Cross-sectional Electron Microscopy Research Articles

Nanoscale stress distributions and microstructural changes at scratch track cross-sections of a deformed brittle-ductile CrN-Cr bilayer

In order to interpret the mechanical response of thin films subjected to scratch tests, it is necessary to elucidate local stress distributions and microstructural changes accompanying deformation across the scratch track area. Here, 50 nm synchrotron cross-sectional X-ray nanodiffraction and electron microscopy are used to characterize nanoscale multiaxial residual stress gradients and irreversible microstructural-morphological changes across a brittle-ductile film consisting of 1.2 and 2 μm thick CrN and Cr sublayers. The experimental results reveal a complex alternation of the original columnar grain microstructure and a formation of pronounced lateral and depth stress gradients, which are complemented by a finite element model. After scratching, steep gradients of in-plane, out-of-plane and shear stress distributions were revealed, ranging from −6 to 1.5 and − 1.5 to 1.5 GPa in CrN and Cr, respectively, which are furthermore correlated with microstructural changes and residual curvatures. The scratch test results in intergranular grain sliding and the formation of nanoscopic intragranular defects within CrN, while Cr sublayer's thickness reduction and pile-up formation are accompanied by a bending of columnar crystallites and localized plastic deformation. In summary, the quantitative stress data elucidate the stabilizing role of the Cr sublayer, which suppresses the bilayer's catastrophic fracture during scratch tests.

Fabrication of Germanium-on-insulator in a Ge wafer with a crystalline Ge top layer and buried GeO2 layer by oxygen ion implantation

The paper reports fabrication of Germanium-on-Insulator (GeOI) wafer by Oxygen ion implantation of an undoped Ge wafer of orientation (100). O+ ions (energy 200 keV) were implanted to a fluence of 1.9 × 1018 ions-cm−2 and the implanted wafer was subjected to Rapid Thermal Annealing. The resulting wafer has a top crystalline Ge layer of ~ 220 nm thickness and resistivity ≈32 Ohm-cm and a buried Oxide layer (BOX) of crystalline GeO2 (thickness ≈0.62 μm). The crystalline GeO2 layer has hexagonal crystal structure with lattice constants close to the standard values. Raman Spectroscopy and cross-sectional electron microscopy established that the top Ge layer was recrystallized during annealing with a residual tensile strain of around +0.4% and an estimated dislocation density of 2.7 × 107cm−2. The crystallinity and electrical characteristics of the top layer and the quality of the BOX layer are such that it can be utilized for device fabrication.

Structure and morphology of CrSi<sub>2</sub> layers formed by rapid thermal treatment

The formation of chromium disilicide layers on n-type single crystal silicon substrates (111) during rapid thermal annealing in heat balance mode by the methods of Rutherford backscattering, X-ray diffraction and transmission electron microscopy of cross sections was investigated. Chromium films of about 30 nm thickness were deposited by magnetron sputtering of a chromium target with argon ions onto silicon substrates at room temperature. The rapid thermal treatment was carried out in a temperature range of 200 to 550 °C in a heat balance mode by irradiating the substrates backside with a non-coherent light flux of quartz halogen lamps in a nitrogen ambient for 7 s. It was established that hexagonal phase of chromium disilicide formation with grain size of 150–300 nm occurs in a threshold manner when the temperature of rapid thermal treatment exceeds 400 °C. At the same time, there are strong changes in the films surface morphology and surface roughness, and a silicide-silicon interface occur. In this case the wavy film surface morphology practically repeats silicide-silicon interface morphology (the surface exactly replicates the interface). The mechanism of CrSi2/Si interface structure roughness formation based on consideration of Kirkendall effect and deformation-stimulated diffusion of vacancies is proposed and discussed. The research results of the structure and morphology of CrSi2 layers on silicon are well-correlated with the results of the Schottky barrier electrophysical measurements. The results obtained can be used in microelectronics for forming rectifying contacts and interconnects metallization for integrated circuits, as well as for thermoelectric and optoelectronic applications.

Metals by Micro-Scale Additive Manufacturing: Comparison of Microstructure and Mechanical Properties.

Many emerging applications in microscale engineering rely on the fabrication of 3D architectures in inorganic materials. Small-scale additive manufacturing (AM) aspires to provide flexible and facile access to these geometries. Yet, the synthesis of device-grade inorganic materials is still a key challenge toward the implementation of AM in microfabrication. Here, a comprehensive overview of the microstructural and mechanical properties of metals fabricated by most state-of-the-art AM methods that offer a spatial resolution ≤10 μm is presented. Standardized sets of samples are studied by cross-sectional electron microscopy, nanoindentation, and microcompression. It is shown that current microscale AM techniques synthesize metals with a wide range of microstructures and elastic and plastic properties, including materials of dense and crystalline microstructure with excellent mechanical properties that compare well to those of thin-film nanocrystalline materials. The large variation in materials' performance can be related to the individual microstructure, which in turn is coupled to the various physico-chemical principles exploited by the different printing methods. The study provides practical guidelines for users of small-scale additive methods and establishes a baseline for the future optimization of the properties of printed metallic objects-a significant step toward the potential establishment of AM techniques in microfabrication.

Determination of the Complex Dielectric Function of Ion-Implanted Amorphous Germanium by Spectroscopic Ellipsometry

Accurate reference dielectric functions play an important role in the research and development of optical materials. Libraries of such data are required in many applications in which amorphous semiconductors are gaining increasing interest, such as in integrated optics, optoelectronics or photovoltaics. The preparation of materials of high optical quality in a reproducible way is crucial in device fabrication. In this work, amorphous Ge (a-Ge) was created in single-crystalline Ge by ion implantation. It was shown that high optical density is available when implanting low-mass Al ions using a dual-energy approach. The optical properties were measured by multiple angle of incidence spectroscopic ellipsometry identifying the Cody-Lorentz dispersion model as the most suitable, that was capable of describing the dielectric function by a few parameters in the wavelength range from 210 to 1690 nm. The results of the optical measurements were consistent with the high material quality revealed by complementary Rutherford backscattering spectrometry and cross-sectional electron microscopy measurements, including the agreement of the layer thickness within experimental uncertainty.

Physico-mechanical behaviour of Oil Palm Broom Fibres (OPBF) as eco-friendly building material

Until recently, the rib of the leaflets of the oil palm tree was only used for making brooms due to its stiffness and durability. However, the mechanical properties of this fibre are unknown. Due to the geometrical variation of the cross-section of the fibres along their length, this study divided them into 4-categories. The result of this study reveals that the fibres have a specific gravity between 0.45-0.84 and diameter varying between 0.20 mm (at the tail) and 4.00 mm (at the cap). Maximum tensile strength of 900 MPa was recorded. Scanning electron microscopy of fibre cross-sections revealed graded cavities concentrated at the core but a densely packed cortex. This radial and longitudinal density gradient is responsible for the phenomenon whereby the fibres are stiffer in bending but possess reduced tensile strength towards the cap. Further investigations carried out on the fibres include water absorption, chemical composition and thermogravimetric analysis. The fibre is proposed for use as natural-fibre reinforcement in cement and polymeric composites as it is cheap, and void of high carbon footprints associated with the use of conventional reinforcement materials in construction.

Studies on growth mechanism of intermediate layer of (Mo,W)5Si3 and interdiffusion in the (Mo,W)-(Mo,W)Si2 system prepared by pack cementation coating

Silicide based oxidation resistant coatings were developed over Mo-30 W wt.% alloy using halide activated pack cementation process. Formation of single-layer [only (Mo,W)Si2] and double-layer [consisting of (Mo,W)Si2 outer and (Mo,W)5Si3 intermediate] coatings were developed below and above a critical coating temperature of 1100 °C using a pack mixture having a composition of 20Si-2.5NH4F-77.5Al2O3 (wt.%). The activation energy for the growth of (Mo,W)Si2 coating layer was found to be 51 kJ/mole. The diffusion mechanism of the species in (Mo,W)-(Mo,W)Si2 system (using single layer coated samples) was studied by conducting isothermal annealing experiments at different temperatures of 700, 800 and 1000 oC for a period of 1000 h. The growth rate of the (Mo,W)5Si3 interlayer during the isothermal annealing treatment was found to be parabolic. The mechanism of the formation of (Mo,W)5Si3 intermediate layer, its microstructure and growth kinetics in (Mo,W)-(Mo,W)Si2 system were investigated by cross-sectional electron microscopy and EBSD analysis. The growth mechanism of the intermediate layer was further discussed using a Physico-chemical approach.

Atomic diffusion bonding of Si wafers using thin Nb films

This study of atomic diffusion bonding of Si wafers in vacuum using thin Nb films reveals remarkably high bonding strength, greater than fracture strength of a Si wafer, obtained using Nb films at thicknesses of 1.7–20 nm on each side. Transmission electron microscopic cross-section images show no vacancy at the bonded interface. Particularly, crystal lattice rearrangement occurs to a remarkable degree at the bonded interface of 10 nm thick Nb films. Surface roughness reduction is likely to enhance the crystal lattice rearrangement at the bonded interface, even with thick Nb films.

Is a Zirconia Dental Implant Safe When It Is Available on the Market?

The market share of zirconia (ZrO2) dental implants is steadily increasing. This material comprises a polymorphous character with three temperature-dependent crystalline structures, namely monoclinic (m), tetragonal (t) and cubic (c) phases. Special attention is given to the tetragonal phase when maintained in a metastable state at room temperature. Metastable tetragonal grains allow for the beneficial phenomenon of Phase Transformation Toughening (PTT), resulting in a high fracture resistance, but may lead to an undesired surface transformation to the monoclinic phase in a humid environment (low-temperature degradation, LTD, often referred to as ‘ageing’). Today, the clinical safety of zirconia dental implants by means of long-term stability is being addressed by two international ISO standards. These standards impose different experimental setups concerning the dynamic fatigue resistance of the final product (ISO 14801) or the ageing behavior of a standardized sample (ISO 13356) separately. However, when evaluating zirconia dental implants pre-clinically, oral environmental conditions should be simulated to the extent possible by combining a hydrothermal treatment and dynamic fatigue. For failure analysis, phase transformation might be quantified by non-destructive techniques, such as X-Ray Diffraction (XRD) or Raman spectroscopy, whereas Scanning Electron Microscopy (SEM) of cross-sections or Focused Ion Beam (FIB) sections might be used for visualization of the monoclinic layer growth in depth. Finally, a minimum load should be defined for static loading to fracture. The purpose of this communication is to contribute to the current discussion on how to optimize the aforementioned standards in order to guarantee clinical safety for the patients.

Double replication for characterizing cracks in surface-hardened polydimethylsiloxane

Surface-hardened polydimethylsiloxane (PDMS) with random cracks is studied by means of double replication. The PDMS samples are prepared under different curing conditions, resulting in a different Young's modulus of the bulk. To modify the surface, an excimer lamp at 172 nm is used. The samples are stretched uniaxially until the first cracks appear. As an evaluation under strain is hard or rather impossible, the idea is to replicate the cracked sample in a curable material and to characterize the cracks by inspection of the replica. To protect the sample from mechanical loading, this replication is done by molding in OrmoStamp on glass; these replicas are used for optical inspection to determine the crack spacing. As a characterization of the depth and width of the cracks is highly facilitated when cleaving is enabled, a second replication is performed into a thin layer of SU-8 on Si; these second replica are analyzed by secondary electron microscopy of cross sections. They provide a realistic picture of the crack shape. It is found that the curing conditions affect the crack shape; a U-shape occurs with a low bulk modulus, whereas a V-shape occurs with a high bulk modulus. The parameters width, depth, and spacing are largely unaffected by the curing conditions. This work provides a background to understand the behavior of random cracks, which is, e.g., useful to design a system with controlled cracks that remain stable. An important finding is that as soon as cracking occurs at a certain strain, already a number of cracks develop, yet without any further stretching. The cracks behave independently from neighboring cracks. Upon further stretching, new cracks develop, and the crack width and depth remain similar. Therefore, these random cracks are stable, and sample failure does not occur up to a strain of at least 40%.

Effect of Copper on the Localized Corrosion and Stress Corrosion Resistance of High Strength Aluminum Alloys

High strength to weight ratio of the 7xxx aluminum alloys offer an exciting alternative to advanced high strength steels in automotive structural applications. Recently, Novelis has developed AdvanzTM 7000 series alloys that not only has higher strength to weight ratio but also improved localized corrosion resistance compared to conventional 7075. In this work, we have studied the effect of copper content on the stress corrosion cracking resistance of the AdvanzTM 7000 in peak-age (T6) conditions. ASTM G110 test showed pitting corrosion morphology in both low and high copper alloys compared to intergranular corrosion in 7075. Electrochemical polarization showed that increase in copper content increased the cathodic activity and pushed the free corrosion potential in noble direction. Slow strain rate tests were carried out according to ASTM G129 in ASTM D1384 solution containing hydrogen peroxide and in silicone oil. The fractured samples were subjected to scanning electron microscopy (SEM) and metallography cross section. These results indicated that the SCC cracks were initiated from the pitting corrosion sites in AdvanzTM 7000, while the intergranular corrosion site acted as crack nucleation site for 7075. The failure mechanism is discussed in terms of shift in corrosion morphology and the role of Al7Cu2Fe particles that had increased with the copper content of the alloys.

Transmission Electron Microscopy Study of Epitaxial Li-Mn-O Films Grown by Pulsed Laser Deposition: The Effect of Temperature on Formation of Phases

Abstract A simple system of epitaxial film electrodes with no additives is desirable for investigation surface reactions and anisotropic electrochemical properties of battery's active materials. In this work we demonstrate the possibility of growing epitaxially by pulsed laser deposition single orientation thin films of two phases, orthorhombic o-LiMnO 2 and monoclinic m-Li 2 MnO 3 ; the phases were and still are of great interest for developing high capacity intercalation cathodes for Li-ion batteries. Orientation of the films was controlled by selecting orientation of SrTiO 3 substrate covered coherently with a layer of SrRuO 3 to provide electrical contact for future electrochemical measurements. Different phases were formed by adjusting temperature of a substrate during deposition, 600 °C for forming m-Li 2 MnO 3 , and 700–800 °C – for o-LiMnO 2 . Detailed high-resolution scanning transmission electron microscopy of cross-sections has determined the unique orientation relationship for both phases, which can be seen as cube-on-cube of cubic and pseudo-cubic oxygen sub-lattices of SRO and Li-Mn-O phases, respectively. Lower symmetry of the Li-Mn-O phases results in formation of structural domains, which are analyzed in detail.

Germanium Incorporation in Silicon Carbide Epitaxial Layers Using Molecular Beam Epitaxy on 4H-SiC Substrates

Ternary (Si1-xCy)Gex+y solid solutions were grown on Si-face 4H-SiC applying atomic layer molecular beam epitaxy at low temperatures. The grown layers consist of twinned 3C-SiC revealed by cross section electron microscopy. The germanium was incorporated on silicon lattice sites as revealed by atomic location by channeling enhanced microanalysis transmission electron microscopy studies. The Ge concentration of the grown 3C-(Si1-xCy)Gex+y heteroepitaxial layers decreases with increasing growth temperatures, but exceeds the solid solubility limit.

Immunity to Contact Scaling in MoS2 Transistors Using in Situ Edge Contacts.

Atomically thin two-dimensional (2D) materials are promising candidates for sub-10 nm transistor channels due to their ultrathin body thickness, which results in strong electrostatic gate control. Properly scaling a transistor technology requires reducing both the channel length (distance from source to drain) and the contact length (distance that source and drain interface with semiconducting channel). Contact length scaling remains an unresolved epidemic for transistor scaling, affecting devices from all semiconductors-silicon to 2D materials. Here, we show that clean edge contacts to 2D MoS2 can provide immunity to the contact-scaling problem, with performance that is independent of contact length down to the 20 nm regime. Using a directional ion beam, in situ edge contacts of various metal-MoS2 interfaces are studied. Characterization of the intricate edge interface using cross-sectional electron microscopy reveals distinct morphological effects on the MoS2 depending on its thickness-from monolayer to few-layer films. The in situ edge contacts also exhibit an order of magnitude higher performance compared to the best-reported ex situ metal edge contacts. Our work provides experimental evidence for a solution to contact scaling in transistors, using 2D materials with clean edge contact interfaces, opening a new way of designing devices with 2D materials.

ZnO/Si nanowires heterojunction array-based nitric oxide (NO) gas sensor with noise-limited detectivity approaching 10 ppb

We report a ZnO/Silicon nanowire (ZnO/Si NWs) heterojunction array-based NO gas sensor operating at room temperature with an extremely high response (noise limited response ∼10 ppb). The sensor shows very high selectivity towards NO gas sensing and limited perturbation in response due to the presence of moisture. The sensor has been fabricated by using cost-effective chemical processing that is compatible with wafer-level processing. The vertically aligned Si NWs array has been made by an electroless etching method and the ZnO nanostructure was made by chemical solution deposition and spin-coating. Extensive cross-sectional electron microscopy and composition analysis by line EDS allowed us to make a physical model. The electrical characteristic of the model was to fit the I–V data before and after exposure to gas and essential changes in electrical parameters were obtained. This was then explained based on a proposal for the mechanism of gas sensing. We observe that the heterostructure leads to a synergetic effect where the sensing response is more than the sum total of the individual components, namely the ZnO and the Si NWs. The response is much enhanced in the p-n junction when the n-ZnO nanostructure interfaces with p-Si NW compared to that in the n-n junction formed by ZnO on n-Si NW.

An unusual disordered alveolar bone material in the upper furcation region of minipig mandibles: A 3D hierarchical structural study

Teeth are subjected to compressive loads during mastication. Under small loads the soft tissue periodontal ligament (PDL) deforms most. However when the loads increase and the PDL is highly compressed, the tooth and the alveolar bone supporting the tooth, begin to deform. Here we report on the structure of this alveolar bone in the upper furcation region of the first molars of mature minipigs. Using light microscopy and scanning electron microscopy (SEM) of bone cross-sections, we show that this bone is hypermineralized, containing abundant small pores around 1–5 μm in diameter, lacunae around 10–20 μm as well as larger spaces. This bone does not possess the typical lamellar motif or other repeating structures normally found in cortical or trabecular mammalian bone. We also use high resolution focused ion beam scanning electron microscopy (FIB-SEM) in the serial surface mode to image the 3D organization of the demineralized bone matrix. We show that the upper furcation bone matrix has a disordered isotropic structure composed mainly of individual collagen fibrils with no preferred orientation, as well as highly staining material that is probably proteoglycans. Much larger aligned arrays of collagen fibers – presumably Sharpey’s fibers – are embedded in this material. This unusual furcation bone material is similar to the disordered material found in human lamellar bone. In the upper furcation region this disordered bone comprises almost all the volume excluding Sharpey’s fibers. We surmise that this most unusual bone type functions to resist the repeating compressive loads incurred by molars during mastication.

Influence of normal velocity on microstructure and density of films produced by nanoparticle impact

We describe the use of the laser ablation of microparticle aerosol process to deposit patterned thick films of Ag by impacting nanoparticles with a mean size of 6 nm at high velocities and systematically study the effects of varying the nanoparticle impact velocity, while fixing the nanoparticle size. A positive relationship between impact velocity and relative density of the deposits is observed, but it is shown that the geometry of the deposits also influences the deposit density, with taller, narrower deposits resulting in lower relative densities. Scanning electron microscopy of film cross-sections shows that there are spatial variations in the relative density, with lower densities near the deposit surfaces. Using molecular dynamics simulations of the impact process for 6 nm particles, it is shown that a critical normal impacting velocity of 600-750 m/s exists for obtaining dense deposits. Normal velocities less than the critical velocity do not result in sufficient plastic deformation to fill interstices between the impacted particles. A geometric model based on this critical normal velocity and the relative deposit height is developed to explain the influence of relative heights and impacting velocity on porosity. Good qualitative agreement between the geometric model and experiments is demonstrated.

Microstructural Characteristics of Vehicle-Aged Heavy-Duty Diesel Oxidation Catalyst and Natural Gas Three-Way Catalyst

Techniques to control vehicle engine emissions have been under increasing need for development during the last few years in the more and more strictly regulated society. In this study, vehicle-aged heavy-duty catalysts from diesel and natural gas engines were analyzed using a cross-sectional electron microscopy method with both a scanning electron microscope and a transmission electron microscope. Also, additional supporting characterization methods including X-ray diffractometry, X-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy and catalytic performance analyses were used to reveal the ageing effects. Structural and elemental investigations were performed on these samples, and the effect of real-life ageing of the catalyst was studied in comparison with fresh catalyst samples. In the real-life use of two different catalysts, the poison penetration varied greatly depending on the engine and fuel at hand: the diesel oxidation catalyst appeared to suffer more thorough changes than the natural gas catalyst, which was affected only in the inlet part of the catalyst. The most common poison, sulphur, in the diesel oxidation catalyst was connected to cerium-rich areas. On the other hand, the severities of the ageing effects were more pronounced in the natural gas catalyst, with heavy structural changes in the washcoat and high concentrations of poisons, mainly zinc, phosphorus and silicon, on the surface of the inlet part.

Effect of Mesoporous TiO2 Thicknesses on the Performance of Solid-State Dye-Sensitized Solar Cells

We report the fabrication of solid-state dye-sensitized solar cells (ss-DSSCs), using a metal-free organic dye (D102) as a sensitizer. Mesoporous TiO2 acting as a photoanode, is prepared from TiO2 nanopaste followed by drafting light-absorbing dye molecules. Spiro-OMeTAD is used as a hole transport material (HTM), which has a potential role in the energy conversion process of solid-state dye-sensitized solar cells (ss-DSSCs). Here, TiO2 mesoporous films are used with three different thicknesses (∼ 1.5 μm, ∼ 1.7 μm and ∼ 2.0 μm) for the device fabrication. Various characterizations of optimum TiO2 porous film (with the thickness of ∼ 2.0 μm) are performed such as ultraviolet–visible (UV–Vis) transmission, x-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and cross-section scanning electron microscopy (SEM). Transmittance analysis reveals that transparency of the TiO2 films ranges from 70% to 80%. The x-ray diffraction (XRD) pattern of TiO2 shows anatase as its major crystalline phase. The porous structure formation of the TiO2 film is confirmed by scanning electron microscopy (SEM) cross section analysis. Photovoltaic performance of the devices was examined in air. Higher power-conversion efficiency (PCE) of 3.5% is obtained with optimum device thickness (∼ 2.3 μm). The device stability test is performed under continuous illumination for 2 h, showing slightly good air and light stability.

Ordered Mesoporous Zeolite Thin Films with Perpendicular Reticular Nanochannels of Wafer Size Area

Highly ordered mesoporous zeolite thin films (MZTFs) are grown at rigid substrates via molecular interaction from ternary structure-directing agents (SDAs) in a Stober-like condition. The ultrathin films (10–20 nm) were deposited layer-by-layer via self-assembly of β-zeolite seeds and SDAs at 40–50 °C, resulting reticular cellular pores and perpendicular nanochannels with average pore sizes (6.7 ± 1.1 nm). The walls of the nanochannels consist of zeolitic structures in microcrystallines. Compared to cylindrical mesochannels of amorphous silica thin films, the MZTFs with reticular mesochannels demonstrate 5R/6R stretching modes (550–650 cm–1) and significantly enhanced hydrothermal stability. The MZTFs were characterized in top-view and cross-sectional electron microscopies (SEM/HRSEM/HRTEM) as well as X-ray scattering techniques (GIXRD/GISAXS), which confirm a body-centered tetragonal (Im3m) unit cell with unique Moire patterns. Upon calcination, the film thickness is reduced to 10.6 nm with a structure ...