arXiv subjects

Jannik C. Meyer

Publications and source records attributed to Jannik C. Meyer.

At least 37 records · Page 2Linked to original sources

arXiv2018

Electron-Beam Manipulation of Silicon Dopants in Graphene

The direct manipulation of individual atoms in materials using scanning probe microscopy has been a seminal achievement of nanotechnology. Recent advances in imaging resolution and sample stability have made scanning transmission electron microscopy a promising alternative for single-atom manipulation of covalently bound materials. Pioneering experiments using an atomically focused electron beam have demonstrated the directed movement of silicon atoms over a handful of sites within the graphene lattice. Here, we achieve a much greater degree of control, allowing us to precisely move silicon impurities along an extended path, circulating a single hexagon, or back and forth between the two graphene sublattices. Even with manual operation, our manipulation rate is already comparable to the state-of-the-art in any atomically precise technique. We further explore the influence of electron energy on the manipulation rate, supported by improved theoretical modeling taking into account the vibrations of atoms near the impurities, and implement feedback to detect manipulation events in real time. In addition to atomic-level engineering of its structure and properties, graphene also provides an excellent platform for refining the accuracy of quantitative models and for the development of automated manipulation.

cond-mat.mtrl-sci↗

arXiv2018

Analysis of point defects in graphene using low dose scanning transmission electron microscopy imaging and maximum likelihood reconstruction

Freestanding graphene displays an outstanding resilience to electron irradiation at low electron energies. Point defects in graphene are, however, subject to beam driven dynamics. This means that high resolution micrographs of point defects, which usually require a high electron irradiation dose might not represent the intrinsic defect population. Here, we capture the inital defects formed by ejecting carbon atoms under electron irradiation, by imaging with very low doses and subsequent reconstruction of the frequently occuring defects via a maximum likelihood algorithm.

cond-mat.mes-hall↗

arXiv2018

Automated image acquisition for low-dose STEM at atomic resolution

Beam damage is a major limitation in electron microscopy that becomes increasingly severe at higher resolution. One possible route to circumvent radiation damage, which forms the basis for single-particle electron microscopy and related techniques, is to distribute the dose over many identical copies of an object. For the acquisition of low-dose data, ideally no dose should be applied to the region of interest prior to the acquisition of data. We present an automated approach that can collect large amounts of data efficiently by acquiring images in an user-defined area-of-interest with atomic resolution. We demonstrate that the stage mechanics of the Nion UltraSTEM, combined with an intelligent algorithm to move the sample, allows the automated acquisition of atomically resolved images from micron-sized areas of a graphene substrate. Moving the sample stage automatically in a regular pattern over the area-of-interest enables the collection of data from pristine sample regions without exposing them to the electron beam before recording an image. Therefore, it is possible to obtain data with minimal dose (no prior exposure from focusing), which is only limited by the minimum signal needed for data processing. This enables us to prevent beam induced damage in the sample and to acquire large datasets within a reasonable amount of time.

cond-mat.mtrl-sci↗

arXiv2018

Insights into radiation damage from atomic resolution scanning transmission electron microscopy imaging of mono-layer CuPcCl$_{16}$ films on graphene

Atomically resolved images of monolayer organic crystals have only been obtained with scanning probe methods so far. On the one hand, they are usually prepared on surfaces of bulk materials, which are not accessible by (scanning) transmission electron microscopy. On the other hand, the critical electron dose of a monolayer organic crystal is orders of magnitudes lower than the one for bulk crystals, making (scanning) transmission electron microscopy characterization very challenging. In this work we present an atomically resolved study on the dynamics of a monolayer CuPcCl\textsubscript{16} crystal under the electron beam as well as an image of the undamaged molecules obtained by low-dose electron microscopy. The results show the dynamics and the radiation damage mechanisms in the 2D layer of this material, complementing what has been found for bulk crystals in earlier studies. Furthermore, being able to image the undamaged molecular crystal allows the characterization of new composites consisting of 2D materials and organic molecules.

cond-mat.mtrl-sci↗

arXiv2018

Atomic structure of intrinsic and electron-irradiation-induced defects in MoTe2

Studying the atomic structure of intrinsic defects in two-dimensional transition metal dichalcogenides is difficult since they damage quickly under the intense electron irradiation in transmission electron microscopy (TEM). However, this can also lead to insights into the creation of defects and their atom-scale dynamics. We first show that MoTe 2 monolayers without protection indeed quickly degrade during scanning TEM (STEM) imaging, and discuss the observed atomic-level dynamics, including a transformation from the 1H phase into 1T', three-fold rotationally symmetric defects, and the migration of line defects between two 1H grains with a 60° misorientation. We then analyze the atomic structure of MoTe2 encapsulated between two graphene sheets to mitigate damage, finding the as-prepared material to contain an unexpectedly large concentration of defects. These include similar point defects (or quantum dots, QDs) as those created in the non-encapsulated material, and two different types of line defects (or quantum wires, QWs) that can be transformed from one to the other under electron irradiation. Our density functional theory simulations indicate that the QDs and QWs embedded in MoTe2 introduce new midgap states into the semiconducting material, and may thus be used to control its electronic and optical properties. Finally, the edge of the encapsulated material appears amorphous, possibly due to the pressure caused by the encapsulation.

cond-mat.mtrl-sci↗

arXiv2017

Towards atomically precise manipulation of 2D nanostructures in the electron microscope

Despite decades of research, the ultimate goal of nanotechnology--top-down manipulation of individual atoms--has been directly achieved with only one technique: scanning probe microscopy. In this Review, we demonstrate that scanning transmission electron microscopy (STEM) is emerging as an alternative method for the direct assembly of nanostructures, with possible applications in plasmonics, quantum technologies, and materials science. Atomically precise manipulation with STEM relies on recent advances in instrumentation that have enabled non-destructive atomic-resolution imaging at lower electron energies. While momentum transfer from highly energetic electrons often leads to atom ejection, interesting dynamics can be induced when the transferable kinetic energies are comparable to bond strengths in the material. Operating in this regime, very recent experiments have revealed the potential for single-atom manipulation using the Angstrom-sized electron beam. To truly enable control, however, it is vital to understand the relevant atomic-scale phenomena through accurate dynamical simulations. Although excellent agreement between experiment and theory for the specific case of atomic displacements from graphene has been recently achieved using density functional theory molecular dynamics, in many other cases quantitative accuracy remains a challenge. We provide a comprehensive reanalysis of available experimental data on beam-driven dynamics in light of the state-of-the-art in simulations, and identify important targets for improvement. Overall, the modern electron microscope has great potential to become an atom-scale fabrication platform, especially for covalently bonded 2D nanostructures. We review the developments that have made this possible, argue that graphene is an ideal starting material, and assess the main challenges moving forward.

cond-mat.mtrl-sci↗

arXiv2017

Buckyball sandwiches

Two-dimensional (2D) materials have considerably expanded the field of materials science in the last decade. Even more recently, various 2D materials have been assembled into vertical van der Waals heterostacks, and it has been proposed to combine them with other low- dimensional structures to create new materials with hybridized properties. Here, we demonstrate the first direct images of a suspended 0D/2D heterostructure incorporating $C_{60}$ molecules between two graphene layers in a buckyball sandwich structure. We find clean and ordered $C_{60}$ islands with thicknesses down to one molecule, shielded by the graphene layers from the microscope vacuum and partially protected from radiation damage during scanning transmission electron microscopy imaging. The sandwich structure serves as a 2D nanoscale reaction chamber allowing the analysis of the structure of the molecules and their dynamics at atomic resolution.

cond-mat.mtrl-sci↗

arXiv2017

Cleaning graphene: comparing heat treatments in air and in vacuum

Surface impurities and contamination often seriously degrade the properties of two-dimensional materials such as graphene. To remove contamination, thermal annealing is commonly used. We present a comparative analysis of annealing treatments in air and in vacuum, both ex situ and "pre-situ", where an ultra-high vacuum treatment chamber is directly connected to an aberration-corrected scanning transmission electron microscope. While ex situ treatments do remove contamination, it is challenging to obtain atomically clean surfaces after ambient transfer. However, pre-situ cleaning with radiative or laser heating appears reliable and well suited to clean graphene without undue damage to its structure.

cond-mat.mtrl-sci↗

arXiv2016

Grain boundary-mediated nanopores in molybdenum disulfide grown by chemical vapor deposition

Molybdenum disulfide (MoS2) is a particularly interesting member of the family of two-dimensional (2D) materials due to its semiconducting and tunable electronic properties. Currently, the most reliable method for obtaining high-quality industrial scale amounts of 2D materials is chemical vapor deposition (CVD), which results in polycrystalline samples. As grain boundaries (GBs) are intrinsic defect lines within CVD-grown 2D materials, their atomic structure is of paramount importance. Here, through atomic-scale analysis of micrometer-long GBs, we show that covalently bound boundaries in 2D MoS2 tend to be decorated by nanopores. Such boundaries occur when differently oriented MoS2 grains merge during growth, whereas the overlap of grains leads to boundaries with bilayer areas. Our results suggest that the nanopore formation is related to stress release in areas with a high concentration of dislocation cores at the grain boundaries, and that the interlayer interaction leads to intrinsic rippling at the overlap regions. This provides insights for the controlled fabrication of large-scale MoS 2 samples with desired structural properties for applications.

cond-mat.mtrl-sci↗

arXiv2016

Doping of metal-organic frameworks towards resistive sensing

Coordination polymerization leads to various metal-organic frameworks (MOFs) in which symmetrical metal nodes exposed to nano voids lead to unique physical properties and chemical functionalities. One of the challenges towards their applications as porous materials is to make MOFs optimally conductive to be used as electronic components. Here, we demonstrate that Co-MOF-74, a honeycomb nano{framework with one{dimensionally arranged cobalt atoms advances its physical properties by accommodating Tetracyanochinodimethan (TCNQ), an acceptor molecule. Strong intermolecular charge transfer narrows the optical band gap down to 1.5 eV of divalent TCNQ and enhances the electrical conduction, which allows the MOF to be utilized for resistive gas- and photo-sensing. Our result provides insight into electronic interactions in doped MOFs and paves the way towards their electronic applications.

cond-mat.mes-hall↗

arXiv2016

Single-atom spectroscopy of phosphorus dopants implanted into graphene

One of the keys behind the success of the modern semiconductor technology has been the ion implantation of silicon, which allows its electronic properties to be tailored. For similar purposes, heteroatoms have been introduced into carbon nanomaterials both during growth and using post-growth methods. However, due to the nature of the samples, it has been challenging to determine whether the heteroatoms have been incorporated into the lattice as intended, with direct observations so far being limited to N and B dopants, and incidental Si impurities. Furthermore, ion implantation of these materials is more challenging due to the requirement of very low ion energies and atomically clean surfaces. Here, we provide the first atomic-resolution imaging and electron energy loss spectroscopy (EELS) evidence of phosphorus atoms incorporated into the graphene lattice by low-energy ion irradiation. The measured P L-edge response of an single-atom EELS spectrum map shows excellent agreement with an ab initio spectrum simulation, conclusively identifying the P in a buckled substitutional configuration. Our results demonstrate the viability of phosphorus as a lattice dopant in $sp^2$-bonded carbon structures and provide its unmistakeable fingerprint for further studies.

cond-mat.mtrl-sci↗

arXiv2016

High-performance hybrid electronic devices from layered PtSe2 films grown at low temperature

Layered two-dimensional (2D) materials display great potential for a range of applications, particularly in electronics. We report the large-scale synthesis of thin films of platinum diselenide (PtSe2), a thus far scarcely investigated transition metal dichalcogenide. Importantly, the synthesis by thermal assisted conversion is performed at 400 °C, representing a breakthrough for the direct integration of this novel material with silicon (Si) technology. Besides the thorough characterization of this new 2D material, we demonstrate its promise for applications in high-performance gas sensing with extremely short response and recovery times observed due to the 2D nature of the films. Furthermore, we realized vertically-stacked heterostructures of PtSe2 on Si which act as both photodiodes and photovoltaic cells. Thus this study establishes PtSe2 as a potential candidate for next-generation sensors and (opto-)electronic devices, using fabrication protocols compatible with established Si technologies.

cond-mat.mtrl-sci↗

arXiv2016

Isotope analysis in the transmission electron microscope

The Ångström-sized probe of the scanning transmission electron microscope can visualize and collect spectra from single atoms. This can unambiguously resolve the chemical structure of materials, but not their isotopic composition. Here we differentiate between two isotopes of the same element by quantifying how likely the energetic imaging electrons are to eject atoms. First, we measure the displacement probability in graphene grown from either $^{12}$C or $^{13}$C and describe the process using a quantum mechanical model of lattice vibrations coupled with density functional theory simulations. We then test our spatial resolution in a mixed sample by ejecting individual atoms from nanoscale areas spanning an interface region that is far from atomically sharp, mapping the isotope concentration with a precision better than 20%. Although we use a scanning instrument, our method should be applicable to any atomic resolution transmission electron microscope and to other low-dimensional materials.

physics.ins-det↗

arXiv2015

Raman Characterization of Platinum Diselenide Thin Films

Platinum diselenide (PtSe2) is a newly discovered 2D material which is of great interest for applications in electronics and catalysis. PtSe2 films were synthesized by thermally-assisted selenization of predeposited platinum films and scanning transmission electron microscopy revealed the crystal structure of these films to be 1T. Raman scattering of these films was studied as a function of film thickness, laser wavelength and laser polarization. Eg and A1g Raman active modes were identified using polarization measurements in the Raman setup. These modes were found to display a clear position and intensity dependence with film thickness, for multiple excitation wavelengths, and their peak positions agree with simulated phonon dispersion curves for PtSe2. These results highlight the practicality of using Raman spectroscopy as a prime characterization technique for newly-synthesized 2D materials.

cond-mat.mtrl-sci↗

arXiv2015

Confined linear carbon chains: A route to bulk carbyne

The extreme instability and strong chemical activity of carbyne, the infinite sp1 hybridized carbon chain, are responsible for its low possibility to survive in ambient conditions. Therefore, much less has been possible to explore about carbyne as compared to other novel carbon allotropes such as fullerenes, nanotubes and graphene. Although end-capping groups can be used to stabilize carbon chains, length limitation is still a barrier for its actual production, and even more for applications. Here, we report a novel route for bulk production of record long acetylenic linear carbon chains protected by thin double-walled carbon nanotubes. A corresponding extremely high Raman band is the first proof of a truly bulk yield formation of very long arrangements, which is unambiguously confirmed by transmission electron microscopy and near-field Raman spectroscopy. Our production establishes a way to exceptionally long stable carbon chains including more than 2300 carbon atoms, and an elegant forerunner towards the final goal of a bulk production of essentially infinite carbyne.

cond-mat.mtrl-sci↗

arXiv2014

Silicon-carbon bond inversions driven by 60 keV electrons in graphene

We demonstrate that 60 keV electron irradiation drives the diffusion of threefold coordinated Si dopants in graphene by one lattice site at a time. First principles simulations reveal that each step is caused by an electron impact on a C atom next to the dopant. Although the atomic motion happens below our experimental time resolution, stochastic analysis of 38 such lattice jumps reveals a probability for their occurrence in a good agreement with the simulations. Conversions from three- to fourfold coordinated dopant structures and the subsequent reverse process are significantly less likely than the direct bond inversion. Our results thus provide a model of non-destructive and atomically precise structural modification and detection for two-dimensional materials.

cond-mat.mes-hall↗

arXiv2014

Atomic structure and energetics of large vacancies in graphene

We present a computational study on the topology, energetics and structural deformations for a large number of experimentally observed defect configurations in graphene. We find that both the number of lost hexagonal carbon rings and introduced non-hexagonal rings increase linearly as a function of the vacancy order (number of missing atoms). The formation energies of the defects increase by about 2.2 eV per missing atom after an initial offset, establishing these defects as the lowest energy vacancy configurations studied in graphene to date. In addition, we find that even small point defects, which have been until now assumed flat, cause graphene to bend out of plane when not restricted into prohibitively confined geometries. This effect reaches to relative long distances even for some of the smallest defects, significantly reducing the stress otherwise imposed on the surrounding lattice.

cond-mat.mtrl-sci↗

arXiv2014

Imaging Atomic-Level Random Walk of a Point Defect in Graphene

Deviations from the perfect atomic arrangements in crystals play an important role in affecting their properties. Similarly, diffusion of such deviations is behind many microstructural changes in solids. However, observation of point defect diffusion is hindered both by the difficulties related to direct imaging of non-periodic structures and by the time scales involved in the diffusion process. Here, instead of imaging thermal diffusion, we stimulate and follow the migration of a divacancy through graphene lattice using a scanning transmission electron microscope operated at 60 kV. The beam-activated process happens on a timescale that allows us to capture a significant part of the structural transformations and trajectory of the defect. The low voltage combined with ultra-high vacuum conditions ensure that the defect remains stable over long image sequences, which allows us for the first time to directly follow the diffusion of a point defect in a crystalline material.

cond-mat.mtrl-sci↗