Konrad Jarausch

Three-dimensional electron microscopy of individual nanoparticles

The characterization of nanomaterials with complex three-dimensional (3D) geometries is required to further research and enable the continuing development of nanotechnology. In this manuscript, we report a protocol which combines focused ion beam (FIB) milling, thin film deposition and solution chemistry to optimize a rotation holder for 3D structural and chemical analysis of nanoparticles. This protocol is used to customize the geometry, surface and chemistry of a scanning transmission electron microscope (STEM) or transmission electron microscope (TEM) rotation holder for the nanoparticle system of interest. To illustrate this concept, rotation holder stubs were optimized to facilitate the 3D STEM imaging and analysis of core-shell nanoparticles used for DNA detection. Using this approach, it was possible to characterize the morphology, optoelectronic properties and chemical composition of individual core-shell nanoparticles in 3D. STEM images were captured at regular angular intervals over a complete 360 degrees rotation to eliminate missing wedge artifacts. Electron energy-loss spectroscopy (EELS) spectrum images were acquired intermittently for comparative chemical analysis. This approach allows the 3D STEM/TEM analysis to be performed with the nanoparticle of interest cantilevered over vacuum to minimize substrate effects. Standard tomography techniques were used to reconstruct the 3D structure of the individual nanoparticles from the STEM HAADF rotation series. EELS spectrum imaging was used to determine the local material properties such as composition, band-gap and plasmon energy. The nanoparticle analysis protocol reported here can easily be adapted to facilitate 3D TEM/STEM analysis of other nanomaterial systems.

The newly installed aberration corrected dedicated STEM (Hitachi HD2700C) at Brookhaven National Laboratory

The Hitachi HD2700C was recently successfully installed at the newly established Center for Functional Nanomaterials, Brookhaven National Lab (BNL). It was the first commercial aberration corrected electron microscope manufactured by Hitachi. The instrument is based on HD2300, a dedicated STEM developed a few years ago to complete with the VG STEMs [1]. The BNL HD2700C has a cold-field-emission electron source with high brightness and small energy spread, ideal for atomically resolved STEM imaging and EELS. The microscope has two condenser lenses and an objective lens with a gap that is slightly smaller than that of the HD2300, but with the same ±30° sample tilts capability. The projector system consists of two lenses that provide more flexibility in choosing various camera lengths and collection angles for imaging and spectroscopy. There are seven fixed and retractable detectors in the microscope. Above the objective lens is the secondary electron detector to image surface morphology of the sample. Below are the Hitachi HAADF and BF detector for STEM, and a Hitachi TV rate (30frame/sec) CCD camera for fast observations and alignment. The Gatan 14bit 2.6k×2.6k CCD camera located further down is for diffraction (both convergent and parallel illumination) and Ronchigram analysis. The Gatan ADF detector and EELS spectrometer (a specially modified high energy resolution Enfina spectrometer incorporating full 2nd and dominant 3rd order corrected optics and low drift electronics, a 16bit 100×1340 pixel CCD) are located at the bottom of the instrument. The CEOS probe corrector has been modified and optimized for this instrument.

The development and characteristics of a high-speed EELS mapping system for a dedicated STEM

A new EELS (electron energy loss spectroscopy) real-time elemental mapping system has been developed for a dedicated scanning transmission electron microscope (STEM). The previous two-window-based jump-ratio system has been improved by a three-window-based system. It is shown here that the three-window imaging method has less artificial intensity in elemental maps than the two-window-based method. Using the new three-window system, the dependence of spatial resolution on the energy window width was studied experimentally and also compared with TEM-based EELS. Here it is shown experimentally that the spatial resolution of STEM-based EELS is independent of the energy window width in a range from 10 eV to 60 eV.

Elemental, Chemical and Physical State Mapping in Three-Dimensions using EELS-SI Tomography

The EELS spectrum provides a wealth of information regarding the elemental, chemical and physical state of the material under investigation with typically nanometre resolution. When coupled with the STEM Spectrum-Imaging mode of acquisition, this information can be spatially resolved allowing properties to be calculated as two dimensional maps [1]. The ability to do this in a fast, automated manner has resulted in the EELS STEM-SI technique to become the method of choice for high resolution microanalysis in the transmission electron microscope. However, information acquired in this way is always projected in the direction parallel to the electron beam.

Uranium single atom imaging and EELS mapping using aberration corrected STEM and LN2 cold stage

Single heavy atoms on a thin carbon substrate represent a nearly ideal test specimen to evaluate STEM performance [1,2]. The single atoms approximate point scatters when imaged with the STEM large angle annular detector. (This is not necessarily true when using small angle scattering in TEM to make a phase contrast image.) The high scattering power relative to the substrate gives a high signal-to-noise ratio, even with relatively low beam current. The thinness of the sample eliminates any issues regarding depth of focus or channelling effects. The specimen was prepared in a manner similar to negative staining, except with a much lower concentration of Uranyl Acetate. The sample shown consisted of tobacco mosaic virus (TMV) on a 2nm thick carbon film substrate supported by holey film. The sample was rinsed several times with 0.01% Uranyl Acetate (compare to 2% normally used for negative staining) and air dried.

Four-dimensional STEM-EELS Tomography

Advances in electron based instrumentation are enabling multi-dimensional data acquisition to explore the unique structure-property relationships of nano-structured materials [1–3]. Here we report a technique for directly probing and analyzing a material’s three-dimensional (3D) electronic structure. A rotation holder [4] is used to vary specimen orientation and record STEM EELS spectrum images at regular angular intervals (using a Hitachi HD2300A FEG-STEM equipped with a Gatan ENFINA spectrometer). Experimental conditions were optimized to facilitate acquisition rates of over 100 spectra/second and thereby make the acquisition of such large data sets practical. By using a pillar shaped sample, and a rotation holder the electronic properties are sampled with a constant projection thickness and over a complete 180 degree rotation to minimize artefacts. Analysis software was developed to align the four-dimensional (4D) data volume, and extract the spectral properties of interest [5]. By combining energy loss information from such a series of spectrum images it is possible to map not only the microstructure, but also the elemental, physical and chemical state information of a material in three dimensions. Details and limitations of the 4D STEM-EELS acquisition will be discussed here, while the options and requirements for analysis of 4D EELS data sets will be discussed separately [5]. This technique has been applied to map the 3D properties of a variety of samples and two examples are reported here.

Interface Characterization for Vertically Aligned Carbon Nanofibers for On-chip Interconnect Applications

Nanostructure characterization of carbon nanofibers (CNFs) for on-chip interconnect applications is presented. We propose a novel technique for characterizing interfacial nanostructures of vertically aligned CNFs, optimally suited for cross-sectional imaging with scanning transmission electron microscopy (STEM). Using this technique, vertically aligned CNFs are selectively grown by plasma-enhanced chemical vapor deposition (PECVD), on a substrate comprising a narrow strip (width ~100nm) formed by focused ion beam (FIB). Using high-resolution STEM, we show that CNFs with diameters ranging from 10-100 nm exhibit very similar graphitic layer morphologies at the base contact interface