques. Some examples are the detection of chemical bonding in direct HRTEM images for N-doped graphene and boron nitride, the possibility to distinguish between different light elements (B, C, N, O) from HRTEM and ADF-STEM and the possibility to perform spectroscopic measurements on single atoms

Vibrational Properties of a Two-Dimensional Silica Kagome Lattice

Kagome lattices are structures possessing fascinating magnetic and vibrational properties, but in spite of a large body of theoretical work, experimental realizations and investigations of their dynamics are scarce. Using a combination of Raman spectroscopy and density functional theory calculations, we study the vibrational properties of two-dimensional silica (2D-SiO2), which has a kagome lattice structure. We identify the signatures of crystalline and amorphous 2D-SiO2 structures in Raman spectra and show that, at finite temperatures, the stability of 2D-SiO2 lattice is strongly influenced by phonon-phonon interaction. Our results not only provide insights into the vibrational properties of 2D-SiO2 and kagome lattices in general but also suggest a quick nondestructive method to detect 2D-SiO2.Peer reviewe

Atomic Structure of Reduced Graphene Oxide

Using high resolution transmission electron microscopy. we identify the specific atomic scale features in chemically derived graphene monolayers that originate from the oxidation-reduction treatment of graphene. The layers are found to comprise defect-free graphene areas with sizes of a few nanometers interspersed with defect areas dominated by clustered pentagons and heptagons. Interestingly, all carbon atoms in these defective areas are bonded to three neighbors maintaining a planar sp(2)-configuration, which makes them undetectable by spectroscopic techniques. Furthermore, we observe that they introduce significant in-plane distortions and strain in the surrounding lattice