Thermodynamic properties and switching dynamics of perpendicular shape anisotropy MRAM.

The power consumption of modern random access memory (RAM) has been a motivation for the development of low-power non-volatile magnetic RAM (MRAM). Based on a CoFeB/MgO magnetic tunnel junction, MRAM must satisfy high thermal stability and a low writing current while being scaled down to a sub-20 nm size to compete with the densities of current RAM technology. A recent development has been to exploit perpendicular shape anisotropy along the easy axis by creating tower structures, with the free layers' thickness (along the easy axis) being larger than its width. Here we use an atomistic model to explore the temperature dependent properties of thin cylindrical MRAM towers of 5 nm diameter while scaling down the free layer from 48 to 8 nm thick. We find thermal fluctuations are a significant driving force for the switching mechanism at operational temperatures by analysing the switching field distribution from hysteresis data. We find that a reduction of the free layer thickness below 18 nm rapidly loses shape anisotropy, and consequently stability, even at 0 K. Additionally, there is a change in the switching mechanism as the free layer is reduced to 8 nm. Coherent rotation is observed for the 8 nm free layer, while all taller towers demonstrate incoherent rotation via a propagated domain wall.

Ultraviolet-Ozone Treatment: An Effective Method for Fine-Tuning Optical and Electrical Properties of Suspended and Substrate-Supported MoS2.

Ultraviolet-ozone (UV-O3) treatment is a simple but effective technique for surface cleaning, surface sterilization, doping, and oxidation, and is applicable to a wide range of materials. In this study, we investigated how UV-O3 treatment affects the optical and electrical properties of molybdenum disulfide (MoS2), with and without the presence of a dielectric substrate. We performed detailed photoluminescence (PL) measurements on 1-7 layers of MoS2 with up to 8 min of UV-O3 exposure. Density functional theory (DFT) calculations were carried out to provide insight into oxygen-MoS2 interaction mechanisms. Our results showed that the influence of UV-O3 treatment on PL depends on whether the substrate is present, as well as the number of layers. Additionally, 4 min of UV-O3 treatment was found to be optimal to produce p-type MoS2, while maintaining above 80% of the PL intensity and the emission wavelength, compared to pristine flakes (intrinsically n-type). UV-O3 treatment for more than 6 min not only caused a reduction in the electron density but also deteriorated the hole-dominated transport. It is revealed that the substrate plays a critical role in the manipulation of the electrical and optical properties of MoS2, which should be considered in future device fabrication and applications.

Directional dependence of band gap modulation via uniaxial strain in MoS2and TiS3.

Strain is widely employed to modulate the band structures of two-dimensional (2D) van der Waals (vdW) materials. Such band engineering with strain applied along different crystallographic directions, however, is less explored. Here, we investigate the band gap modulation of layered chalcogenides, MoS2and TiS3, and the dependence of their band gaps on the directions of applied strain, using first-principles calculations. The band gap transition in MoS2is found to reduce in energy linearly as a function of increasing tensile strain, with a weakly directional-dependent gradient, varying by 4.6 meV/% (from -52.7 ± 0.6 to -57.3 ± 0.1 meV/%) from the zigzag to armchair directions. Conversely, the band gap in TiS3decreases with strain applied along the a lattice vector, but increases with strain applied in the perpendicular direction, with a non-linear strain-band gap relationship found between these limits. Analysis of the structure of the materials and character of the band edge states under strain helps explain the origins of the stark differences between MoS2and TiS3. Our results provide new insights for strain engineering in 2D materials and the use of the direction of applied strain as another degree of freedom.

On the Importance of Nanoparticle Necks and Carbon Impurities for Charge Trapping in TiO2.

Particle attachment and neck formation inside TiO2 nanoparticle networks determine materials performance in sensing, photo-electrochemistry, and catalysis. Nanoparticle necks can feature point defects with potential impact on the separation and recombination of photogenerated charges. Here, we investigated with electron paramagnetic resonance a point defect that traps electrons and predominantly forms in aggregated TiO2 nanoparticle systems. The associated paramagnetic center resonates in the g factor range between g = 2.0018 and 2.0028. Structure characterization and electron paramagnetic resonance data suggest that during materials processing, the paramagnetic electron center accumulates in the region of nanoparticle necks, where O2 adsorption and condensation can occur at cryogenic temperatures. Complementary density functional theory calculations reveal that residual carbon atoms, which potentially originate from synthesis, can substitute oxygen ions in the anionic sublattice, where they trap one or two electrons that mainly localize at the carbon. Their emergence upon particle neck formation is explained by the synthesis- and/or processing-induced particle attachment and aggregation facilitating carbon atom incorporation into the lattice. This study represents a substantial advance in linking dopants, point defects, and their spectroscopic fingerprints to microstructural features of oxide nanomaterials.

Predictive Removal of Interfacial Defect-Induced Trap States between Titanium Dioxide Nanoparticles via Sub-Monolayer Zirconium Coating.

First principles modeling of anatase TiO2 surfaces and their interfacial contacts shows that defect-induced trap states within the band gap arise from intrinsic structural distortions, and these can be corrected by modification with Zr(IV) ions. Experimental testing of these predictions has been undertaken using anatase nanocrystals modified with a range of Zr precursors and characterized using structural and spectroscopic methods. Continuous-wave electron paramagnetic resonance (EPR) spectroscopy revealed that under illumination, nanoparticle-nanoparticle interfacial hole trap states dominate, which are significantly reduced after optimizing the Zr doping. Fabrication of nanoporous films of these materials and charge injection using electrochemical methods shows that Zr doping also leads to improved electron conductivity and mobility in these nanocrystalline systems. The simple methodology described here to reduce the concentration of interfacial defects may have wider application to improving the efficiency of systems incorporating metal oxide powders and films including photocatalysts, photovoltaics, fuel cells, and related energy applications.

Controlling the Formation of Conductive Pathways in Memristive Devices.

Resistive random-access memories are promising candidates for novel computer architectures such as in-memory computing, multilevel data storage, and neuromorphics. Their working principle is based on electrically stimulated materials changes that allow access to two (digital), multiple (multilevel), or quasi-continuous (analog) resistive states. However, the stochastic nature of forming and switching the conductive pathway involves complex atomistic defect configurations resulting in considerable variability. This paper reveals that the intricate interplay of 0D and 2D defects can be engineered to achieve reproducible and controlled low-voltage formation of conducting filaments. The author find that the orientation of grain boundaries in polycrystalline HfOx is directly related to the required forming voltage of the conducting filaments, unravelling a neglected origin of variability. Based on the realistic atomic structure of grain boundaries obtained from ultra-high resolution imaging combined with first-principles calculations including local strain, this paper shows how oxygen vacancy segregation energies and the associated electronic states in the vicinity of the Fermi level govern the formation of conductive pathways in memristive devices. These findings are applicable to non-amorphous valence change filamentary type memristive device. The results demonstrate that a fundamental atomistic understanding of defect chemistry is pivotal to design memristors as key element of future electronics.

Unveiling the Electronic Structure of Grain Boundaries in Anatase with Electron Microscopy and First-Principles Modeling.

Polycrystalline anatase titanium dioxide has drawn great interest, because of its potential applications in high-efficiency photovoltaics and photocatalysts. There has been speculation on the electronic properties of grain boundaries but little direct evidence, because grain boundaries in anatase are challenging to probe experimentally and to model. We present a combined experimental and theoretical study of anatase grain boundaries that have been fabricated by epitaxial growth on a bicrystalline substrate, allowing accurate atomic-scale models to be determined. The electronic structure in the vicinity of stoichiometric grain boundaries is relatively benign to device performance but segregation of oxygen vacancies introduces barriers to electron transport, because of the development of a space charge region. An intrinsically oxygen-deficient boundary exhibits charge trapping consistent with electron energy loss spectroscopy measurements. We discuss strategies for the synthesis of polycrystalline anatase in order to minimize the formation of such deleterious grain boundaries.

Rubbing Powders: Direct Spectroscopic Observation of Triboinduced Oxygen Radical Formation in MgO Nanocube Ensembles.

Powder compaction-induced surface chemistry in metal oxide nanocrystal ensembles is important for very diverse fields such as triboelectrics, tribocatalysts, surface abrasion, and cold sintering of ceramics. Using a range of spectroscopic techniques, we show that MgO nanocube powder compaction with uniaxial pressures that can be achieved by gentle manual rubbing or pressing (p ≥ 5 MPa) excites energetic electron-hole pairs and generates oxygen radicals at interfacial defect structures. While the identification of paramagnetic O- radicals and their adsorption complexes with O2 point to the emergence of hole centers, triboemitted electrons become scavenged by molecular oxygen to convert into adsorbed superoxide anions O2 - as measured by electron paramagnetic resonance (EPR). By means of complementary UV-photoexcitation experiments, we found that photon energies in the range between 3 and 6 eV produce essentially the same EPR spectroscopic fingerprints and optical absorption features. To provide insights into this effect, we performed density functional theory calculations to explore the energetics of charge separation involving the ionization of low-coordinated anions and surface-adsorbed O2 - radicals at points of contact. For all selected configurations, charge transfer is not spontaneous but requires an additional driving force. We propose that a plausible mechanism for oxygen radical formation is the generation of significant surface potential differences at points of contact under loading as a result of the highly inhomogeneous elastic deformations coupled with the flexoelectric effect.

Hole Polaron Migration in Bulk Phases of TiO2 Using Hybrid Density Functional Theory.

Understanding charge-carrier transport in semiconductors is vital to the improvement of material performance for various applications in optoelectronics and photochemistry. Here, we use hybrid density functional theory to model small hole polaron transport in the anatase, brookite, and TiO2-B phases of titanium dioxide and determine the rates of site-to-site hopping as well as thermal ionization into the valance band and retrapping. We find that the hole polaron mobility increases in the order TiO2-B < anatase < brookite and there are distinct differences in the character of hole polaron migration in each phase. As well as having fundamental interest, these results have implications for applications of TiO2 in photocatalysis and photoelectrochemistry, which we discuss.

Anataselike Grain Boundary Structure in Rutile Titanium Dioxide.

The formation of nanoscale phases at grain boundaries in polycrystalline materials has attracted much attention, since it offers a route toward targeted and controlled design of interface properties. However, understanding structure-property relationships at these complex interfacial defects is hampered by the great challenge of accurately determining their atomic structure. Here, we combine advanced electron microscopy together with ab initio random structure searching to determine the atomic structure of an experimentally fabricated Σ13 (221) [11̅0] grain boundary in rutile TiO2. Through careful analysis of the atomic structure and complementary electron energy-loss spectroscopy analysis we identify the existence of a unique nanoscale phase at the grain boundary with striking similarities to the bulk anatase crystal structure. Our results show a path to embed nanoscale anatase into rutile TiO2 and showcase how the atomic structure of even complex internal interfaces can be accurately determined using a combined theoretical and experimental approach.

Density Functional Theory and Experimental Determination of Band Gaps and Lattice Parameters in Kesterite Cu2ZnSn(SxSe1-x)4.

The structures and band gaps of copper-zinc-tin selenosulfides (CZTSSe) are investigated for a range of anion compositions through experimental analysis and complementary first-principles simulations. The band gap was found to be extremely sensitive to the Sn-anion bond length, with an almost linear correlation with the average Sn-anion bond length in the mixed anion phase Cu2ZnSn(SxSe1-x)4. Therefore, an accurate prediction of band gaps using first-principles methods requires the accurate reproduction of the experimental bond lengths. This is challenging for many widely used approaches that are suitable for large supercells. The HSE06 functional was found to predict the structure and band gap in good agreement with the experiment but is computationally expensive for large supercells. It was shown that a geometry optimization with the MS2 meta-GGA functional followed by a single point calculation of electronic properties using HSE06 is a reasonable compromise for modeling larger supercells that are often unavoidable in the study of point and extended defects.

Evidence for Self-healing Benign Grain Boundaries and a Highly Defective Sb2Se3-CdS Interfacial Layer in Sb2Se3 Thin-Film Photovoltaics.

The crystal structure of Sb2Se3 gives rise to unique properties that cannot otherwise be achieved with conventional thin-film photovoltaic materials, such as CdTe or Cu(In,Ga)Se2. It has previously been proposed that grain boundaries can be made benign provided only the weak van der Waals forces between the (Sb4Se6)n ribbons are disrupted. Here, it is shown that non-radiative recombination is suppressed even for grain boundaries cutting across the (Sb4Se6)n ribbons. This is due to a remarkable self-healing process, whereby atoms at the grain boundary can relax to remove any electronic defect states within the band gap. Grain boundaries can, however, impede charge transport due to the fact that carriers have a higher mobility along the (Sb4Se6)n ribbons. Because of the ribbon misorientation, certain grain boundaries can effectively block charge collection. Furthermore, it is shown that CdS is not a suitable emitter to partner Sb2Se3 due to Sb and Se interdiffusion. As a result, a highly defective Sb2Se3 interfacial layer is formed that potentially reduces device efficiency through interface recombination.

First-Principles Investigation of the Structure and Properties of Au Nanoparticles Supported on ZnO.

We present a first-principles investigation of the structure, stability, and reactivity of Au nanoparticles (NPs) supported on ZnO. The morphologies of supported Au NPs are predicted using the formation energy of Au surfaces and the adhesion energy between Au and the dominant ZnO surfaces exposed on ZnO tetrapods. We show how Zn interstitials (a stable intrinsic defect in ZnO) are attracted toward the Au/ZnO interface and in the presence of oxygen can lead to the encapsulation of Au by ZnO, an effect that is observed experimentally. We find that O2 molecules absorb preferentially at the perimeter of the NP in contact with the ZnO support. These results provide atomistic insight into the structure of ZnO-supported Au NPs with relevance to CO oxidation.

Screening Doping Strategies To Mitigate Electron Trapping at Anatase TiO2 Surfaces.

Nanocrystalline anatase titanium dioxide is an efficient electron transport material for solar cells and photocatalysts. However, low-coordinated Ti cations at surfaces introduce low-lying Ti 3d states that can trap electrons, reducing charge mobility. Here, a number of dopants (V, Sb, Sn, Zr, and Hf) are examined to replace these low-coordinated Ti cations and reduce electron trapping in anatase crystals. V, Sb, and Sn dopants act as electron traps, while Zr and Hf dopants are found to prevent electron trapping. We also show that alkali metal dopants can be used to fill surface traps by donating electrons into the 3d states of low-coordinated Ti ions. These results provide practical guidance on the optimization of charge mobility in nanocrystalline TiO2 by doping.

Passivating Grain Boundaries in Polycrystalline CdTe.

Using first-principles density functional calculations, we investigate the structure and properties of three different grain boundaries (GBs) in the solar absorber material CdTe. Among the low ∑ value symmetric tilt GBs ∑3 (111), ∑3 (112), and ∑5 (310), we confirm that the ∑3 (111) is the most stable one but is relatively benign for carrier transport as it does not introduce any new states into the gap. The ∑3 (112) and ∑5 (310) GBs, however, are detrimental due to gap states induced by Te-Te and Cd-Cd dangling bonds. We systematically investigate the segregation of O, Se, Cl, Na, and Cu to the GBs and associated electronic properties. Our results show that co-doping with Cl and Na is predicted to be a viable approach passivating all gap states induced by dangling bonds in CdTe.

Exposure of mass-selected bimetallic Pt-Ti nanoalloys to oxygen explored using scanning transmission electron microscopy and density functional theory.

The response of nanoparticles to exposure to ambient conditions and especially oxidation is fundamental to the application of nanotechnology. Bimetallic platinum-titanium nanoparticles of selected mass, 30 kDa and 90 kDa, were produced using a magnetron sputtering gas condensation cluster source and deposited onto amorphous carbon TEM grids. The nanoparticles were analysed with a Cs-corrected Scanning Transmission Electron Microscope (STEM) in High Angle Annular Dark Field (HAADF) mode. It was observed that prior to full Ti oxidation, Pt atoms were dispersed within a Ti shell. However, after full oxidation by prolonged exposure to ambient conditions prior to STEM, the smaller size 30 kDa particles form a single Pt core and the larger size 90 kDa particles exhibit a multi-core structure. Electron beam annealing induced a single core morphology in the larger particles. First principles density functional theory (DFT) calculations were employed to calculate the lowest energy structure of the Pt-Ti nanoparticles with and without the presence of oxygen. It was demonstrated that, as the concentration of oxygen increases, the lowest energy structure changes from dispersed Pt to multiple Pt cores and finally a single Pt core, which is in good agreement with the experimental observations.

Atomic structure and electronic properties of MgO grain boundaries in tunnelling magnetoresistive devices.

Polycrystalline metal oxides find diverse applications in areas such as nanoelectronics, photovoltaics and catalysis. Although grain boundary defects are ubiquitous their structure and electronic properties are very poorly understood since it is extremely challenging to probe the structure of buried interfaces directly. In this paper we combine novel plan-view high-resolution transmission electron microscopy and first principles calculations to provide atomic level understanding of the structure and properties of grain boundaries in the barrier layer of a magnetic tunnel junction. We show that the highly [001] textured MgO films contain numerous tilt grain boundaries. First principles calculations reveal how these grain boundaries are associated with locally reduced band gaps (by up to 3 eV). Using a simple model we show how shunting a proportion of the tunnelling current through grain boundaries imposes limits on the maximum magnetoresistance that can be achieved in devices.

Modification of Charge Trapping at Particle/Particle Interfaces by Electrochemical Hydrogen Doping of Nanocrystalline TiO2.

Particle/particle interfaces play a crucial role in the functionality and performance of nanocrystalline materials such as mesoporous metal oxide electrodes. Defects at these interfaces are known to impede charge separation via slow-down of transport and increase of charge recombination, but can be passivated via electrochemical doping (i.e., incorporation of electron/proton pairs), leading to transient but large enhancement of photoelectrode performance. Although this process is technologically very relevant, it is still poorly understood. Here we report on the electrochemical characterization and the theoretical modeling of electron traps in nanocrystalline rutile TiO2 films. Significant changes in the electrochemical response of porous films consisting of a random network of TiO2 particles are observed upon the electrochemical accumulation of electron/proton pairs. The reversible shift of a capacitive peak in the voltammetric profile of the electrode is assigned to an energetic modification of trap states at particle/particle interfaces. This hypothesis is supported by first-principles theoretical calculations on a TiO2 grain boundary, providing a simple model for particle/particle interfaces. In particular, it is shown how protons readily segregate to the grain boundary (being up to 0.6 eV more stable than in the TiO2 bulk), modifying its structure and electron-trapping properties. The presence of hydrogen at the grain boundary increases the average depth of traps while at the same time reducing their number compared to the undoped situation. This provides an explanation for the transient enhancement of the photoelectrocatalytic activity toward methanol photooxidation which is observed following electrochemical hydrogen doping of rutile TiO2 nanoparticle electrodes.

Polar Spinel-Perovskite Interfaces: an atomistic study of Fe3O4(111)/SrTiO3(111) structure and functionality.

Atomic resolution scanning transmission electron microscopy and electron energy loss spectroscopy combined with ab initio electronic calculations are used to determine the structure and properties of the Fe3O4(111)/SrTiO3(111) polar interface. The interfacial structure and chemical composition are shown to be atomically sharp and of an octahedral Fe/SrO3 nature. Band alignment across the interface pins the Fermi level in the vicinity of the conduction band of SrTiO3. Density functional theory calculations demonstrate very high spin-polarization of Fe3O4 in the interface vicinity which suggests that this system may be an excellent candidate for spintronic applications.