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Since the introduction of the dual beam system, many active research projects have been accelerated at Missouri S&T, e.g.

(1) Epitaxial electrodeposition of Fe₃O₄/Zn superlattices;

(2) Focused ion beam for micromachining and nanodevices;

(3) Multi-energy laser processing;

(4) Environmentally benign corrosion coatings;

(5) Nanostructured multi-layer capacitors;

(6) Platinum catalysts dispersion on the carbon nanotubes. 

Moreover, the Oxford Energy Dispersive Spectrometer (EDS) and the HKL Electron Backscatter Diffraction (EBSD) system were installed onto the Helios 600 dual beam instrument in March 2009.  This update would further improve research capabilities for composition and orientation analyses at Missouri S&T.

 

1.      Epitaxial Electrodeposition of Zn/ Fe₃O₄ Superlattices on Single –Crystal Au (111)

 

Fig. 1 Cross-sectional STEM image of Zn/Fe₃O₄ superlattices on Au (111).

 

Tailoring structures at the atomic level and nanoscale to specific functions relevant to the engineering of material properties is one of the fundamental challenges in materials research.  The aim of the Dr. Stwitzer’s group at the Missouri S&T is to probe ordered nanostructures of metal oxides with tunable optical, electrical, or magnetic properties.  Because Fe₃O₄ is a ferromagnetic oxide of great interest for use in spintronic devices, recent work has focused on Fe₃O₄/Zn superlattices grown on single crystal gold (111) substrate.  Gold (111) substrate has good oxidation stability and excellent expitaxial compatibility with cubic Fe₃O₄ lattice and hexagonal Zn lattice.  The advanced dual beam Helios 600 system is a critical tool to prepare cross-sections of epitaxial films or superlattices in the site-specific and orientation-specific areas.  For example, the orientation relationship between the electrodeposited films or suerplattices and Au (111) substrate was predetermined by X-ray diffraction (XRD).  Then, the state-of-the-art focused ion beam was used to prepare a cross-sectional TEM specimen along Au [110] direction.  Figure 1 is a cross-sectional bright field STEM image with incident electron beam parallel to the [112] direction of Au substrate.  Thickness of the present ZITO film is directly measured to be about 2 μm.  The interface between Fe₃O₄/Zn superlattices and Au substrate is sharp and flat.  Columnar structures and domain structures can be found in the film.   Next step, we plan to further study the Fe₃O₄/Zn superlattices using the Helios 600 dual beam system with EDS/EBSD.   With the increased modulation wavelength of 50-100 nm, it is expected to distinguish each Fe₃O₄ and Zn layer using EDS line scan or EDS mapping.

 

2.      Focused ion beam lithography for micromachining and nanodevices

 

Fig. 2 Patterned ion beam lithographic images.

 

Fig. 3 High resolution SEM images of laser-assisted patterning at the micron level.

 

FIB lithography is becoming increasingly important in the area of nanoprototyping and nanofrabrication with the advantage of a maskless process and extremely high flexibility.  Pattern generation software with graphic bitmap file importability in the dual beam system allows the users to mill complex structures.  Pixel dimension in the bitmap image defines the beam location and color value of each pixel defines the beam dwell time and beam blanking.  Fig. 2 (a) shows an example of FIB milled structure from a bitmap file of a cross-grid pattern.  Fig. 2 (b) is the corresponding high magnification FIB image showing the minimum line width of FIB milling is measured to be around 50 nm.  This patterned catalyst layer were then used to guide nanostructure growth on top of it. 

Another project is related to tiny lab-on-a-chip device fabrication, that is, many functional components such as microfludics, microoptics, microelectronics, and micromechanics are assembled on a chip.  FEI Helios 600 dual beam system was used to check the pattern quality and surface smoothness of three-dimensional micro cylindrical and hemispherical lens arrays fabricated on Foturan glasses using a home-integrated femtosecond laser micromachining system.  Fig. 3 is the SEM images of microoptical (a) cylindrical and (b) hemispherical lenses with around 100 μm in diameters.  It should be noted that both the spherical and the hemispherical patterns were fabricated simultaneously in a single process.  The smooth surfaces and decent laser-machined arrays indicate that dual beam SEM/FIB system is an efficient method to optimize the laser processing parameters for high accuracy and reproducibility of material ablation. 

 

3.      Multi-energy laser processing

 

Fig. 4 (a) high resolution top-view SEM image of polycrystalline diamond-like carbon (DLC) film; (b) A lift-out cross-section TEM specimen welded on the top of a copper grid; (c) cross-sectional dark field STEM image of the polycrystalline DLC film on single-crystal Si substrate; (d) cross-sectional dark field STEM image of the polycrystalline DLC film on polycrystalline Tungsten Carbide (WC) substrate.

 

Because of many attractive properties such as extremely high hardness, low friction, chemical inertness, wear resistance, and biological compatibility, diamond-like carbon (DLC) coatings have drawn almost unparalleled attention both economically and technically in a number of years.  The objective of this project is to economically deposit hard DLC coatings onto Tungsten Carbide (WC) or Silicon substrates using multiple laser sources at ambient environments.  FEI Helios 600 dual beam system was used as a critical tool to expose cross sections of hard coatings on WC or Si substrate.  Exposed cross sections prepared by FIB are very helpful to characterize crystal structure, morphology, grain size, chemical composition, uniformity of the films and understand their relationship with crystal nucleation and growth characteristics.  Two DLC hard coatings were deposited at a low deposition rate of around 5 micrometers per hour on the WC and Si substrates, respectively.  On both substrates, polycrystalline diamond films were found as shown in Fig. 4.  Fig. 4 (a) is a high resolution top-view SEM image of polycrystalline diamond-like carbon (DLC) film. Very large grains of single crystal material, up to a micrometer in size can be seen.  A typical lift-out cross-section TEM specimen welded on the top of a TEM copper grid was shown in Fig. 4(b).  Fig. 4 (c) and (d) are the cross-sectional morphologies of the polycrystalline DLC films on single-crystal Si substrate and on polycrystalline WC substrate. 

 

4.      Environmentally benign corrosion coatings

 

Fig. 5 Site-specific cross-sectioning and in situ high resolution SEM imaging of Cerium oxide coatings on (a) Magnesium and (b) Aluminum substrates with the sample stage tilted at 45 degree.

 

Although chromate conversion coatings have been widely used to provide corrosion protection for aluminum or magnesium alloy components on air craft or light vehicles, many attempts have been made to develop non-toxic, environmentally friendly corrosion inhibitors based on rare-earth and other benign compounds because hexavalent chromium (Cr⁶⁺) is a known carcinogen which can unfortunately form many toxic compounds.  This project focuses on cerium-based conversion coatings.  FEI dual beam system was used to characterize the coating thickness, surface morphology and cross-sectional microstructures.  Fig. 5 is the preliminary results of cerium oxide coatings on magnesium and aluminum substrates.  The coatings appeared to completely cover the surface of the panel but with many cracks.   Coating thicknesses can be easily calculated from the cross-sectional SEM image with the sample stage tilted at 45 degree.  Many cavities were found in the alloy substrates just beneath the coatings.  They appeared to have a close relationship with the coating cracks.  Further comparative experiments will make full use of dual beam system to expose cross sections of cerium oxide coatings at specific positions with different process parameters such as substrate treatment and the number of spray deposition cycles.   The detailed studies of cross-sectional cerium oxide coatings using dual beam high resolution SEM and FIB analysis will shed a light on the protection mechanisms and help us have a more fundamental knowledge of the coating systems as well as novel corrosion-against coating development.

 

5.      Nanostructured multi-layer capacitors

 

Fig. 6 Cross-sectional STEM image of BaTiO₃/Ni multi-layers grown on SiO2/Si substrate.

 

As part of the NSF supported center for studies between Pennsylvania State University and Missouri S&T, the project objective is to establish processing-structure-property relationships in the dielectric material systems.  Focused ion beam was used as a very unique instrument to locate the crossing position, expose the cross-sections of multilayers, and create a cross-sectional specimen for characterization in a high resolution transmission electron microscope (TEM).  As shown in Figure 6, three layers of BaTiO₃ with 100-150 nm thickness in each layer and two layers of Ni electrodes approximately 20 nm in thickness were deposited on Si/SiO₂ wafers. 

 

6.      Platinum catalysts dispersion on the carbon nanotubes

 

Fig. 7 (a) Pt nano-particles doped carbon nanotubes;  (b) High resolution dark field STEM image taken at the magnification of 1,500,000 times.

 

The primary goal of this NSF-supported project is to develop nanostructured electrodes for proton exchange membrane fuel cells.  Catalyst Pt nano-particle deposition on carbon nanotubes is a very important step in nanodevice fabrication for energy conversion in micro fuel cells.  Dark field scanning transmission electron microscopy (STEM) was carried out on the FEI Helios 600 dual beam system at an operating voltage of 30 kV to characterize the grain size, morphology, and dispersivity of Pt nano-paricles because their microstructures and interactions with supported surfaces of the carbon nanotubes have a great influence on the catalytic activity.  Fig. 7 (a) is a typical dark field STEM image of the carbon nanotube supported Pt catalysts.  Well-dispersed Pt nano-particles can be found with spherical morphologies.  It should be noted in Fig. 7 (b) that diameters of the carbon nanotube and the Pt particle can be directly measured to be around 15 nm and 2 nm, respectively when the STEM image was zoomed into the magnification of 1,500,000 times.