Determining local modulus and strength of heterogeneous films by force-deflection mapping of microcantilevers.

Estimating the elastic modulus and strength of heterogeneous films requires local measurement techniques. For local mechanical film testing, microcantilevers were cut into suspended many-layer graphene using a focused ion beam. An optical transmittance technique was used to map thickness near the cantilevers, and multipoint force-deflection mapping with an atomic force microscope was used to record the compliance of the cantilevers. These data were used to estimate the elastic modulus of the film by fitting the compliance at multiple locations along the cantilever to a fixed-free Euler-Bernoulli beam model. This method resulted in a lower uncertainty than is possible from analyzing only a single force-deflection. The breaking strength of the film was also found by deflecting cantilevers until fracture. The average modulus and strength of the many-layer graphene films are 300 and 12 GPa, respectively. The multipoint force-deflection method is well suited to analyze films that are heterogeneous in thickness or wrinkled.

Quantitative STEM: A method for measuring temperature and thickness effects on thermal diffuse scattering using STEM/EELS, and for testing electron scattering models.

In the last two decades, advances in the dark field detectors and microscopes of scanning transmission electron microscopy (STEM) have inspired a resurgence of interest in quantitative STEM analysis. One promising avenue is the use of STEM as a nanothermometric probe. In this application, thermal diffuse scattering, captured by a CCD camera or an annular dark field detector, acts as an indirect measurement of the specimen temperature. One challenge with taking such a measurement is achieving adequate sensitivity to quantify a change in scattered electron signal on the order of 1% or less of the full electron beam. Another difficulty is decoupling the thermal effect on electron scattering from scattering changes due to differing specimen thicknesses and materials. To address these issues, we have developed a method using STEM, combined with electron energy loss spectroscopy (EELS), to produce a material-specific calibration curve. On silicon, across the range 89 K to 294 K, we measured a monotonically increasing HAADF signal ranging from 4.0% to 4.4% of the direct beam intensity at a thickness-to-mean-free-path ratio of 0.5. This yielded a calibration curve of temperature versus full-beam-normalized, thickness-normalized HAADF signal. The method enables thermal measurements on a specimen of varying local thickness at a spatial resolution of a few nanometers. We demonstrated the potential of the technique for testing electron scattering models by applying single-electron scattering theory to the data collected to extract a measurement of the mean atomic vibration amplitude in silicon at 294 K. The measured value, 0.00738 ± 0.00002 nm, agrees well with reported measurement using X-rays.

Visible and short-wavelength infrared light collimation through carbon nanotube, parallel-hole collimators.

Traditional collimators typically require large optics and/or long pathlengths which makes miniaturization difficult. Carbon nanotube templated microfabrication offers a solution to pattern small 3D structures, such as parallel hole collimators. Here we present the characterization of a carbon nanotube parallel hole collimator design and its efficacy in visible and short wavelength infrared light. Comparison to geometric and far field diffraction models are shown to give a close fit, making this a promising technology for miniaturized diffuse light collimation.

High surface-area carbon microcantilevers.

Microscale porous carbon mechanical resonators were formed using carbon nanotube templated microfabrication. These cantilever resonators exhibited nanoscale porosity resulting in a high surface area to volume ratio which could enable sensitive analyte detection in air. These resonators were shown to be mechanically robust and the porosity could be controllably varied resulting in densities from 102 to 103 kg m-3, with pore diameters on the order of hundreds of nanometers. Cantilevers with lengths ranging from 500 µm to 5 mm were clamped in a fixture for mechanical resonance testing where quality factors from 102 to 103 were observed at atmospheric pressure in air.

Tissue-susceptibility matched carbon nanotube electrodes for magnetic resonance imaging.

Test disk electrodes were fabricated from carbon nanotubes (CNT) using the Carbon Nanotube Templated Microfabrication (CNT-M) technique. The CNT-M process uses patterned growth of carbon nanotube forests from surfaces to form complex patterns, enabling electrode sizing and shaping. The additional carbon infiltration process stabilizes these structures for further processing and handling. At a macroscopic scale, the electrochemical, electrical and magnetic properties, and magnetic resonance imaging (MRI) characteristics of the disk electrodes were investigated; their microstructure was also assessed. CNT disk electrodes showed electrical resistivity around 1â€¯Ω·cm, charge storage capacity between 3.4 and 38.4 mC/cm2, low electrochemical impedance and magnetic susceptibility of -5.9 to -8.1 ppm, closely matched to that of tissue (∼-9 ppm). Phantom MR imaging experiments showed almost no distortion caused by these electrodes compared with Cu and Pt-Ir reference electrodes, indicating the potential for significant improvement in accurate tip visualization.

Fabrication of High Aspect Ratio Millimeter-Tall Free-Standing Carbon Nanotube-Based Microelectrode Arrays.

Microelectrode arrays of carbon nanotube (CNT)/carbon composite posts with high aspect ratio and millimeter-length were fabricated using carbon-nanotube-templated microfabrication with a sacrificial "hedge". The high aspect ratio, mechanical robustness, and electrical conductivity of these electrodes make them a potential candidate for next-generation neural interfacing. Electrochemical measurements were also demonstrated using an individual CNT post microelectrode with a diameter of 25 µm and a length of 1 mm to perform cyclic voltammetry on both methyl viologen and dopamine in a phosphate-buffered saline solution. In addition to detection of the characteristic peaks, the CNT post microelectrodes show a fast electrochemical response, which may be enabling for in vivo and/or in vitro measurements. The CNT post electrode fabrication process was also integrated with other microfabrication techniques, resulting in individually addressable electrodes.

Thin-Film Carbon Nanofuses for Permanent Data Storage.

In this study, we have fabricated nanofuses from thin-film, arc-deposited carbon for use in permanent data storage. Thin-film carbon fuses have fewer fabrication barriers and retain the required resistivity and structural stability to act as a data-storage medium. Carbon thin films were characterized for their electrical, microstructural, and chemical bonding properties. Annealing these films in an argon environment at 400 °C reduced the resistivity from about 4 × 10-2 Ω cm as deposited to about 5 × 10-4 Ω cm, allowing a lower blowing voltage. Nanofuses with widths ranging from 200 to 60 nm were fabricated and tested. They blow with voltages between 2 and 5.5 V, and the nanofuses remain stable in both "1" and "0" states under a constantly applied read voltage of 1 V for over 90 h.

Multi-instrument characterization of five nanodiamond samples: a thorough example of nanomaterial characterization.

Here, we report the most comprehensive characterization of nanodiamonds (NDs) yet undertaken. Five different samples from three different vendors were analyzed by a suite of analytical techniques, including X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (ToF-SIMS), inductively coupled plasma mass spectrometry (ICP-MS), diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, X-ray diffraction (XRD), transmission electron microscopy (TEM), electron energy loss spectroscopy (EELS), Brunauer-Emmett-Teller (BET) surface area measurements, and particle size distribution (PSD) measurements. XPS revealed the elemental compositions of the ND surfaces (83-87 at.% carbon and 12-14 at.% oxygen) with varying amounts of nitrogen (0.4-1.8 at.%), silicon (0.1-0.7 at.%), and tungsten (0.3 at.% only in samples from one vendor). ToF-SIMS and ICP showed metal impurities (Al, Fe, Ni, Cr, etc. with unexpectedly high amounts of W in one vendor's samples: ca. 900 ppm). Principal component analyses were performed on the ToF-SIMS and ICP data. DRIFT showed key functional groups (-OH, C=O, C-O, and C=C). BET showed surface areas of 50-214 m(2)/g. XRD and TEM revealed PSD (bimodal distribution and a wide PSD, 5-100 nm, for one vendor's samples). XRD also provided particle sizes (2.7-27 nm) and showed the presence of graphite. EELS gave the sp(2)/sp(3) contents of the materials (37-88% sp(3)). PSD measurements were performed via differential sedimentation of the particles (mean particle size ca. 17-50 nm). This comprehensive understanding should allow for improved construction of nanodiamond-based materials.

High Aspect Ratio Carbon Nanotube Membranes Decorated with Pt Nanoparticle Urchins for Micro Underwater Vehicle Propulsion via H2O2 Decomposition.

The utility of unmanned micro underwater vehicles (MUVs) is paramount for exploring confined spaces, but their spatial agility is often impaired when maneuvers require burst-propulsion. Herein we develop high-aspect ratio (150:1), multiwalled carbon nanotube microarray membranes (CNT-MMs) for propulsive, MUV thrust generation by the decomposition of hydrogen peroxide (H2O2). The CNT-MMs are grown via chemical vapor deposition with diamond shaped pores (nominal diagonal dimensions of 4.5 × 9.0 µm) and subsequently decorated with urchin-like, platinum (Pt) nanoparticles via a facile, electroless, chemical deposition process. The Pt-CNT-MMs display robust, high catalytic ability with an effective activation energy of 26.96 kJ mol(-1) capable of producing a thrust of 0.209 ± 0.049 N from 50% [w/w] H2O2 decomposition within a compact reaction chamber of eight Pt-CNT-MMs in series.

Alternative FIB TEM sample preparation method for cross-sections of thin metal films deposited on polymer substrates.

Transmission electron microscopy (TEM) and focused ion beam (FIB) are proven tools to produce site-specific samples in which to study devices from initial processing to causes for failure, as well as investigating the quality, defects, interface layers, etc. However, the use of polymer substrates presents new challenges, in the preparation of suitable site-specific TEM samples, which include sample warping, heating, charging, and melting. In addition to current options that address some of these problems such as cryo FIB, we add an alternative method and FIB sample geometry that address these challenges and produce viable samples suitable for TEM elemental analysis. The key feature to this approach is a larger than usual lift-out block into which small viewing windows are thinned. Significant largely unthinned regions of the block are left between and at the base of the thinned windows. These large unthinned regions supply structural support and thermal reservoirs during the thinning process. As proof-of-concept of this sample preparation method, we also present TEM elemental analysis of various thin metallic films deposited on patterned polycarbonate, lacquer, and poly-di-methyl-siloxane substrates where the pattern (from low- to high-aspect ratio) is preserved.

Stable, microfabricated thin layer chromatography plates without volume distortion on patterned, carbon and Al2O3-primed carbon nanotube forests.

Some of us recently described the fabrication of thin layer chromatography (TLC) plates from patterned carbon nanotube (CNT) forests via direct infiltration/coating of the CNTs by low pressure chemical vapor deposition (LPCVD) of silicon from SiH4, followed by high temperature oxidation of the CNTs and Si. Herein we present an improved microfabrication process for the preparation of these TLC plates. First, a few nanometers of carbon and/or a thin film of Al2O3 is deposited on the CNTs. This method of priming the CNTs for subsequent depositions appears to be new. X-ray photoelectron spectroscopy confirms the presence of additional oxygen after carbon deposition. After priming, the plates are coated by rapid, conformal deposition of an inorganic material that does not require subsequent oxidation, i.e., by a fast pseudo atomic layer deposition (ψ-ALD) of SiO2 from trimethylaluminum and tris(tert-butoxy)silanol. Unlike devices described previously, faithful reproduction of the features in the masks is still observed after oxidation. A bonded, amino phase on the resulting plates shows fast, highly efficient separations of fluorescent dyes (plate heights in the range of 1.6-7.7 µm). Extensive characterization of the new materials by TEM, SEM, EDAX, DRIFT, and XPS is reported. A substantially lower process temperature for the removal of the CNT scaffold is possible as a result of the already oxidized materials used.

Dynamical diffraction simulations in FePt--I.

A series of multislice simulations to quantify the effect of various degrees of order, composition, and thickness on the electron diffracted intensities were performed using the L10 FePt system as the case study. The dynamical diffraction studies were done in both a convergent electron beam diffraction and selected area electron diffraction condition. The L10 symmetry demonstrated some peculiar challenges in the simulation, in particular between the {111} plane normal and the <111> direction, which are not equivalent because of tetragonality. A hybrid weighting function atom of Fe-Pt was constructed to account for S < 1 or nonequiatomic compositions. This statistical approach reduced the complexity of constructing a crystal with the probability that a particular atom was at a particular lattice site for a given order parameter and composition. Considerations of accelerating voltage, convergent angle, and thermal effects are discussed. The simulations revealed significant differences in intensity ratios between films of various compositions but equivalent unit cell numbers and degree of order.

Comparison of simulated and experimental order parameters in FePt--II.

Eight FePt thin film specimens of various thicknesses, compositions, and order parameters have been analyzed to determine the robustness and fidelity of multislice simulations in determining the chemical order parameter via electron diffraction (ED). The shape of the simulated curves depends significantly on the orientation and thickness of the specimen. The ED results are compared to kinematical scattering order parameters, from the same films, acquired from synchrotron X-ray diffraction (XRD). For the specimens analyzed with convergent beam electron diffraction conditions, the order parameter closely matched the order parameter as determined by the XRD methodology. However, the specimens analyzed by selected area electron diffraction conditions did not show good agreement. This has been attributed to substrate effects that hindered the ability to accurately quantify the intensity values of the superlattice and fundamental reflections.

A modified back-etch method for preparation of plan-view high-resolution transmission electron microscopy samples.

A modified back-etch method is described that has been successfully used to prepare samples of thin films and nanoparticles on Si wafer substrates for examination by high-resolution transmission electron microscopy (HRTEM). This process includes ultrasonic cutting, abrasive pre-thinning and a two-stage etching procedure. Unlike previous reports of back-etching methods, tetramethyl ammonium hydroxide, which has a very high-etching selectivity of Si to SiO(2), is used for the final etching to allow removal of the Si without degradation of the SiO(2) membrane. An innovative wrapping method is also described. This novel approach reduces the preparation time for HRTEM samples to <1 h per sample for groups of 10 or more samples. As an example, the preparation of FePt nanoparticle samples for HRTEM imaging is described.