Departmental Papers (MSE)

Demonstration and Modeling of Multi-Bit Resistance Random Access Memory

Xiang Yang et al. — Wed, 27 Feb 2013 13:38:16 PST

Although intermediates resistance states are common in resistance random access memory (RRAM), two-way switching among them has not been demonstrated. Using a nanometallic bipolar RRAM, we have illustrated a general scheme for writing/rewriting multi-bit memory using voltage pulses. Stability conditions for accessing intermediate states have also been determined in terms of a state distribution function and the weight of serial load resistance. A multi-bit memory is shown to realize considerable space saving at a modest decrease of switching speed.

Simulations and Generalized Model of the Effect of Filler Size Dispersity on Electrical Percolation in Rod Networks

Rose M. Mutiso et al. — Wed, 27 Feb 2013 13:38:14 PST

We present a three-dimensional simulation of electrical conductivity in isotropic, polydisperse rod networks from which we determine the percolation threshold (ϕ_c). Existing analytical models that account for size dispersity are formulated in the slender-rod limit and are less accurate for predicting ϕ_c in composites with rods of modest L/D. Using empirical approximations from our simulation data, we generalized the excluded volume percolation model to account for both finite L/D and size dispersity, providing a solution for ϕ_c of polydisperse rod networks that is quantitatively accurate across the entire L/D range.

The Role of Confinement on Stress-Driven Grain Boundary Motion in Nanocrystalline Aluminum Thin Films

Daniel S. Gianola et al. — Wed, 27 Feb 2013 13:38:13 PST

3D molecular dynamics simulations are performed to investigate the role of microstructural confinement on room temperature stress-driven grain boundary (GB) motion for a general population of GBs in nanocrystalline Al thin films. Detailed analysis and comparison with experimental results reveal how coupled GB migration and GB sliding are manifested in realistic nanoscale networks of GBs. The proximity of free surfaces to GBs plays a significant role in their mobility and results in unique surface topography evolution. We highlight the effects of microstructural features, such as triple junctions, as constraints to otherwise uninhibited GB motion. We also study the pinning effects of impurities segregated to GBs that hinder their motion. Finally, the implications of GB motion as a deformation mechanism governing the mechanical behavior of nanocrystalline materials are discussed.

Atomic and Electronic Structure of the BaTiO₃(001) (√5×√5)R26.6° Surface Reconstruction

John Mark P. Martirez et al. — Wed, 27 Feb 2013 13:38:11 PST

This contribution presents a study of the atomic and electronic structure of the (√5×√5)R26.6° surface reconstruction on BaTiO₃ (001) formed by annealing in ultrahigh vacuum at 1300 K. Through density functional theory calculations in concert with thermodynamic analysis, we assess the stability of several BaTiO₃ surface reconstructions and construct a phase diagram as a function of the chemical potential of the constituent elements. Using both experimental scanning tunneling microscopy (STM) and scanning tunneling spectroscopy measurements, we were able to further narrow down the candidate structures, and conclude that the surface is either TiO₂-Ti_3/5, TiO₂-Ti_4/5, or some combination, where Ti adatoms occupy hollow sites of the TiO₂ surface. Density functional theory indicates that the defect states close to the valence band are from Ti adatom 3d orbitals (≈1.4 eV below the conduction band edge) in agreement with scanning tunneling spectroscopy measurements showing defect states 1.56±0.11 eV below the conduction band minimum (1.03±0.09 eV below the Fermi level). STM measurements show electronic contrast between empty and filled states’ images. The calculated local density of states at the surface shows that Ti 3d states below and above the Fermi level explain the difference in electronic contrast in the experimental STM images by the presence of electronically distinctive arrangements of Ti adatoms. This work provides an interesting contrast with the related oxide SrTiO₃, for which the (001) surface (√5×√5)R26.6° reconstruction is reported to be the TiO₂ surface with Sr adatoms.

First-Principles Calculation of the Bulk Photovoltaic Effect in Bismuth Ferrite

Steve M. Young et al. — Wed, 27 Feb 2013 13:38:10 PST

We compute the bulk photovoltaic effect (BPVE) in BiFeO3 using first-principles shift current theory, finding good agreement with experimental results. Furthermore, we reconcile apparently contradictory observations: by examining the contributions of all photovoltaic response tensor components and accounting for the geometry and ferroelectric domain structure of the experimental system, we explain the apparent lack of BPVE response in striped polydomain samples that is at odds with the significant response observed in monodomain samples. We reveal that the domain-wall-driven response in striped polydomain samples is partially mitigated by the BPVE, suggesting that enhanced efficiency could be obtained in materials with cooperative rather than antagonistic interaction between the two mechanisms.

Statistical Topology of Cellular Networks in Two and Three Dimensions

J. K. Mason et al. — Wed, 27 Feb 2013 13:38:09 PST

Cellular networks may be found in a variety of natural contexts, from soap foams to biological tissues to grain boundaries in a polycrystal, and the characterization of these structures is therefore a subject of interest to a range of disciplines. An approach to describe the topology of a cellular network in two and three dimensions is presented. This allows for the quantification of a variety of features of the cellular network, including a quantification of topological disorder and a robust measure of the statistical similarity or difference of a set of structures. The results of this analysis are presented for numerous simulated systems including the Poisson-Voronoi and the steady-state grain growth structures in two and three dimensions.

Lattice Anharmonicity in Defect-Free Pd Nanowhiskers

Lisa Y. Chen et al. — Wed, 27 Feb 2013 13:38:08 PST

We have investigated anharmonic behavior of Pd by applying systematic nanoscale tensile testing to near defect-free nanowhiskers offering a large range of elastic strain. We measured size-dependent deviations from bulk elastic behavior in nanowhiskers with diameters as small as ∼30 nm. In addition to size-dependent variations in Young’s modulus in the small strain limit, we measured nonlinear elasticity at strains above ∼1%. Both phenomena are attributed to higher-order elasticity in the bulklike core upon being biased from its equilibrium configuration due to the role of surface stresses in small volumes. Quantification of the size-dependent second- and third-order elastic moduli allows for calculation of intrinsic material nonlinearity parameters, e.g., δ. Comparison of the size-independent values of δ in our nanowhiskers with studies on bulk fcc metals lends further insight into the role of length scales on both elastic and plastic mechanical behavior.

First Principles Calculation of the Shift Current Photovoltaic Effect in Ferroelectrics

Steve M. Young et al. — Wed, 27 Feb 2013 13:38:07 PST

We calculate the bulk photovoltaic response of the ferroelectrics BaTiO₃ and PbTiO₃ from first principles by applying the “shift current” theory to the electronic structure from density functional theory. The first principles results for BaTiO₃ reproduce experimental photocurrent direction and magnitude as a function of light frequency, as well as the dependence of current on light polarization, demonstrating that shift current is the dominant mechanism of the bulk photovoltaic effect in BaTiO₃. Additionally, we analyze the relationship between response and material properties in detail. Photocurrent does not depend simply or strongly on the magnitude of material polarization, as has been previously assumed; instead, electronic states with delocalized, covalent bonding that is highly asymmetric along the current direction are required for strong shift current enhancements. The complexity of the response dependence on both external and material parameters suggests applications not only in solar energy conversion, but in photocatalysis and sensor and switch type devices as well.

Direct Determination of the Effect of Strain on Domain Morphology in Ferroelectric Superlattices with Scanning Probe Microscopy

K. Kathan-Galipeau et al. — Wed, 27 Feb 2013 13:38:06 PST

A variant of piezo force microscopy was used to characterize the effect of strain on polarization in [(BaTiO₃)_n/(SrTiO₃)_m]_p superlattices. The measurements were compared to theoretical predictions based on phase-field calculations. When polarization is constrained to be perpendicular to the substrate, the measured polarization and domain morphology agree quantitatively with the predictions. This case allows the presence of an internal electric field in the thin film to be identified. The measured trend in piezoelectric response with strain state was in qualitative agreement with predictions, and the differences were consistent with the presence of internal electrical fields. Clear differences in domain morphology with strain were observed; and in some cases, the lateral anisotropic strain appeared to influence the domain morphology. The differences in magnitude and morphology were attributed to the internal electric fields and anisotropic strains.

Thermomechanical Stability of Ultrananocrystalline Diamond

Vivekananda P. Adiga et al. — Mon, 26 Mar 2012 09:52:21 PDT

We have measured mechanical stiffness and dissipation in ultrananocrystalline diamond (UNCD) from 63 K to 450 K using microcantilever resonators in a custom ultrahigh vacuum (UHV) atomic force microscope. UNCD exhibits a temperature coefficient of modulus that is found to be extremely low: -26 ppm/K, which is close to the previously measured value of -24 ppm/K for single crystal diamond. The magnitude and the temperature dependence of dissipation are consistent with the behavior of disordered systems. The results indicate that defects, most likely at the grain boundaries, create the dominant contribution to mechanical dissipation. These measurements of modulus and dissipation versus temperature in this temperature range in UNCD establish the nanostructure’s effect on the thermomechanical stability and suggest routes for tailoring these properties.

A size-dependent nanoscale metal–insulator transition in random materials

Albert B.K. Chen et al. — Mon, 26 Mar 2012 09:52:18 PDT

Insulators and conductors with periodic structures can be readily distinguished, because they have different band structures, but the differences between insulators and conductors with random structures are more subtle. In 1958, Anderson provided a straightforward criterion for distinguishing between random insulators and conductors, based on the 'diffusion' distance ζ for electrons at 0 K (ref. 3). Insulators have a finite ζ, but conductors have an infinite ζ. Aided by a scaling argument, this concept can explain many phenomena in disordered electronic systems, such as the fact that the electrical resistivity of 'dirty' metals always increases as the temperature approaches 0 K (refs 4–6). Further verification for this model has come from experiments that measure how the properties of macroscopic samples vary with changes in temperature, pressure, impurity concentration and applied magnetic field, but, surprisingly, there have been no attempts to engineer a metal–insulator transition by making the sample size less than or more thanζ. Here, we report such an engineered transition using six different thin-film systems: two are glasses that contain dispersed platinum atoms, and four are single crystals of perovskite that contain minor conducting components. With a sample size comparable to ζ, transitions can be triggered by using an electric field or ultraviolet radiation to tune ζ through the injection and extraction of electrons. It would seem possible to take advantage of this nanometallicity in applications.

Diffusive Molecular Dynamics and its Application to Nanoindentation and Sintering

Ju Li et al. — Mon, 26 Mar 2012 09:52:14 PDT

The interplay between diffusional and displacive atomic movements is a key to understanding deformation mechanisms and microstructure evolution in solids. The ability to handle the diffusional time scale and the structural complexity in these problems poses a general challenge to atomistic modeling. We present here an approach called diffusive molecular dynamics (DMD), which can capture the diffusional time scale while maintaining atomic resolution, by coarse-graining over atomic vibrations and evolving a smooth site-probability representation. The model is applied to nanoindentation and sintering, where intimate coupling between diffusional creep, displacive dislocation nucleation, and grain rotation are observed.

Effect of Electric and Stress Field on Structures and Quantum Conduction of Cu Nanowires

C. He et al. — Mon, 26 Mar 2012 09:52:11 PDT

The ballistic transport properties of Cu nanowires under different electric and stress fields are investigated for future application in microelectronics using first-principles density-function theory. Relative to the case with the electric field only, the stability and quantum conduction of both nonhelical and helical atomic strands are enhanced by applying a stress field F. Under V = 1 V/Å, the most excellent quantum conductivity is exhibited at F = 1.5 nN for the nonhelical atomic strands while at F = 2 nN for the helical ones, and the latter is more stable with collapse-resistant F high as 3 nN compared to the former as 2 nN.

Electron localization and magnetism in SrRuO₃ with non-magnetic cation substitution

W. Tong et al. — Sun, 18 Sep 2011 20:23:42 PDT

The destruction of the ferromagnetism of alloyed SrRuO₃ can be caused by electron localization at the substitution sites. Among all the non-magnetic cations that enter the B site, Zr⁴⁺ is the least disruptive to conductivity and ferromagnetism. This is because Zr⁴⁺ does not cause any charge disorder, and its empty d electron states which are poorly matched in energy with the Ru t_2g⁴ states cause the least resonance scattering of Ru’s d electrons. Conducting Sr(Ru, Zr)O₃ may be used as an electrode for perovskite-based thin film devices, while its insulating counterpart provides unprecedented magnetoresistance, seldom seen in other non-manganite and non-cobaltite perovskites.

(Some figures in this article are in colour only in the electronic version)

Graded Interface Models for More Accurate Determination of van der Waals-London Dispersion Interactions Across Grain Boundaries

Klaus van Benthem et al. — Thu, 07 Jul 2011 08:56:35 PDT

Attractive van der Waals–London dispersion interactions between two half crystals arise from local physical property gradients within the interface layer separating the crystals. Hamaker coefficients and London dispersion energies were quantitatively determined for Σ5 and near-Σ13 grain boundaries in SrTiO₃ by analysis of spatially resolved valence electron energy-loss spectroscopy (VEELS) data. From the experimental data, local complex dielectric functions were determined, from which optical properties can be locally analyzed. Both local electronic structures and optical properties revealed gradients within the grain boundary cores of both investigated interfaces. The results show that even in the presence of atomically structured grain boundary cores with widths of less than 1 nm, optical properties have to be represented with gradual changes across the grain boundary structures to quantitatively reproduce accurate van der Waals–London dispersion interactions. London dispersion energies of the order of 10% of the apparent interface energies of SrTiO₃ were observed, demonstrating their significance in the grain boundary formation process. The application of different models to represent optical property gradients shows that long-range van der Waals–London dispersion interactions scale significantly with local, i.e., atomic length scale property variations.

Lattice Dynamics of Metal-Organic Frameworks: Neutron Inelastic Scattering and First-Principles Calculations

Wei Zhou et al. — Thu, 07 Jul 2011 08:56:32 PDT

By combining neutron inelastic scattering (NIS) and first-principles calculations, we have investigated the lattice dynamics of metal-organic framework-5 (MOF5). The structural stability of MOF5 was evaluated by calculating the three cubic elastic constants.We find that the shear modulus, c₄₄=1.16 GPA, is unusually small, while two other moduli are relatively large (i.e., c₁₁=29.42 GPa and c₁₂=12.56 GPa). We predict that MOF5 is very close to structural instability and may yield interesting phases under high pressure and strain. The phonon dispersion curves and phonon density of states were directly calculated and our simulated NIS spectrum agrees very well with our experimental data. Several interesting phonon modes are discussed, including the softest twisting modes of the organic linker.

Size-Dependent Phase Transition Memory Switching Behavior and Low Writing Currents in GeTe Nanowires

Se-Ho Lee et al. — Thu, 07 Jul 2011 08:56:29 PDT

Synthesis and device characteristics of highly scalable GeTe nanowire-based phase transition memory are reported. The authors have demonstrated reversible phase transition memory switching behavior in GeTe nanowires, and obtained critical device parameters, such as write and erase currents, threshold voltage, and programming curves. The diameter dependence of memory switching behavior in GeTe nanowires was studied and a systematic reduction of writing currents with decreasing diameter was observed, with currents as low as 0.42 mA for a 28 nm nanowire. Results show that nanowires are very promising for scalable memory applications and for studying size-dependent phase transition mechanisms at the nanoscale.

Structure and Hydrogen Bonding in CaSiD_{1+ x}: Issues About Covalent Bonding

Hui Wu et al. — Thu, 07 Jul 2011 08:56:26 PDT

We report here our high-resolution neutron powder diffraction and neutron vibrational spectroscopy study of CaSiD_1+x along with first-principles calculations, which reveal the deuterium structural arrangements and bonding in this novel alloy hydride. Both the structural and spectroscopic results show that, for x > 0, D atoms start occupying a Ca₃Si interstitial site. The corresponding Si-D bond length is determined to be 1.82 Å, fully 0.24 Å larger than predicted by theory. Thus, our neutron measurements are at odds with the strongly covalent Si-H bonding in CaSiH_1+x that such calculations suggest, a result which may have implications for a number of ongoing studies of metal-hydrogen systems destabilized by Si alloying.

Transition-Metal-Ethylene Complexes as High-Capacity Hydrogen-Storage Media

Engin Durgun et al. — Thu, 07 Jul 2011 08:56:23 PDT

with two transition metals (TM) such as Ti. The resulting TM-ethylene complex then absorbs up to ten hydrogen molecules, reaching to gravimetric storage capacity of ~ 14 wt%. Dimerization, polymerizations, and incorporation of the TM-ethylene complexes in nanoporous carbon materials are also discussed. Our results are quite remarkable and open a new approach to high-capacity hydrogen-storage materials discovery.

In Situ Nanomechanical Testing in Focused Ion Beam and Scanning Electron Microscopes

Daniel S. Gianola et al. — Wed, 15 Jun 2011 13:33:34 PDT

The recent interest in size-dependent deformation of micro- and nanoscale materials has paralleled both technological miniaturization and advancements in imaging and small-scale mechanical testing methods. Here we describe a quantitative in situ nanomechanical testing approach adapted to a dualbeam focused ion beam and scanning electron microscope. A transducer based on a three-plate capacitor system is used for high-fidelity force and displacement measurements. Specimen manipulation, transfer, and alignment are performed using a manipulator, independently controlled positioners, and the focused ion beam. Gripping of specimens is achieved using electron-beam assisted Pt-organic deposition. Local strain measurements are obtained using digital image correlation of electron images taken during testing. Examples showing results for tensile testing of single-crystalline metallic nanowires and compression of nanoporous Au pillars will be presented in the context of size effects on mechanical behavior and highlight some of the challenges of conducting nanomechanical testing in vacuum environments.