Electron holographic observation of micro-magnetic fields current-generated from single carbon coil

In situ biasing and off-axis electron holography of a ZnO nanowire

Quantitative characterization of electrically active dopants and surface charges in nano-objects is challenging, since most characterization techniques using electrons [1-3], ions [4] or field ionization effects [5-7] study the chemical presence of dopants, which are not necessarily electrically active. We perform cathodoluminescence and voltage contrast experiments on a contacted and biased ZnO nanowire with a Schottky contact and measure the depletion length as a function of reverse bias. We compare these results with state-of-the-art off-axis electron holography in combination with electrical in situ biasing on the same nanowire. The extension of the depletion length under bias observed in scanning electron microscopy based techniques is unusual as it follows a linear rather than square root dependence, and is therefore difficult to model by bulk equations or finite element simulations. In contrast, the analysis of the axial depletion length observed by holography may be compared with three-dimensional simulations, which allows estimating an n-doping level of 1 × 10¹⁸ cm^-3 and negative sidewall surface charge of 2.5 × 10¹² cm^-2 of the nanowire, resulting in a radial surface depletion to a depth of 36 nm. We found excellent agreement between the simulated diameter of the undepleted core and the active thickness observed in the experimental data. By combining TEM holography experiments and finite element simulation of the NW electrostatics, the bulk-like character of the nanowire core is revealed.

Calibration of multi-layered probes with low/high magnetic moments

We present a comprehensive method for visualisation and quantification of the magnetic stray field of magnetic force microscopy (MFM) probes, applied to the particular case of custom-made multi-layered probes with controllable high/low magnetic moment states. The probes consist of two decoupled magnetic layers separated by a non-magnetic interlayer, which results in four stable magnetic states: ±ferromagnetic (FM) and ±antiferromagnetic (A-FM). Direct visualisation of the stray field surrounding the probe apex using electron holography convincingly demonstrates a striking difference in the spatial distribution and strength of the magnetic flux in FM and A-FM states. In situ MFM studies of reference samples are used to determine the probe switching fields and spatial resolution. Furthermore, quantitative values of the probe magnetic moments are obtained by determining their real space tip transfer function (RSTTF). We also map the local Hall voltage in graphene Hall nanosensors induced by the probes in different states. The measured transport properties of nanosensors and RSTTF outcomes are introduced as an input in a numerical model of Hall devices to verify the probe magnetic moments. The modelling results fully match the experimental measurements, outlining an all-inclusive method for the calibration of complex magnetic probes with a controllable low/high magnetic moment.

Advanced electron holography techniques for in situ observation of solid-state lithium ion conductors

Advanced techniques for overcoming problems encountered during in situ electron holography experiments in which a voltage is applied to an ionic conductor are reported. The three major problems encountered were 1) electric-field leakage from the specimen and its effect on phase images, 2) high electron conductivity of damage layers formed by the focused ion beam method, and 3) chemical reaction of the specimen with air. The first problem was overcome by comparing experimental phase distributions with simulated images in which three-dimensional leakage fields were taken into account, the second by removing the damage layers using a low-energy narrow Ar ion beam, and the third by developing an air-tight biasing specimen holder.

Synthesis of Helical Carbon Fibers and Related Materials: A Review on the Past and Recent Developments

Helical carbon fibers (HCFs) have been widely studied due to their unique helical morphology and superior properties, which make them efficient materials for several potential applications. This review summarizes the past and current advancement on the synthesis of HCFs. The review focuses and discusses synthesis strategies and effect of experimental parameters on the growth of HCFs. The effect of preparation method of catalyst, catalyst nature, catalyst composition, catalyst size, catalyst initial and final shape, reaction temperature, reaction time, carbon source, impurities, and electromagnetic field on the growth of HCFs is reviewed. We also discuss the growth mechanism for HCFs and the synthesis of HCFs related materials. Finally, we conclude with a brief summary and an outlook on the challenges and future prospects of HCFs.

Analysis of nonlinear intensity attenuation in bright-field TEM images for correct 3D reconstruction of the density in micron-sized materials

To obtain the correct tomographic reconstruction of micron-sized materials, the nonlinear intensity attenuation of bright-field transmission electron microscopy (BF-TEM) images was analyzed as a function of the sample thickness using a high-voltage electron microscope. The intensity attenuation was precisely measured relative to the projection thickness of carbon microcoils (CMCs) at acceleration voltages of 400-1000 kV using objective apertures (OAs) with radii of 2.1-28 nm(-1). The results show that the nonlinearity is strongly dependent on the OA size and the acceleration voltage. The influence of nonlinearity on tomographic reconstructions was also examined using a specially developed 360°-tilt sample holder. Sliced images of the reconstructed volumes indicated that an increase in the nonlinearity caused artificial fluctuations in the internal density of materials and inaccurate shapes of the objects in more significant cases. Conditions sufficient for reconstruction with the correct density have been estimated to be 0.67 of the minimum electron transmittance, and for reconstructions with correct shapes, 0.4. This information enables foreseeing the quality of the reconstruction from a single BF-TEM image prior to the tilt-series acquisition. As a result to demonstrate the appropriateness of these conditions, a CMC with a diameter of 3.7 µm was reconstructed successfully; i.e. not only the shape but also the internal density were correctly reproduced using electron tomography.

Fabrication of high sensitivity carbon microcoil pressure sensors

This work demonstrates a highly sensitive pressure sensor that was fabricated using carbon microcoils (CMCs) and polydimethylsiloxane (PDMS). CMCs were grown by chemical vapor deposition using various ratios of Fe-Sn catalytic solution. The pressure sensor has a sandwiched structure, in which the as-grown CMCs were inserted between two PDMS layers. The pressure sensor exhibits piezo-resistivity changes in response to mechanical loading using a load cell system. The yields of the growth of CMCs at a catalyst proportion of Fe:Sn = 95:5 reach 95%. Experimental results show that the sensor achieves a high sensitivity of 0.93%/kPa from the CMC yield of 95%. The sensitivity of the pressure sensor increases with increasing yield of CMCs. The demonstrated pressure sensor shows the advantage of high sensitivity and is suitable for mass production.

Facile preparation of magnetic carbonaceous nanoparticles for Pb2+ ions removal

Magnetic carbonaceous nanoparticles were prepared by a facile two-step solution phase thermal synthesis. Magnetic nanoparticles (MNPs) with size less than 100 nm were first generated from FeCl(3) in a solvothermal reaction. The size could be significantly reduced to approximately 30 nm when 1,6-hexanediamine was employed in the reaction solution to functionalize the surface of MNPs with amine. Both the plain and amine-functionalized MNPs (MH) were effectively encapsulated in the carbonaceous shell by hydrothermal treatment in 0.5 M glucose solution. The saturation magnetization of MH decreased significantly from 70 to 25 emu/g after carbonaceous shell was formed. The as-prepared magnetic carbonaceous nanoparticles (MH@C) carries a negative surface charge (-30 mV) at neutral pH and has a point of zero charge (PZC) at pH 2. The carbonaceous shell not only can protect the magnetic nanoparticles (MNP) from the corrosive environment but also possesses a high adsorption capacity towards Pb(II). The adsorption isotherm at room temperature can be well-fitted by Langmuir model with a maximum adsorption capacity of 123 mg/g.

Coil-in-coil carbon nanocoils: 11 gram-scale synthesis, single nanocoil electrical properties, and electrical contact improvement

Coil-in-coil carbon nanocoils (CNCs) were synthesized by means of acetylene decomposition using nickel nanoparticles as catalysts. The investigations revealed that there are often several CNCs self-assembled in one nanospring. The yield of coil-in-coil CNCs was high up to 11 g in each run at the decomposition temperature of 450 degrees C. CNC nanodevices were fabricated for systematical examinations of charge conduction in the single CNC and in the electrical contacts. A focused laser beam of about 70 mum in diameter was applied for selective annealing CNC nanodevices so as to improve the electrical contacts to the CNC. Our study showed that the selective focused laser annealing technique is an effective route to improve the electrical contacts to the nanodevice. Temperature-dependent CNC resistances are analyzed with the Mott-variable range hopping (VRH) and Efros-Shklovskii VRH model, revealing electron hopping conduction in the disordered CNCs with a characteristic length of about 5-50 nm.