Support-based transfer and contacting of individual nanomaterials for in-situ nanoscale investigations

Although in-situ transmission electron microscopy (TEM) of nanomaterials has been gaining importance in recent years, difficulties in sample preparation have limited the number of studies on electrical properties. Here, a support-based preparation method of individual 1D and 2D materials is presented, which yields a reproducible sample transfer for electrical investigation by in-situ TEM. Using a mechanically rigid support grid allows the reproducible transfer and contacting to in-situ chips by focused ion beam with minimum damage and contamination. The transfer quality is assessed by exemplary studies of different nanomaterials, including a monolayer of WS2. Preliminary results from in-situ test experiments give an overview of possible studies, which concern the interplay between structural properties and electrical characteristics on the individual nanomaterial level as well as failure analysis under electrical current or studies of electromigration, Joule heating and related effects. The TEM measurements can be enriched by additional correlative microscopy techniques, which allow the study with a spatial resolution in the range of a few microns. Although developed for in-situ TEM, the present transfer method is also applicable to transferring nanomaterials to similar chips for performing further studies or even for using them in potential electrical/optoelectronic/sensing devices.Comment: 23 pages, 15 figure

Stable CoO2_2 Nanoscrolls With Outstanding Electrical Properties

Layered CoO$_2$ is of great interest for its promising properties but is meta-stable in its bulk form. CoO$_2$ was synthesized in a long-term stable nanotubular or scrolled form by converting the quasi-one-dimensional crystal structure of bulk Ca$_3$Co$_2$O$_6$ via a hydrothermal treatment. The resulting one-dimensional nanostructures with very thin walls are investigated in detail. The CoO_$2$ is found to crystallize in monoclinic form, similar to the related CaCoO$_2$-CoO$_2$ misfit structure. Individual nanoscrolls are characterized electrically and show a p-type semiconducting nature with a high current-carrying capacity of $7.6 \times 10^6$ A/cm$^2$ and an extremely high breakdown voltage of 27 kV/cm. The results demonstrate the possibility to stabilize meta-stable materials in low-dimensional forms and a promising application of the nanoscrolls as interconnect in high-voltage electronic circuitry

Fabrication of phase masks from amorphous carbon thin films for electron-beam shaping

Background: Electron-beam shaping opens up the possibility for novel imaging techniques in scanning (transmission) electron microscopy (S(T)EM). Phase-modulating thin-film devices (phase masks) made of amorphous silicon nitride are commonly used to generate a wide range of different beam shapes. An additional conductive layer on such a device is required to avoid charging under electron-beam irradiation, which induces unwanted scattering events. Results: Phase masks of conductive amorphous carbon (aC) were successfully fabricated with optical lithography and focused ion beam milling. Analysis by TEM shows the successful generation of Bessel and vortex beams. No charging or degradation of the aC phase masks was observed. Conclusion: Amorphous carbon can be used as an alternative to silicon nitride for phase masks at the expense of a more complex fabrication process. The quality of arbitrary beam shapes could benefit from the application of phase masks made of amorphous C

Charging of electron beam irradiated amorphous carbon thin films at liquid nitrogen temperature.

We studied the charging behavior of an amorphous carbon thin film kept at liquid-nitrogen temperature under focused electron-beam irradiation. Negative charging of the thin film is observed. The charging is attributed to a local change in the work function of the thin film induced by electron-stimulated desorption similar to the working principle of the hole free phase plate in its Volta potential implementation at elevated temperature. The negative bias of the irradiated film arises from the electron beam induced desorption of water molecules from the carbon film surface. The lack of positive charging, which is expected for non-conductive materials, is explained by a sufficient electrical conductivity of the carbon thin film even at liquid-nitrogen temperature as proven by multi-probe scanning tunneling microscopy and spectroscopy measurements

Analyzing contrast in cryo-transmission electron microscopy: Comparison of electrostatic Zach phase plates and hole-free phase plates

Phase plates (PPs) are beneficial devices to improve the phase contrast of life-science objects in cryo-transmission electron microscopy (TEM). The development of the hole-free (HF) PP, which consists of a thin carbon film, has led to impressive results due to its ease in fabrication, implementation and application. However, the phase shift of the HFPP can be controlled only indirectly. The electrostatic Zach PP uses a strongly localized and adjustable electrostatic potential to generate well-defined and variable phase shifts between scattered and unscattered electrons. However, artifacts in phase-contrast TEM images are induced by the presence of the PP rod in the diffraction plane. We present a detailed analysis and comparison of the contrast-enhancing capabilities of both PP types and their emerging artifacts. For this purpose, cryo-TEM images of a standard T4-bacteriophage test sample were acquired with both PP types. Simulated images reproduce the experimental images well and substantially contribute to the understanding of contrast formation. An electrostatic Zach PP was used in this work to acquire cryo-electron tomograms with enhanced contrast, which are of similar quality as tomograms obtained by HFPP TEM

Contrast of Backscattered Electron SEM Images of Nanoparticles on Substrates with Complex Structure

This study is concerned with backscattered electron scanning electron microscopy (BSE SEM) contrast of complex nanoscaled samples which consist of SiO2 nanoparticles (NPs) deposited on indium-tin-oxide covered bulk SiO2 and glassy carbon substrates. BSE SEM contrast of NPs is studied as function of the primary electron energy and working distance. Contrast inversions are observed which prevent intuitive interpretation of NP contrast in terms of material contrast. Experimental data is quantitatively compared with Monte-Carlo- (MC-) simulations. Quantitative agreement between experimental data and MC-simulations is obtained if the transmission characteristics of the annular semiconductor detector are taken into account. MC-simulations facilitate the understanding of NP contrast inversions and are helpful to derive conditions for optimum material and topography contrast

Room-Temperature Anomalous Hall Effect in Graphene in Interfacial Magnetic Proximity with EuO Grown by Topotactic Reduction

We show that thin layers of EuO, a ferromagnetic insulator, can be achieved by topotactic reduction under titanium of a Eu2O3 film deposited on top of a graphene template. The reduction process leads to the formation of a 7-nm thick EuO smooth layer, without noticeable structural changes in the underlying chemical vapor deposited (CVD) graphene. The obtained EuO films exhibit ferromagnetism, with a Curie temperature that decreases with the initially deposited Eu2O3 layer thickness. By adjusting the thickness of the Eu2O3 layer below 7 nm, we promote the formation of EuO at the very graphene interface: the EuO/graphene heterostructure demonstrates the anomalous Hall effect (AHE), which is a fingerprint of proximity-induced spin polarization in graphene. The AHE signal moreover persists above Tc up to 350K due to a robust super-paramagnetic phase in EuO. This original high-temperature magnetic phase is attributed to magnetic polarons in EuO: we propose that the high strain in our EuO films grown on graphene stabilizes the magnetic polarons up to room temperature. This effect is different from the case of bulk EuO in which polarons vanish in the vicinity of the Curie temperature Tc= 69K.Comment: 29 page

Onion-like Fe3O4/MgO/CoFe2O4 magnetic nanoparticles: new ways to control magnetic coupling between soft/hard phases

The control of the magnetization inversion dynamics is one of the main challenges driving the design of new nanostructured magnetic materials for magnetoelectronic applications. Nanoparticles with onion-like architecture offer a unique opportunity to expand the possibilities allowing to combine different phases at the nanoscale and also modulate the coupling between magnetic phases by introducing spacers in the same structure. Here we report the fabrication, by a three-step high temperature decomposition method, of Fe3O4/MgO/CoFe2O4 onio-like nanoparticles and their detailed structural analysis, elemental compositional maps and magnetic response. The core/shell/shell nanoparticles present epitaxial growth and cubic shape with overall size of (29+-6) nm. These nanoparticles are formed by cubic iron oxide core of (22+-4) nm covered by two shells, the inner of magnesium oxide and the outer of cobalt ferrite of ~1 and ~2.5 nm of thickness, respectively. The magnetization measurements show a single reversion magnetization curve and the enhancement of the coercivity field, from HC~608 Oe for the Fe3O4/MgO to HC~5890 Oe to the Fe3O4/MgO/CoFe2O4 nanoparticles at T=5 K, ascribed to the coupling between both ferrimagnetic phases with a coupling constant of =2 erg/cm2. The system also exhibits exchange bias effect, where the exchange bias field increases up to HEB~2850 Oe at 5 K accompanied with the broadening of the magnetization loop of HC~6650 Oe. This exchange bias effect originates from the freezing of the surface spins below the freezing temperature TF=32 K that pinned the magnetic moment of the cobalt ferrite shell.Comment: 39 pages, 8 figure