Faculty Publications from Nebraska Center for Materials and Nanoscience

PCED2.0—A computer program for the simulation of polycrystalline electron diffraction pattern

Xingzhong Li — Wed, 21 Dec 2011 09:59:04 PST

A computer program for the simulation of polycrystalline electron diffraction patterns is described. PCED2.0, an upgraded version of the previous JECP/PCED, can be used as a teaching aid and research tool for phase identification, microstructure texture analysis, and phase fraction determination. In addition to kinematical theory for diffraction intensity calculation of polycrystalline samples, Blackman two-beam dynamical correction is included. March model is used for out-of-plane and in-plane texture simulation. A pseudo-Voigt function is used for the peak profile fitting of diffraction rings. User-friendly interface is improved in the handling of experimental diffraction data and the flexibility of indexing. Application of the program for the analysis of FePt thin films is given as an example.

JECP—a Java Electron Crystallography Project

Xingzhong Li — Wed, 16 Nov 2011 13:08:49 PST

JECP stands for a java electron crystallography project that includes a series of practical java stand-alone programs for electron diffraction/microscopy and crystallography applications. The aim of the JECP project is twofold, i) as teaching tools to show students the principles of electron diffraction/microscopy and crystallography, ii) as research tools to analyze experimental results. Although there are commercial and public domain computer programs available that allow a user to simulate electron diffraction patterns or processing HREM images, there are always situations when we need to perform operations that are not a feature of any of the existing programs. The programs in the JECP can be modified and extended to meet the need in experiments.

JECP/HOLZ — An interactive computer program for simulation of HOLZ patterns

Xingzhong Li — Wed, 16 Nov 2011 11:38:08 PST

In the present work, an interactive computer program for simulation of the HOLZ line patterns, JECP/HOLZ, is reported. A feature of this program is the integration of the simulation of the HOLZ reflections and the HOLZ (curved and straight) lines, which is interactive with respect to changes of the lattice parameter, the accelerator voltage, the zone axis, and the beam tilt. The program can be used as a teaching aid for students as well as a tool for scientists working on electron diffraction experiments.

JECP/SP: A computer program for generating stereographic projections, applicable to specimen orientation adjustment in TEM experiments

Xingzhong Li — Wed, 16 Nov 2011 11:28:39 PST

In the present work, a computer program was written for generating stereographic projections and was also extended as an application for specimen orientation adjustment in TEM experiments. JECP/SP is a fully functional tool for generating stereographic projections in direct and reciprocal space. JECP/SP has been extended as an application for specimen orientation adjustment using a TEM holder. JECP/SP can be used as a teaching aid for students in crystallography as well as a practical tool for scientists performing TEM experiments.

JECP/ED — A computer program for simulation of selected-area and precession electron diffraction patterns

Xingzhong Li — Wed, 16 Nov 2011 11:18:50 PST

Crystal structure determination by electron crystallography has been very successful in many fields, ranging from organic to inorganic materials. Collection of electron diffraction data is still a crucial step in electron crystallography. The curvature of the Ewald sphere limits the data available in each electron diffraction pattern. The dynamical multi-scattering effect of electron diffraction increases the complexity of the structure determination procedure.

A precession method (Vincent & Midgley, 1994) has been used to solve the crystal structure of AlmFe by Gjonnes and coworker (Berg et al., 1998; Gjonnes et al., 1998). The advantages of precession electron diffraction are (i) the effective reduction of the dynamical multi-scattering effects, (ii) an increase in the amount of electron diffraction data due to the precession of the electron beam, (iii) the ability to obtain integrated intensity of each electron diffraction spot.

In the present work, an electron diffraction simulation program, JECP/ED, is reported. The program can be used as a teaching aid for students as well as a tool for scientists working on electron diffraction experiments.

Synthesis and characterization of highly textured Pt–Bi thin films

Xingzhong Li et al. — Wed, 16 Nov 2011 08:46:07 PST

Pt–Bi films were synthesized on glass and thermally oxidized silicon substrates by e-beam evaporation and annealing. The structures were characterized using X-ray diffraction (XRD) and transmission electron microscopy/selected area electron diffraction (TEM/SAED) techniques. Single-phase PtBi was obtained at an annealing temperature of 300°C, whereas a higher annealing temperature of 400°C was required to obtain the highly textured γ-PtBi2 phase. TEM/SAED analysis showed that the films annealed at 400°C contain a dominant γ-PtBi2 phase with a small amount of β-PtBi2 and α-PtBi2 phases. Both the PtBi and γ-PtBi2 phases are highly textured in these two kinds of film: the c-axis of the hexagonal PtBi phase is mostly in the film plane, whereas the c-axis of the trigonal γ-PtBi2 phase is perpendicular to the film plane. The electrical resistivity of the film with the γ-PtBi2 phase was smaller by one order of magnitude than that of the film with the PtBi phase.

Resistive hysteresis and interface charge coupling in BaTiO3-ZnO heterostructures

V. M. Voora et al. — Wed, 02 Dec 2009 11:40:52 PST

We report on temperature, time, and voltage dependent resistive hysteresis measurements of BaTiO₃-ZnO heterostructures grown on (001) Si substrates by pulsed laser deposition. We observe a diodelike behavior and cycling-voltage dependent hysteresis formation under forward bias. We explain these effects with depletion layer formation between the ZnO and BaTiO₃ layers, an additional barrier due to the spontaneous polarization of ZnO and the ferroelectric nature of BaTiO₃. The disappearance of the resistive hysteresis above the ferroelectric-paraelectric phase transition temperature of BaTiO₃ conformed that the hysteresis is related to the ferroelectricity of BaTiO₃. Time dependent resistance measurements reveal memory effects.

Electrical properties of ZnO–BaTiO3–ZnO heterostructures with asymmetric interface charge distribution

V. M. Voora et al. — Wed, 02 Dec 2009 11:33:11 PST

We report on capacitance-voltage, current-voltage, Sawyer–Tower, and transient current switching measurements for a ZnO–BaTiO₃–ZnO heterostructure deposited on (001) silicon by using pulsed laser deposition. The triple-layer structure reveals asymmetric capacitance- and current-voltage hysteresis and cycling-voltage dependent Sawyer–Tower polarization drift. We explain our findings by coupling of the ferroelectric (BaTiO₃) and piezoelectric (ZnO) interface charges and parallel polarization orientation of the ZnO layers causing asymmetric space charge region formation under positive and negative bias. The transient current characteristics suggest use of this structure as nonvolatile memory device.

Optical, structural, and magnetic properties of cobalt nanostructure thin films

D. Schmidt et al. — Wed, 02 Dec 2009 11:24:47 PST

We report on optical, structural, and magnetic properties of two substantially different cobalt nanostructure thin films deposited at an oblique angle of incidence of 85° away from the substrate normal. Comparison is made between an achiral columnar thin film grown without substrate rotation and a chiral nanocoil sculptured thin film by glancing angle deposition with substrate rotation. Generalized spectroscopic ellipsometry is employed to determine geometrical structure properties and the anisotropic optical constants of the films in the spectral range from 400 to 1000 nm. The magnetic properties are analyzed with a superconducting quantum interference device magnetometer. Both nanostructure thin films show highly anisotropic optical properties such as strong form birefringence and large dichroism. In particular, Co slanted columnar thin films are found to possess monoclinic optical properties. Magnetic measurements at room temperature show hysteresis anisotropy with respect to a magnetic field either parallel or perpendicular to the nanostructures’ long axis. We find extremely large coercive fields of approximately 3 kOe for our achiral columnar nanostructures.

Hole diffusion profile in a p-p+ silicon homojunction determined by terahertz and midinfrared spectroscopic ellipsometry

Tino Hofmann et al. — Tue, 01 Dec 2009 13:22:22 PST

The noncontact and nondestructive optical determination of spatial distributions of free-charge-carriers in low-doped semiconductor homo- and heterojunctions addresses fundamental physical properties of device related structures. For low-density free-charge-carriers, particularly for hole densities with their intrinsically lower mobility parameters than electron densities, optical characterization is a challenge. The carrier density plasma frequencies are located at long wavelength equivalents within the terahertz spectral region.

Annealing effects on the optical properties of semiconducting boron carbide

R. B. Billa et al. — Tue, 01 Dec 2009 13:10:11 PST

Infrared vibrations of as-deposited and annealed semiconducting boron carbide thin films were investigated by midinfrared spectroscopic ellipsometry. The strong boron-hydrogen resonance at ~2560 cm⁻¹ in as-deposited films reveals considerable hydrogen incorporation during plasma-enhanced chemical vapor deposition. Extended annealing at 600 °C caused significant reduction in film thickness, substantial reduction of boron-hydrogen bond resonance absorption, and development of distinct blue-shifted boron-carbon and icosahedral vibration mode resonances. Our findings suggest that annealing results in substantial loss of hydrogen and in development of icosahedral structure, accompanied by strain relaxation and densification.

A low-temperature specific heat study of giant dielectric constant materials

C. P. Sun et al. — Fri, 09 Oct 2009 13:25:35 PDT

A low-temperature specific heat study has been performed on the insulating giant dielectric constant material CaCu₃Ti₄O₁₂ and two related compounds, Bi_2/3Cu₃Ti₄O₁₂ and La_0.5Na_0.5Cu₃Ti₄O₁₂, from 0.6 to 10 K. From analyzing the specific heat data in a very low-temperature range, 0.6–1.5 K, and moderately low-temperature range, 1.5–5 K, in addition to the expected Debye terms, we observed significant contributions originating from the linear and Einstein terms, which we attributed as the manifestation of low-lying elementary excitations due to lattice vibrations occurring at the grain boundaries and induced by local defects. Together with the findings on electronic and mechanical properties, a phenomenological model is proposed to explain the high dielectric constant behavior in both low and high frequency regions.

Geometrical Interpretation and Simulation of HOLZ Patterns

Xingzhong Li — Fri, 11 Sep 2009 11:52:02 PDT

Higher-order Laue zone (HOLZ) pattern is very useful in measurement of crystalline lattice parameters, strain and composition variation in a small area of sample. Bloch dynamical theory on convergent-beam electron diffraction (CBED), including the formation of HOLZ lines, has been well developed and documented in Ref. and references therein. Although CBED patterns and included HOLZ lines are best modeled using a dynamical formulation, models based on a geometrical interpretation for the HOLZ-line positions have remained in widespread use because of their simplicity. Moreover, the geometrical interpretation is helpful to understand the formation of HOLZ lines for beginners. The geometrical interpretation of HOLZ lines can be carried out in either Ewald sphere construction or dispersion surface construction.

Quantitative Analysis of Polycrystalline Electron Diffraction Patterns

X. Z. Li — Fri, 11 Sep 2009 11:48:54 PDT

Image processing and computer-assisted analysis are very important in electron microscopy/crystallography, e.g., computer programs have been well developed for image processing and simulation for high-resolution electron microscopy images, and are widely used in research. Unfortunately, few computer programs have been developed for analysis and processing electron diffraction ring patterns. Facing this situation, Labar [1] developed ProcessDiffraction, a computer program to process electron diffraction patterns from polycrystalline and amorphous samples, with an aim to provides good angular resolution, quantified intensities, and reproducible identification of discontinuous and/or faint rings. For phase identification, it is to compare the generated diffractogram to the X-ray diffraction data bases. Li [2] developed JECP/PCED, a computer program for simulation of polycrystalline electron diffraction pattern and phase identification, which uses the raw or processed diffraction pattern as the input file and directly match to the simulated ring pattern. Both of the programs have proved to be fast yet accurate for phase identification.

United States Patent: Nano-Crystalline, Homo-Metallic, Protective Coatings

Fereydoon Namavar — Fri, 05 Jun 2009 07:22:19 PDT

The present invention provides orthopedic prosthesis having at least one metallic component that includes a metallic substrate on which an integrally formed nano-crystalline coating is formed. The coating and the substrate have at least one metallic constituent in common having an average atomic concentration in the coating that differs from an average atomic concentration in the substrate by less than about 10 percent. Further, the nano-crystalline coatings includes crystalline grains with an average size in a range of about 1 to 999 nanometers, and more preferably in a range of about 10 to 200 nanometers. A transition region that exhibits a graded reduction in average grain size separates the coating from the substrate. The coating advantageously exhibits an enhanced hardness, and a high degree of resistance to corrosion and wear. In one application, the nanocrystalline coatings of the invention are utilized to form articulating surfaces of various orthopedic devices.

Free Electron Behavior in InN: On the Role of dislocations and Surface Electron Accumulation

V. Darakchieva et al. — Wed, 08 Apr 2009 11:20:18 PDT

The free electron behavior in InN is studied on the basis of decoupled bulk and surface accumulation electron densities in InN films measured by contactless optical Hall effect. It is shown that the variation in the bulk electron density with film thickness does not follow the models of free electrons generated by dislocation-associated nitrogen vacancies. This finding, further supported by transmission electron microscopy results, indicates the existence of a different thickness-dependent doping mechanism. Furthermore, we observe a noticeable dependence of the surface electron density on the bulk density, which can be exploited for tuning the surface charge in future InN based devices.

Rapidly Solidified Sm–Co–V Nanocomposite Permanent Magnets

R. S. K. Valiveti et al. — Wed, 08 Apr 2009 11:20:17 PDT

Alloys around the Sm–Co eutectic composition provide an opportunity to form two-phase nanocomposite permanent magnets consisting of nanoscale Co fibers embedded in Sm₂Co₁₇.While ternary alloying elements may refine the scale of the structure, they may also disrupt the eutectic growth and lead to the formation of primary Co. Thus, microstructural selection maps were constructed for conventionally solidified Sm–Co–V alloys. It was found that V additions enlarged the primary Sm₂Co₁₇-forming region and, at (Sm_0.09Co_0.91)₉₇ V₃, resulted in a eutectic structure. Upon rapid solidification, this alloy was determined to have a coercivity of 5 kOe with a high remanent ratio. However, the V addition reduced the magnetization, which limited the energy product to 4.3 MG Oe. The rapidly solidified structure consisted of primary SmCo₇ dendrites along with an intergranular Co region, suggesting that eutectic structure formation is skewed by underlying metastable phase relationships.

Enhanced Blocking Temperature and Isothermal Control of Hysteresis Loop Shifts in Co/NiO/[Co/Pt] Heterostructures with Orthogonal Easy Axes

A. Baruth et al. — Wed, 08 Apr 2009 11:10:36 PDT

Heterostructures of Co 4 nm/NiO 1.1 nm/[Co 0.4 nm/Pt 0.6 nm] with mutually orthogonal easy axes allow for isothermal tuning of the hysteresis loop shifts along the applied field axis at room temperature, as well as displaying a greatly enhanced blocking temperature. The loop shifts can be varied up to 35 Oe through the application of moderate dc magnetic fields of 3 kOe. The presence of the [Co/Pt] multilayer with perpendicular anisotropy is responsible for a significant enhancement of the blocking temperature. For this thickness of NiO, the blocking temperature is expected to be well below 50 K, in contrast to the observed blocking temperature of 225 K. These effects may be tailored by a judicious choice of materials. The dependence of the loop shift on applied field will vary depending on the coercivity and remanance of the ferromagnetic layers. The enhancement of the blocking temperature is highly dependent on the in-plane and out-of-plane anisotropy constants of the intervening antiferromagnet. Both effects are directly relevant to a variety of modern spintronic applications.

Role of Covalent Fe-As Bonding in the Magnetic Moment Formation and Exchange Mechanisms in Iron-Pnictide Superconductors

Kirill D. Belashchenko et al. — Wed, 08 Apr 2009 11:10:35 PDT

The electronic origin of the huge magnetostructural effect in layered Fe-As compounds is elucidated using LiFeAs as a prototype. The crucial feature of these materials is the strong covalent bonding between Fe and As, which tends to suppress the exchange splitting. The bonding-antibonding splitting is very sensitive to the distance between Fe and As nuclei. We argue that the fragile interplay between bonding and magnetism is universal for this family of compounds. The exchange interaction is analyzed in real space, along with its correlation with covalency and doping. The range of interaction and itinerancy increase as the Fe-As distance is decreased. Superexchange makes a large antiferromagnetic contribution to the nearest-neighbor coupling, which develops large anisotropy when the local moment is not too small. This anisotropy is very sensitive to doping.

Creating Micro- and Nanostructures on Tubular and Spherical Surfaces

O. Lima et al. — Wed, 08 Apr 2009 11:10:33 PDT

The authors developed a new technique to create micro- and nanometer scale structures on curved free-standing objects by combining embossing/imprinting lithography approaches with mechanical loadings on elastic films. Embossing/imprinting generates small structures and mechanical loading determines shape or geometry of the final object. As a result, a portion of the tubes with a radius between 0.5 and 3.5 mm and a portion of the spheres with a radius between 2.4 and 7.0 mm were fabricated with grating line features (period of 700 nm) and microlens array features (lens radius of 2.5 µm) atop, respectively. It was found that both static analyses and finite element models can give good estimates on the radii of those curved objects, based on the dimension of the two layers, loading format, as well as mechanic strains. Thus, good control over shape and dimension of the free-standing structure can be achieved.