Low-voltage Aberration-corrected Transmission Electron Microscopy Research Articles

Growth Anisotropy and Morphology Evolution of Line Defects in Monolayer MoS2 : Atomic-Level Observation, Large-Scale Statistics, and Mechanism Understanding.

Understanding the growth behavior and morphology evolution of defects in 2D transition metal dichalcogenides is significant for the performance tuning of nanoelectronic devices. Here, the low-voltage aberration-corrected transmission electron microscopy with an in situ heating holder and a fast frame rate camera to investigate the sulfur vacancy lines in monolayer MoS2 is applied. Vacancy concentration-dependent growth anisotropy is discovered, displaying first lengthening and then broadening of line defects as the vacancy densifies. With the temperature increase from 20 °C to 800 °C, the defect morphology evolves from a dense triangular network to an ultralong linear structure due to the temperature-sensitive vacancy migration process. Atomistic dynamics of line defect reconstruction on the millisecond time scale are also captured. Density functional theory calculations, Monte Carlo simulation, and configurational force analysis are implemented to understand the growth and reconstruction mechanisms at relevant time and length scales. Throughout the work, high-resolution imaging is closely combined with quantitative analysis of images involving thousands of atoms so that the atomic-level structure and the large-area statistical rules are obtained simultaneously. The work provides new ideas for balancing the accuracy and universality of discoveries in the TEM study and will be helpful to the controlled sculpture of nanomaterials.

Texture, Nanotexture, and Structure of Carbon Nanotube-Supported Carbon Cones.

Graphene-based carbon micro-/nano-cones were prepared by depositing pyrolytic carbon onto individual carbon nanotubes as supports using a specific chemical vapor deposition process. They were investigated by means of high-resolution scanning electron microscopy, low-voltage aberration-corrected transmission electron microscopy, Raman spectroscopy, and molecular dynamics modeling. While the graphenes were confirmed to be perfect, the cone texture was determined to be preferably scroll-like, with the scroll turns being parallel to the cone axis. Correspondingly, many of the concentrically displayed graphenes (actually scroll turns) exhibit the same helicity vector. When radii of curvature are large enough, this could allow for coherent stacking to locally take place in spite of the lattice shift induced by the curvature. A particular care was taken on investigating the cone apexes, in which a specific type of graphene termination was observed, here designated as the "zip" defect. Calculations determined a plausible stable structure that such a defect type may correspond to. This defect was found to generate a very low Raman ID/ID' band ratio (1.5), for which physical reasons are proposed. Combining our results and that of the literature allowed proposing an identification chart for a variety of defects able to affect the graphene lattice or edges.

Properties of two dimensional materials obtained from experiments in a low-voltage aberration-corrected TEM

We report on structural and electronic properties of two-dimensional materials ob-tained by analytical low-voltage aberration-corrected transmission electron microscopy. Basic crystallographic defects and their peculiarities will be discussed for two-dimensional materials at the atomic level. Thus, we report the atomic structure of point defect and -clusters [1], the full life circle of dislocations [2] and the movements of grain boundaries in grapheme [3]. In addition, we unravel the atomic structure of the amorphous phase (graphene, SiO2) in direct space just from single-atom-based analysis of high-resolution TEM images [5, 6]. As the energetic electron beam is interacting with the specimen via transferring energy to the atoms, structural transformation between different phases can be followed atom-by-atom [7, 8, 9]. In addition, physical properties such as the knock-on damage threshold is determined from controlled direct space experiments and precise measurements of high-resolution TEM images of graphene and MoS2[8, 7]. However beam-electron interactions with the specimen are also restricting imaging the pristine structure of a sample. It can be suppressed by simply limiting the total electron doses on the samples. Limited electron doses, however, result in worse signal to noise ratios. Here, a quantitative approach for estimating the visibility of objects in TEM images with limited doses will be presented [10]. Another traditional approach to suppress electron-induced damage during HRTEM observation is to employ an efficient cleaning procedure [11] and the protective coating of sensitive materials. This old approach will be taken to its extreme, when radiation sensitive materials are enclosed inside carbon nanotubes [12] and between two graphene layers [13]. We show moreover the advantage of lowering the accelerating voltage for imaging the pristine structure of low-dimensional materials [14]. [4] P. Wachsmuth, R. Hambach, M.K. Kinyanjui, et al., Phys. Rev. B B 88, 075433, (2013) [5] P. Y. Huang, S. Kurasch, A. Srivastava, et al. Nano Lett. 12(2), 1081, (2012) [6] P. Y. Huang, S. Kurasch, J.S. Alden, et al., Science 342, 224, (2013) [7] H.-P. Komsa, J. Kotakoski, S. Kurasch, et al., Phys. Rev. Lett. 109, 035503 (2012) [8] C Meyer, F Eder, S Kurasch, et al. Physical Review Letters, 108, 196102. 2012. [9] B. Westenfelder, J. C. Meyer, J. Biskupek, et al., Transformations of Carbon Adsorbates on Graphene Substrates under Extreme Heat, Nano Letters, 11 (12), 5123-5127, 2011 [10] Z. Lee, H. Rose, O. Lehtinen, et al., Ultramicroscopy (2014), DOI 10.1016/j.ultramic.2014.01.010 [11] G. Algara-Siller, S. Kurasch, M. Sedighi, et al., Appl. Phys. Lett. 103 (2013) 203107 [12] T. Zoberbier, T. W. Chamberlain, J. Biskupek, et al., J. Am. Chem. Soc. 134 (2012) 3073-3079 [13] G. Algara-Siller, S. Kurasch, M. Sedighi, et al., Appl. Phys. Lett. 103. 203107, (2013) [14] U. Kaiser et al. Ultramicroscopy, 111, 8, 1239, (2011) [15] Fruitful cooperation within the SALVE project and financial support by the DFG (German Research Foundation) and by the Ministry of Science, Research, and the Arts (MWK) of Baden-Württemberg are gratefully acknowledged.

High-precision imaging of an encapsulated Lindqvist ion and correlation of its structure and symmetry with quantum chemical calculations

Low-voltage aberration-corrected transmission electron microscopy (AC-TEM) of discrete Lindqvist [W(6)O(19)](2-) polyoxometalate ions inserted from an ethanolic solution of [NBu(4)](2)[W(6)O(19)] into double walled carbon nanotubes (DWNTs) allows a higher precision structural study to be performed than previously reported. W atom column separations within the constituent W(6) tungsten cage can now be visualized with sufficient clarity that reliable correlation with structural predictions from density functional theory (DFT) can be achieved. Calculations performed on [W(6)O(19)](2-) anions encapsulated in carbon nanotubes show good agreement with measured separations between pairs of W(2) atom columns imaged within equatorial WO(6) polyhedral pairs and also single W atom positions located within individual axial WO(6) octahedra. Structural data from the tilted chiral encapsulating DWNT were also determined simultaneously with the anion structural measurements, allowing the influence of the conformation of the encapsulating tubule to be included in the DFT calculation and compared against that of other candidate encapsulating nanotubes. Additional DFT calculations performed using Li(+) cations as a model for the [NBu(4)](+) counterions indicate that the latter may help to induce charge transfer between the DWNT and the [W(6)O(19)](2-) ion and this may help to constrain the motion of the ion in situ.

Motion of Light Adatoms and Molecules on the Surface of Few-Layer Graphene

Low-voltage aberration-corrected transmission electron microscopy (TEM) is applied to investigate the feasibility of continuous electron beam cleaning of graphene and monitor the removal of residual species as present on few-layer graphene (FLG) surfaces. This combined approach allows us to detect light adatoms and evaluate their discontinuous sporadic motional behavior. Furthermore, the formation and dynamic behavior of isolated molecules on the FLG surface can be captured. The preferential source of adatoms and adsorbed molecules appeared to be carbonaceous clusters accumulated from residual solvents on the graphene surface. TEM image simulations provide potential detail on the observed molecular structures. Molecular dynamics simulations confirm the experimentally observed dynamics occurring on the energy scale imposed by the presence of the 80 kV electron beam and help elucidate the underlying mechanisms.

Understanding the Metal-Carbon Interface in FePt Catalyzed Carbon Nanotubes

Any tip functionalization of carbon nanotubes, for which the relative orientation between their (metallic) catalyst particle and the nanotube axis is essential, requires a detailed knowledge of the nature of the internal interface between the particle and the outgrown tube. In the present work, this interface is characterized with atomic precision using state-of-the-art low-voltage aberration-corrected transmission electron microscopy in combination with molecular dynamics simulations for the case of hard-magnetically terminated carbon nanotubes. Our results indicate that the physical principle based upon which the interfacial metal facet is chosen is a reduction of the desorption energy for carbon.

In-situ Observations of Restructuring Carbon Nanotubes via Low-voltage Aberration-corrected Transmission Electron Microscopy

ABSTRACTThe molecular structure and dynamics of carbon nanostructures is much discussed throughout the literature, mostly from the theoretical side because of a lack of suitable experimental techniques to adequately engage the problem. A technique that has recently become available is low-voltage aberration-corrected transmission electron microscopy. It is a valuable tool with which to directly observe the atomic structure and dynamics of the specimen in situ. Time series aberration-corrected low-voltage transmission electron microscopy is used to study the dynamics of single-wall carbon nanotubes in situ. We confirm experimentally previous theoretical predictions for the agglomeration of adatoms forming protrusions and subsequent removal. A model is proposed how lattice reconstruction sites spread. In addition, the complete healing of a multi-vacancy consisting of ca. 20 missing atoms in a nanotube wall is followed.

Examining Co-Based Nanocrystals on Graphene Using Low-Voltage Aberration-Corrected Transmission Electron Microscopy

We present a method to produce graphene and few layer graphene sheets using solution phase chemistry, which are used as ultrathin support membranes for enhanced imaging of nanomaterials using transmission electron microscopy. We demonstrate this by decorating the surface of the graphene sheets with Co-based nanocrystals (CoCl(2) and hcp Co). Low-voltage aberration-corrected high resolution transmission electron microscopy at 80 kV is used to image the nanocrystals on the thin graphene supports. We show that electron beam irradiation causes the CoCl(2) nanocrystals to become mobile on the graphene surface and exhibit both rotational and translational motion. We provide real-time in situ monitoring with atomic resolution of the coalescence of two CoCl(2) nanocrystals on the graphene surface, driven by electron beam irradiation. The CoCl(2) nanocrystals are then annealed in vacuum and transform into Co nanocrystals with hcp crystal structure. We show that these Co nanocrystals are catalytic and electron beam irradiation leads to the etching of the graphene surface, not observed for the CoCl(2) nanocrystals.