In recent years, many scientific discoveries and technological developments have been of interdisciplinary nature connecting the classical disciplines of physics, chemistry, biology and medicine. An important category concerns "Nano-Science", in particular carbon-based soft matter and soft/hard matter’s interface structures. Their atomic structure is requested at the pristine stage, ideally at atomic level.
Experiments within our SALVE I-III project comprises the practical application of low-voltage microscopy for materials science problems. We investigated and will investigate low-dimensional and bulk electron-beam sensitive materials such as carbon nanomaterials, thin polymers and single molecules, but also oxides and/or doped semiconductors. In SALVE I-II about 70 publications with experimental data have been published.
Our general aim is to determine the optimum experimental TEM conditions (e.g., acceleration voltage, current density) to achieve highest possible specimen resolution. Consequently, one major interest is to understand the electron - material interactions in the TEM. The most important radiation damage processes are
- Atom knock-on, which can be reduced or nullified by reducing the electron energy
- Surface etching, which can be reduced by lowering electron energy
- Ionization and radiolysis (bond breaking), which increases with decreasing electron energy
- Heating, which increases at lower electron beam energy
- Chemical etching due to residual molecules (water), which is nearly independent of the electron energy
Studies carried out in Work Package (A): HRTEM during SALVE I-II [1 - 4] showed in agreement with studies from others [5 - 8] that carbon nanotubes are excellent substrates for molecules, providing mechanical support, efficient reduction of ionisation and suppression of molecular motion, thus enabling atomically resolved analysis at the single-molecule level and further insight into radiation damage processes. For instance, it was observed that the reduction of the accelerating voltage from 80 to 20 kV increases the life time of C60 molecules in carbon nanotubes by two orders of magnitude . New strategies which may open routes to study currently inaccessible beam-sensitive objects also comprise sandwiching between graphene sheets [9, 10].
Experiments in Work Package (B): Spectroscopy have been conducted during SALVE I-II. Low acceleration voltages have several important advantages for spectroscopic applications, in particular electron energy-loss spectroscopy (EELS) and energy-filtered TEM. Because slower electrons interact more strongly with the sample, the spectroscopic signal is considerably enhanced. In addition to the long acquisition times usable at lower voltages this effect increases the achievable S/N-ratio further. It has been shown that using low acceleration voltages, the signal even of individual atoms can be recorded [11 - 13]. For graphene it was observed in SALVE I-II that the upper limit of available scattering angles in momentum-resolved energy-loss spectra can be considerably extended at 40 kV without damaging the sample .
In Work Package (D): In-situ TEM and devices, the intrinsic optical properties and local band-gaps of selected bulk materials could be measured with high accuracy using low voltages during SALVE I-II .
Topics in Experiments (SALVE III)
Experimental research in SALVE III has topics in Work Package (A), (B) and (D), which will be described here in detail.
Work Package (A: HRTEM) - Understanding radiation damage mechanisms at low voltages and investigation of beam sensitive materials.
In order to be able to separate the contributions of the diverse effects on radiation damage, several parameters will be varied. Materials/test-objects will be selected in such a way that different damage mechanisms are dominant. In particular, the effect of different voltages as well as the comparison of room temperature and liquid nitrogen temperature measurements will be performed.
Consequently, the studies in this Work Package experiments-(A) are based on a carefully selected set of sample materials. The first set of sample materials are two-dimensional materials, such as single-layer graphene or transition-metal-dichalcogenide. In a single-layered material progression of radiation damage can be observed at the atom-by-atom level, as the exact atomic structure of the sample is directly interpretable from the projection TEM images, and consequently the radiation damage cross-sections can be determined accurately. The second set of sample materials are single wall carbon nanotubes which host the objects such as C60 fullerenes, coronene or other simple organic molecules, preferably those which are investigated on graphene. One further highly efficient method for reducing radiation damage is still relatively new and was introduced by SALVE I-II researchers  and another group  in parallel, that is, sandwiching a sample between two graphene layers. This method will be applied in topic (A3). In topic (A4) we will use selected two-dimensional materials as substrate for simple biological materials such as organic molecules. The last topic in Work Package (A) is dedicated to beam-sensitive bulk materials.
The formation of defects as a function of voltage and current density will be studied by evaluating HRTEM images using the tools of this project. The images will be analyzed and properties such as damage cross section will be obtained. We will focus on the determination of the transitions between crystalline − semi-amorphous − up to the amorphous state of each two-dimensional materials and if progress is well, we define property-related fingerprints in the nearest-neighbour evaluations of different amorphous phases. The different 2D materials are for instance graphene, MoS2, MoSe2 and NbSe2; the accelerating voltage will be varied inbetween 20 and 80 kV. Selected specimens will be investigated at room and LN2 temperature.
For different types of species the usage of CNT as containers may be the only way to protect them against electron beam damage during HRTEM imaging. In SALVE III we will study:
- the synthesis of peapod systems for incorporation of organic and organometallic derivatives and metals
- molecules with increasing complexity of organic functional groups bearing conformationally rigid or flexible units
- technologically important organic molecules, such as high aspect ratio molecular wires and disk-shaped polyaromatic molecules
As demonstrated in the case of MoS2 during SALVE I-II , constructing samples in different graphene/transition-metal-dichalcogenide heterostructure configurations allows experimental separation of the different damage channels and repeating such experiments with different materials and different voltages will grant valuable insights into the role of the damage mechanisms and allow to avoid damage in later investigations. These studies will be continued in SALVE III.
The resolution of biological objects is decisively limited by radiation damage because they consist of weakly bonded and light atoms. Imaging the pristine atomic structure of a biological object is a topic of enormous relevance in materials and life sciences. Based on analysis in topics experiments-(A1) and (A2) we will select thin sample substrates for imaging simple molecules and biological objects. First high-resolution TEM imaging on biological objects at low voltages will be performed in the second half of SALVE III. Such studies will be performed in 2D (projections) as well as in 3D (electron tomographic reconstruction). Selected specimens will be investigated at room and LN2 temperature.
In addition to single molecule studies, which rely on thin samples, we also want to investigate the chromatic aberration corrected imaging of thicker samples. Because such specimens will be thicker than one elastic mean free path length plus one inelastic one they may enable high-resolution imaging with inelastic scattered electrons. Here we will study the contrast mechanisms for multiple elastic and inelastic scattering, in particular the relative contributions of amplitude, phase, and higher-order interaction contrast. First experiments have been performed by Rasmus Schröder (University Heidelberg) at the ACAT – TEAM instrument at Argonne National Lab. It will be interesting to see how beam-sensitive samples will behave under a tomographic data collection scheme when all interacting electrons can be used for imaging. Again we expect less damage to the sample as we do not need to filter out inelastic scattered electrons (zero-loss imaging mode) which is currently the preferred mode of operation for tomography on thick samples. These experiments require advancement of sample preparation, which is SALVE III topic preparation-(A5).
Work Package (B: Spectroscopy) - Optimization and application of EELS at low acceleration voltages
The first part of this work package in SALVE III is dedicated to methodological developments for low-dose measurements. The second part will be devoted to spectroscopic applications for two-dimensional and beam-sensitive materials. These experiments will be conducted on the FEI Titan 80-300 microscope (topic B1) and on the FEI SALVE III microscope (topic B2).
Our developments for the ω-x and ω-q mode conducted during SALVE I-II will be in the first half of SALVE III applied for core-loss measurements using the existing standard Titan TEM at Ulm university (energy resolution of 0.5eV).
The CC/CS corrector of the latter decreases Cerenkov-radiation effect, which reduces noise at lower voltages. This enables valence-loss EELS. With the availability of the monochromated SALVE machine in beginning of 2016, valence-loss excitations can be studied with an energy resolution ≤0.1 eV. Combined with the advantages of low-voltage microscopy and the methods developed in SALVE, this machine will be an ideal tool for a wide range of applications.
Using in-situ microscopy, we are focusing on imaging structural variations under the supply of an electrical current. It has been shown, that low acceleration voltages have important advantages for the measurement of the intrinsic optical properties and local band-gaps of bulk materials [1, 18]. In order to characterize the electrical properties of graphene sheets separately from those of the electrical contact, we will perform 4-terminal electrical measurements by separate pairs of current and voltage contacts on the membranes. One major goal will be the evaluation of the dependence of electrical conductivity on the defect-state of graphene and coverage of its surface with various deposits. Electron irradiation at higher voltages will be applied to pattern graphene membranes to contain regular arrays of amorphous surrounded by crystalline regions. Hall experiments will be performed in situ in the TEM by making use of the magnetic field of the TEM lenses. In previous experiments, the field-strength at the sample position has been determined as function of acceleration voltage and objective lens excitation: The field strength at the sample can be gradually changed by raising the current in the objective lens coils from zero to 1.4 T (at 80 kV). Measurements using the Hall-effect will be used to determine electrical properties such as charge carrier concentration and mobility. The in-situ experimental measuring procedure once optimized for the new generation of cartridges will we applied not only to graphene but also to other 2D samples like BN and 2D heterostructures like BN-graphene or MoS2-graphene.
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