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Correlation between precipitates evolution and mechanical properties of Al-Sc-Zr alloy with Er additions during isothermal ageing were investigated by microhardness measurements, transmission electron microscopy, atom probe tomography and density functional theory-based simulations. The results demonstrate that the Er additions significantly improve the hardness during elevated temperature ageing, especially at 400°C. This is mainly because Er additions increase the nucleation rate of the Al3(Er,Sc,Zr) precipitates, resulting in a higher density of fine and uniform dispersion of L12 structured nanoparticles. First-principles calculations demonstrate that the second nearest neighboring solute-solute interactions for the species Sc, Zr and Er are energetically favored – a key feature to rationalize the observed precipitate structure and the underlying formation mechanism. The sequential formation of the core/shell precipitates in the Er-free alloy and core/double-shell precipitates in the Er-containing alloy arises due to the different solute-solute and solute-vacancy interaction energies, and the relative diffusivities of the Er, Sc and Zr species in Al. These results shed light on the beneficial effects of Er additions on the age-hardening behavior of Al-Sc-Zr alloy and provide guidance for designing the ageing treatments for the Al-Sc-Zr(-Er) alloys.

Identification and passivation of defect-induced electron-hole recombination centers are currently crucial for improving the efficiency of hybrid perovskite solar cells. Besides general intrinsic defects, experimental reports have indicated that hydrogen interstitials are also abundant in hybrid perovskite layers; however, few reports have evaluated the effect of such defects on the charge carrier recombination and device efficiencies. Here, we reveal that under I-poor synthesis conditions, the negatively charged monatomic hydrogen interstitial, Hi-, will form in the prototypical CH3NH3PbI3 perovskite layer, acting as a detrimental deep-level defect, which leads to efficient electron-hole recombination and lowers the cell performance. We further rationalize that Br doping can mitigate the large atomic displacement caused by the presence of Hi- and hence suppress the formation of the deep localized state. The results advance the knowledge of the deep-level defects in hybrid perovskites and provide useful information for enhancing solar cell performance by defect engineering.

All-inorganic lead-free CsSnBr3 is attractive for applications in solar cells due to its nontoxicity and stability, but the device performance to date has been poor. Besides the intrinsic properties, impurities induced from electrodes may significantly influence the device performance. Here, we systematically studied the stability, transition energy levels, and diffusion of impurities from the most commonly used electrodes (Au, Ag, Cu, graphite, and graphene) in CsSnBr3 based on density functional theory calculations. Our results reveal that, whereas graphite and graphene electrodes exhibit negligible influence on CsSnBr3 due to the relatively high formation energies for carbon impurities in CsSnBr3, atoms from the metal electrodes can effectively diffuse into CsSnBr3 along interstice and form electrically active impurities in CsSnBr3. In this case, a significant amount of donor interstitial impurities, such as $$Ag_i^ +$$ A g i + , $$Cu_i^ +$$ C u i + , and $$Au_i^ +$$ A u i + , will be formed under p-type conditions, whereas the Sn-site substitutional acceptor impurities, namely $$Au_{Sn}^{2 - }$$ A u S n 2 − , $$Ag_{Sn}^{2 - }$$ A g S n 2 − , and $$Cu_{Sn}^{2 - }$$ C u S n 2 − , are the dominant impurities, especially under n-type conditions. In particular, except for $$Au_i^ +$$ A u i + , all these major impurities from the metal electrodes act as nonradiative recombination centers in CsSnBr3 and significantly degrade the device performance. Our work highlights the distinct behaviors of the electrode impurities in CsSnBr3 and their influence on the related devices and provides valuable information for identifying suitable electrodes for optoelectronic applications.

Accurate simulation of semiconductor nanowires (NWs) under strain is challenging, especially for bent NWs. Here, we propose a simple yet efficient unit-cell model to simulate strain-mediated bandgap modulation in both straight and bent NWs. This is with consideration that uniaxlly bent NWs experience continuous compressive and tensile strains through their cross-sections. A systematic investigation of a series of III-V and II-VI semiconductors NWs in both wurtzite and zinc blende polytypes is performed using hybrid density functional theory methods. The results reveal three common trend in bandgap evolution upon application of strain. Existing experimental measurements corroborate with our predictions concerning bandgap evolution as well as direct-indirect bandgap transitions upon strain. By examining the variation of previous theoretical studies, our result further highlights the significance of geometrical relaxtion in NW simulation. This simplified model is expected to be applicable to investigations of the electronic, optoelectronic, and sensorial properties of all semiconductor NWs.

Here, the segregation and accumulation of hydrogen in Ni grain boundaries, and its effects on cohesion and tensile mechanical strength were studied by means of density functional theory simulations. Three model grain boundaries were considered: the Σ 3 ( 1 1 ¯ 1 ) [ 110 ] , Σ 5 ( 120 ) [ 001 ] and Σ 11 ( 1 1 ¯ 0 ) [ 113 ] , as representatives for the highly coherent twin, high energy random high angle, and “special” low energy highly coherent grain boundaries, respectively. Hydrogen segregation was found to be favourable in the Σ 5 and Σ 11 grain boundaries, but not in the Σ 3. Hydrogen accumulation studied via a comprehensive site-permutation analysis revealed the mechanisms for how H accumulation capacity varies as a function of grain boundary character. We show that the interfacial cohesion of boundaries can diminish by between 6.7-37.5% at varying levels of H-accumulation. The cohesion of the grain boundaries was analysed using a novel chemical bond-order based approach, enabling a quantitative atomistic determination of the fracture paths arising from hydrogen embrittlement. These simulations explain the details of why grain boundary character is the principal determinant of the likelihood of hydrogen segregation and accumulation, and hence their vulnerability to hydrogen-enhanced decohesion. This knowledge can be used in the design of thermomechanical processes to achieve grain boundary engineering for resistance to hydrogen embrittlement.

The exceptionally low deformation potential is proposed as the key determinant for the high carrier mobility in α-phosphorene. This is related to its unique corrugated two-dimensional structure. Herein, we present a systematic first-principles density functional theory study on ten α-phosphorene isostructures by calculating the three key parameters of the carrier mobility. An electron mobility in the armchair direction with a value comparable to α-phosphorene is found in α-PAs, α-PCH, and α-AsCH, due to the structure-caused low deformation potential. The highest carrier mobility is predicted in α-graphane because of a two-orders-of-magnitude smaller deformation potential than the other isostructures. The low deformation potential can be correlated to the separation of charge carriers from neighbouring unit cells. This study highlights a feasible route to generating high mobility materials through deformation potential engineering.

High magnetization materials are in great demand for the fabrication of advanced multifunctional magnetic devices. Notwithstanding this demand, the development of new materials with these attribute...

Boron (B) and phosphorous (P) co-doped colloidal silicon nanocrystals (Si NCs) have unique size-dependent optical properties, which lead to potential applications in optoelectronic and biomedical applications. However, the microstructure of the B and P co-doped colloidal Si NCs – in particular, the exact location of the dopant atoms in real space, has not been studied. A lack of understanding of this underlying question limits our ability to better control sample fabrication, as well as our ability to further develop the optical properties. To study the microstructure, a process enabling atom probe tomography (APT) of colloidal Si NCs was developed. A dispersion of colloidal Si NCs in a SiO2 sol-gel solution and a low temperature curing are demonstrated as the key sample preparation steps. Our APT results demonstrate that a B-rich region exists at the surface of the Si NCs, while P atoms are distributed within the Si NCs. First principles density functional theory calculations of a Si NC embedded in SiO2 matrix reveal that P atoms, which always prefer to reside inside a Si NC, significantly influence the distribution of B atoms. Specifically, P atoms lower the B diffusion barrier at Si/SiO2 interface and stabilize B atoms to reside within individual Si NCs. We propose that the B-modified surface changes the chemical properties of the Si NCs by (i) offering chemical resistance to attack by HF and (ii) enabling dispersibility in solution without aggregation.

Thermal annealing temperature and time dictate the microstructure of semiconductor materials such as silicon nanocrystals (Si NCs). Herein, atom probe tomography (APT) and density functional theory (DFT) calculations are used to understand the thermal annealing temperature effects on Si NCs grown in a SiO2 matrix and the distribution behaviour of boron (B) and phosphorus (P) dopant atoms. The APT results demonstrate that raising the annealing temperature promotes growth and increased P concentration of the Si NCs. The data also shows that the thermal annealing does not promote the incorporation of B atoms into Si NCs. Instead, B atoms tend to locate at the interface between the Si NCs and SiO2 matrix. The DFT calculations support the APT data and reveal that oxygen vacancies regulate Si NC growth and dopant distribution. This study provides the detailed microstructure of p-type, intrinsic, and n-type Si NCs with changing annealing temperature and highlights how B and P dopants preferentially locate with respect to the Si NCs embedded in the SiO2 matrix with the aid of oxygen vacancies. These findings will be useful towards future optoelectronic applications.

Plastic deformation in ceramic materials is normally only observed in nanometre-sized samples. However, we have observed unprecedented levels of plasticity (>50% plastic strain) and excellent elasticity (6% elastic strain) in perovskite oxide Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 (PIN-PMN-PT), under compression along pc pillars up to 2.1 μm in diameter. The extent of this deformation is much higher than has previously been reported for ceramic materials, and the sample size at which plasticity is observed is almost an order of magnitude larger. Bending tests also revealed over 8% flexural strain. Plastic deformation occurred by slip along {110} . Calculations indicate that the resulting strain gradients will give rise to extreme flexoelectric polarization. First principles models predict that a high concentration of oxygen vacancies (Vo) weaken the covalent/ionic bonds, giving rise to the unexpected plasticity. Mechanical testing on Vo-rich Mn-doped PIN-PMN-PT confirmed this prediction. These findings will facilitate the design of plastic ceramic materials and the development of flexoelectric-based nano-electromechanical systems.

Lead-free perovskites $\mathrm{CsSn}{\mathrm{Br}}_{3}$ were predicted to be promising for photovoltaic applications, yet the measured performance is rather disappointing. To understand the mechanism behind the large discrepancy and to further improve the functionality, we study the stability, transition energy levels, and diffusion of different intrinsic defects in pure $\ensuremath{\alpha}\ensuremath{-}\mathrm{CsSn}{\mathrm{Br}}_{3}$ using density-functional theory calculations. Our results reveal the unique characteristics of the intrinsic defects and their effects on the optoelectronic properties of perovskite $\mathrm{CsSn}{\mathrm{Br}}_{3}$. Among the low formation energy defects, the vacancy defects ${V}_{\mathrm{Sn}}$ and ${V}_{\mathrm{Cs}}$ can form shallow energy levels to provide $p$-type conductivity, while the antisite defects ${\mathrm{Br}}_{\mathrm{Sn}}$ and $\mathrm{Br}{}_{\mathrm{Cs}}$ can act as detrimental recombination centers to deteriorate the performance of devices. Moreover, the mobile defects ${V}_{\mathrm{Sn}}^{2\ensuremath{-}}$ and ${V}_{\mathrm{Cs}}^{\ensuremath{-}}\phantom{\rule{0.28em}{0ex}}$ also degrade device performance by the field-screening effect. The effect of the chemical potential modulation of $\mathrm{Sn}{\mathrm{F}}_{2}$ addition into $\mathrm{CsSn}{\mathrm{Br}}_{3}$ is also discussed. These results provide in-depth insights into defect behaviors in the Sn-based perovskites and shed light on related optoelectronic applications.

The mechanical behavior of $\ensuremath{\langle}111\ensuremath{\rangle}$ ultrathin gold nanowires (NWs) and their dependence on the correlated parameters of size (i.e., diameter $\ensuremath{\sim}1--1.6$ nm), morphology (rectangular and hexagonal prism), and ultrahigh density twin boundaries was investigated using density functional theory calculations. The parameters of size, morphology, and the presence of twins all significantly influence the mechanical behavior of Au NWs, and their effects are interdependent. Importantly, an ultrahigh density of twins in NWs enhances their strength in both morphologies, and across all sizes studied. Our calculations reveal a remarkable range of Young's modulus (60--158 GPa) and ultimate tensile strengths (3.7--14.0 GPa), suggesting that the scatter observed in experimental research involving ultrathin gold nanowires is a feature of intrinsic complexity. In particular, a ``wrinkling'' of the atomic planes along the wire axis due to a combination of anisotropic surface stresses and wire core response, is demonstrated to be largely suppressed by the presence of high-density twins. The high tensile strengths of the studied nanowires with twins is attributed to a combination of the suppression of this anisotropic phenomena and slip plane disruption. This work demonstrates that precise control of these parameters is required during synthesis to achieve target mechanical behaviour in industrial device applications. Moreover, the work demonstrates that this system has the potential to be tuned for access to a vast range of materials design space.

Two-dimensional auxetic materials with negative Poisson's ratio expand in response to a tensile strain in cross-section. Such counter-intuitive behaviours have been mainly ascribed to concave structures with ideal rigid ball-stick models. Here, based on first-principles calculations, we systematically analyze the mechanical behaviours of three life-boat structured two-dimensional (2D) materials and report two new auxetic materials: δ-arsenic and δ-graphane. The calculated Poisson's ratio values are correlated with Young's modulus, cohesive energy, and valence shell electron pair repulsion of isostructures. Combining with previous research, we provide a self-consistent explanation of the origin of the 2D in-plane negative Poisson's ratio and an algorithmic route to discover new auxetic materials by comparing the energy restored in bond rotation and stretch.

The capability of precisely determining the atomic identity and position, coupled with an understanding of the inter-atomic interactions is crucial to enabling microstructure-property relationships in modern materials science. Two approaches to the investigation of materials at the atomic-scale are atom probe microscopy (APM) and density functional theory (DFT). On the one hand, APM offers an ultimate in 3D elemental analysis by determining the chemical identity and atomic position in a material. On the other, first principles DFT simulations allow the prediction of atomic structure and, critically, the electronic structure which is the gateway to calculating the real-world properties of materials. Both approaches are undertaken at the atomic-scale, and so are intrinsically complementary. This paper provides a review of the opportunities and challenges in combining these approaches to do materials science. We first summarise the status of these approaches and set out a framework for the current main workflows by which APM experiments inform DFT simulations and vice-versa. We point out certain gaps between the approaches and ways that these may be bridged via a series of case-studies organised into the aforementioned workflow framework.

Density functional theory calculations investigate the mechanical properties of: cubic (c), tetragonal (t) and monoclinic (m) zirconia (ZrO2); cubic yttria-stabilized zirconia (c-YSZ); c-YSZ doped with TiO2, MnO2, CaO and NiO. These find the elastic constants, elastic compliances, bulk, shear and Young's modulus for the three phases of zirconia together with the ideal strength of c-ZrO2. The ideal strength of c-ZrO2 for strain in the [ 100 ] direction (84.3 GPa) being significantly higher than in the [ 110 ] (30.5 GPa) and [ 111 ] directions (9.87 GPa) is attributed to change in the bond angle reducing the internal strain on the bonds. Adding Y2O3 to c-ZrO2 decreases the ideal strength, particularly in the [100] direction, namely to respectively 11.1 and 28.8 GPa for 6.67 and 14.29 mol % Y2O3. Doping c-YSZ with TiO2, MnO2, CaO and NiO further reduces the ideal strength, with the lowest values for TiO2 (7.16–7.88 GPa). The significant decrease in the ideal strength of c-YSZ and doped c-YSZ compared to c-ZrO2 in the [100] direction is attributed to the weakening of the ZrO2 framework by the oxygen vacancies.

The magnetic performance of nanomaterials depends on size, shape, and surface of the nanocrystals. Here, the exposed crystal planes of Co3O4 nanocrystals were analyzed to research the dependence of magnetic properties on the configuration environment of the ions exposed on different surfaces. The Co3O4 nanocrystals with exposed (1 0 0), (1 1 0), (1 1 1), and (1 1 2) planes were synthesized using a hydrothermal method in the shapes of nanocube, nanorod, hexagonal nanoplatelet, and nanolaminar, respectively. Ferromagnetic performance was detected in the (1 0 0) and (1 1 1) plane-exposed samples. First-principles calculation results indicate that unlike the nonmagnetic nature in the bulk, the Co3+ ions exposed on or close to the surface possess sizable magnetic moments because of the variation of coordination numbers and lattice distortion, which is responsible for the ferromagnetic-like behavior. The (1 1 0)-exposed sample keeps the natural antiferromagnetic behavior of bulk Co3O4 because either the surface Co3+ ions have no magnetic moments or their moments are in antiferromagnetic coupling. The (1 1 2)-exposed sample also displays antiferromagnetism because the interaction distances between the magnetized Co3+ ions are too long to form effective ferromagnetic coupling.

Chemical short-range ordering is an important structural feature that would significantly affect materials properties, including mechanical properties. Chemical short-range ordering has been discovered in many alloy systems and usually forms at high temperatures during the materials synthesis or thermal annealing processes. Here we report chemical short-range ordering in an ultrafine-grained supersaturated Al-Zn alloy introduced by severe plastic deformation (strain >1) at room temperature. The chemical short-range ordering results in an ordered L10-AlZn structure with the dimensions of 1–3 nm. It is believed that vacancies introduced by severe plastic deformation play an important role in the formation of the chemical short-range ordering at room temperature.

Solute clustering is a technologically important microstructural process in Al alloys. Exerting control over this process in order to enhance the alloy properties and reduce the energy costs of production is a major scientific and technological focus, with automotive sheet applications serving as a key driver. In this work, we detail changes in the state of clustering arising from the insertion of a thermomechanical pre-ageing process, via a coiling step, immediately after solution treatment. This pre-ageing step effectively mitigates the negative effects of the natural ageing on the mechanical properties that would otherwise occur after solution treatment and ahead of the final paint-bake step. Our work sought to raise the strength of AA6016, which nominally contains little or no Cu, to levels equivalent to the higher Cu-bearing 6xxx grade alloys, whilst preserving excellent ductility. This novel combination of process design and alloy selection has been studied in detail using a combination of atom probe tomography (APT) and first principles density functional theory (DFT) simulations. The thermomechanical process described here invokes a strong cluster strengthening phenomenon. We report on the details of the single-species Si-Si and Mg-Mg clusters and those of the Mg-Si co-clusters and relate the APT observations to our DFT calculations. Mg-Si co-clusters ≥ 20 solute atoms are found to be responsible for the excellent properties in the material exposed to the pre-ageing and paint-baking process. Our results revealed that vacancies can effectively stabilise single-species Si clusters enabling them to attract further solute and serving as a pathway to formation of the larger Mg-Si co-clusters that are so beneficial to alloy properties.

A survey of published literature reveals a difference in the density of amorphous and crystalline solids (organic and inorganic) on the order of 10%-15%, whereas for metallic alloys, it is found to be typically less than 5%. Standard geometric models of atomic packing can account for the polymeric and inorganic glasses without requiring changes in interatomic separations (bond lengths). By contrast, the relatively small difference in density between crystalline and glassy metals (and metallic alloys) implies variations in interatomic separations due to merging orbitals giving rise to reduced atomic volumes. To test this hypothesis, quantum density functional theory computations were carried out on ordered and irregular clusters of aluminum. The results point to decreasing interatomic distances with decreasing coordination, from which one can deduce that the geometrical method of random hard sphere packing significantly underestimates the densities of amorphous metallic alloys.

Solute clustering is a technologically important microstructural process in Al alloys. Exerting control over this process to enhance the alloy properties and reduce the energy costs of production is a major scientific and technological focus, with automotive sheet applications serving as a key driver. In this work, we detail changes in the state of clustering arising from the insertion of a thermomechanical pre-ageing process, via a coil-cooling step in a commercial production line, immediately after solution treatment. This pre-ageing step effectively mitigates the negative effects of the natural ageing on the mechanical properties that would otherwise occur after solution treatment and ahead of the final paint-bake step. Our work was focussed on a short duration/low temperature single-step pre-ageing process on a Cu-lean grade of AA6016. Using a combination of atom probe tomography and first principles density functional theory simulations, we are able to reveal and explain the atomic-scale microstructure arising from this thermomechanical process. Mg–Si co-clusters ≥ 20 solute atoms were found responsible for the excellent properties in the material exposed to the pre-ageing plus paint-baking process. Our simulations revealed that vacancies can effectively stabilise single-species Si clusters enabling them to attract further solute and serving as a pathway to formation of the larger Mg–Si co-clusters that are so beneficial to alloy properties. Our simulations also revealed that the nearest neighbour configurations are a critical aspect to the stability of the clusters.

In this Letter, we report on the evolution of electronic properties governed by epitaxial misfit strain in cubic In2O3 epilayers grown on sapphire. At elevated growth temperature, the competition between the film/substrate lattice mismatch and the thermal expansion mismatch alters the macroscopic biaxial strain from compressive to tensile. Simultaneously, the electron concentration is tuned from degeneration to non-degeneration density below the Mott criterion. The observed surface electron accumulation and metal-insulator transition result from the oxygen deficiency formed at low growth temperature, while high-temperature epitaxy is favorable to achieve remarkably enhanced mobility. The effective strain-property coupling suggests that the improved oxygen stoichiometry and the Fermi level movement controlled by the biaxial strains are responsible for the Mott transition. The strain-mediated reduction of the electron effective mass contributes to the enhanced intrinsic mobility in tensile-strained In2O3 epilayers. These results highlight that strain engineering is an effective stimulus to manipulate the transport properties of oxide semiconductors with improved performance and unexpected functionalities.

The surface of diamond is reported to undergo nonablative photochemical etching when exposed to ultraviolet (UV) radiation which allows controlled single and partial layer removal of lattice layers. Oxygen termination of surface dangling bonds is known to be crucial for the etching process; however, the exact mechanism of carbon ejection remains unclear. We investigate the interaction of UV laser pulses with oxygen-terminated diamond surfaces using atomic-scale surface characterization combined with first-principles time-dependent density functional theory calculations. We present evidence for laser-induced desorption (LID) from carbonyl functional groups at the diamond ${001}$ surface. The doubly bonded carbonyl group is photoexcited into a triply bonded CO-like state, including scission of the underlying $\mathrm{C}─\mathrm{C}$ bonds. The carbon removal process in LID is atom by atom; therefore, this mechanism provides a novel ``top-down'' approach for creating nanostructures on the surface of diamond and other carbon-containing semiconductors.

Identifying stable systems with high CO2 adsorption capacity is an essential goal in CO2 capture and storage technologies. We have carried out a comprehensive first-principles study to explore the CO2 capture capacity of 16 representative metal-doped graphene systems where the metal dopants can be stabilized by single- and double-vacancies. The maximum number of adsorbed CO2 molecules was determined by a combination of adsorption energy and bond distance criteria. Generally, while the double-vacancy can bind metal dopants more strongly than the single-vacancy, single-vacancy graphene with metal dopants are better sorbents, with each Ca, Sc and Y dopant binding up to 5 CO2 molecules. CO2 capture involves significant charge transfer between the CO2 molecule and the dopant–vacancy complexes, where defective graphene acts as a charge reservoir for binding CO2 molecules. Some systems are predicted to involve the formation of a bent CO2 anion. Ca-doped single- and double-vacancy graphene systems, however, readily form oxides upon reaction with CO2, thus they are less reusable for CO2 capture.

It has been often believed that substitutional solute atoms undergo preferential segregation to stacking faults (Suzuki effect). Here, density functional theory calculations reveal a rather diverse spatial distribution of the alloying atoms with respect to faults in binary Mg alloys. Interestingly, while some solutes encounter energy barriers when approaching faults, there is almost no barrier for Al and Sn. These findings advance our understanding concerning the interaction between solutes and extended defects, and provide guidance for stacking fault engineering in Mg alloys.

III-V ternary InGaAs nanowires have great potential for electronic and optoelectronic device applications; however, the 3D structure and chemistry at the atomic-scale inside the nanowires remain unclear, which hinders tailoring the nanowires for specific applications. Here, atom probe tomography is used in conjunction with a first-principles simulation to investigate the 3D structure and chemistry of InGaAs nanowires, and reveals i) the nanowires form a spontaneous core-shell structure with a Ga-enriched core and an In-enriched shell, due to different growth mechanisms in the axial and lateral directions; ii) the shape of the core evolves from hexagon into Reuleaux triangle and grows larger, which results from In outward and Ga inward interdiffusion occurring at the core-shell interface; and iii) the irregular hexagonal shell manifests an anisotropic growth rate on {112}A and {112}B facets. Accordingly, a model in terms of the core-shell shape and chemistry evolution is proposed, which provides fresh insights into the growth of these nanowires.

Density-functional theory calculations are performed to investigate the electronic and vibrational density-of-states (DOS) for a series of recently predicted stable and metastable structures of yttria-stabilised zirconia (YSZ) with yttria (Y2O3) concentrations of 14, 17 and 20 mol%. Analogous quantities are also studied for the so-called δ-phase, which forms for 40 mol% yttria, as well as for model structures with ≈10.3 mol% yttria. These calculated results, together with those for the cubic, tetragonal and monoclinic phases of ZrO2, afford a comparison of structural, electronic and vibrational properties as a function of yttria concentration. With increasing yttria content, the electronic DOS exhibit a decrease in the valence band-width (of about 2.0 eV relative to the cubic phase) and a corresponding increase of the band-gap of 0.73 eV (from cubic to 40 mol% yttria containing ZrO2). Regarding the vibrational DOS (vDOS), the addition of yttria causes the peaks in the vDOS of ZrO2 to become less distinct, reflecting the more dense occupation of states due to the larger number of atoms in each primitive cell, and to the lower symmetry. The vDOS of the various YSZ structures appear qualitatively similar with contributions from O atoms spanning the whole frequency range and cation related contributions present for frequencies 40 − 45 meV . With increasing yttria content, more Zr atoms become seven-fold coordinated as in monoclinic ZrO2, concominantly the vDOS increasingly resembles that of m-ZrO2, but with notable contributions from Y atoms, which has a main peak at about 17 meV, similar to that of Zr atoms.

Solute segregation of Zn, Mg and Cu in and around Al3(Sc,Zr) precipitates in an Al-Zn-Mg-Cu-Sc-Zr alloy was systematically studied using atom probe microscopy, aberration-corrected scanning transmission electron microscopy and first-principles simulations. Results show that the Al3(Sc,Zr) precipitates occur as a Sc-rich ‘core’ and Zr-rich ‘shell’ structure. Zn segregates to the Zr-rich shell of Al3(Sc,Zr) precipitates and substitute mainly for the Al atoms, and the Zn segregation intensifies with increasing ageing time. Mg and Cu atoms segregate to the Zr-rich shell during the early stages of ageing, while the Mg atoms prefer the Al-matrix with longer ageing time. Density functional theory calculations demonstrate that such segregation is energetically favored and highlight the diverse role of Al3(Sc,Zr) in influencing the distribution of the major alloying elements.

A combined experimental and theoretical investigation into the vibrational properties of cubic 8–9 mol% yttria-stabilized zirconia (YSZ) is presented. Measurements of acoustic phonon dispersion curves have been obtained from inelastic neutron scattering investigations using a triple axis spectrometer, as well as calculations of the vibrational density-of-states (vDOS) using density-functional theory. The present measurements agree closely with, and extend, previously published results. The phonons become broader and decrease in intensity as the Brillouin zone boundary is approached, particularly in the Γ–Δ–X direction. Interestingly, there is evidence of a previously unreported low energy phonon band (8–9 meV) in the Γ–Σ–X direction, which could possibly be related to the stabilization (by yttria doping) of the imaginary mode of cubic ZrO2 about the X-point. Compared to pure cubic ZrO2, the vDOS of YSZ are broader and extend to higher frequency. Furthermore, the prominent Zr-related feature in the vDOS of c-ZrO2 at ≈14 meV is shifted to higher energy in the vDOS of YSZ. This behavior is consistent with the measured dispersion bands (first acoustic branch in the Γ–X direction, about the X-point) of YSZ which is higher in energy by a similar amount relative to that of c-ZrO2, thus providing support for the structural model considered.

Due to their graphene-like properties after oxygen reduction, incorporation of graphene oxide (GO) sheets into correlated-electron materials offers a new pathway for tailoring their properties. Fabricating GO nanocomposites with polycrystalline MgB2 superconductors leads to an order of magnitude enhancement of the supercurrent at 5 K/8 T and 20 K/4 T. Herein, we introduce a novel experimental approach to overcome the formidable challenge of performing quantitative microscopy and microanalysis of such composites, so as to unveil how GO doping influences the structure and hence the material properties. Atom probe microscopy and electron microscopy were used to directly image the GO within the MgB2, and we combined these data with computational simulations to derive the property-enhancing mechanisms. Our results reveal synergetic effects of GO, namely, via localized atomic (carbon and oxygen) doping as well as texturing of the crystals, which provide both inter- and intra-granular flux pinning. This study opens up new insights into how low-dimensional nanostructures can be integrated into composites to modify the overall properties, using a methodology amenable to a wide range of applications.

Much effort has been devoted to the development of computational techniques in atom probe microscopy over the past decade. There have been several drivers for this effort. Firstly, there has been effort devoted to addressing the challenges of discerning information from the increasingly large size of the data, and capturing the opportunities that this large data presents. Secondly, there has been significant new effort devoted to the simulation of atom probe data so that pristine datasets that contain microstructural features of increasing complexity can be generated in-silico , and subjected to complex data-mining algorithms. This has enabled the benchmarking of various algorithms, guided the setting of parameters for particular analyses, and exposed the effects of instrumentation parameters such as detector efficiency and aberrations in ionic flight path. The authors are especially interested in the prospects of converging atomic-scale microscopy with atomic-scale materials modelling via first principles approaches. This involves excising parts of the APM data and using these as super-cell inputs to calculations of materials properties via density functional theory. It is our opinion that this represents a major advance for materials science because it enables microscopy to advance microstructure–property relationships to the direct mapping of such relationships based on many-body interactions. As such, this approach has great potential for materials design and development. The final part of this paper focuses on how cloud-based computing represents an exciting frontier of the computational aspects of atom probe microscopy. We discuss the opportunities and the barriers for conducting new materials science through the analysis and visualisation of atom probe data via new generation tools that are cloud-based, and which are managed, curated and governed with significant user-community input and integrated with contemporary electronic laboratory notebook technology.

Controllable doping of semiconductor nanowires is critical to realize their proposed applications, however precise and reliable characterization of dopant distributions remains challenging. In this article, we demonstrate an atomic-resolution three-dimensional elemental mapping of pristine semiconductor nanowires on growth substrates by using atom probe tomography to tackle this major challenge. This highly transferrable method is able to analyze the full diameter of a nanowire, with a depth resolution better than 0.17 nm thanks to an advanced reconstruction method exploiting the specimen's crystallography, and an enhanced chemical sensitivity of better than 8-fold increase in the signal-to-noise ratio.

Graphene based quantum dots and antidots are two nanostructures of primary importance for their fundamental physics and technological applications, particularly in the emerging field of graphene-based nanoelectronics and nanospintronics. Herein, based on first principles density functional theory calculations, we report a comparative study on the electronic structure of these two structurally complementary entities, where the bandgap opening, edge magnetism and the role of hydrogenation are investigated. Our results show the diversity of electronic structures of various dots and antidots, whose properties are sensitive to the edge detailed geometry (including size and shape and edge type). Hydrogen passivation plays an essential roal in affecting the related properties, in particular, it leads to larger bandgap values and suppress the edge magnetism. The frontier orbital analysis is employed to rationalize and compare the complicated nature of dots and antidots. Based on the specific geometrical consideration and the total energy competition of the ground antiferromagnetic and the ferromagnetic states, some magnetic structures (the unpassivated 42-atom-antidot and 54-atom-dot) are proposed to be useful as magnetic switches.

Here we present a study of magnetism in Co_{0.05}Ti_{0.95}O_{2-δ} anatase films grown by pulsed laser deposition under a variety of oxygen partial pressures and deposition rates. Energy-dispersive spectrometry and transmission electron microscopy analyses indicate that a high deposition rate leads to a homogeneous microstructure, while a very low rate or postannealing results in cobalt clustering. Depth resolved low-energy muon spin rotation experiments show that films grown at a low oxygen partial pressure (≈10^{-6} torr) with a uniform structure are fully magnetic, indicating intrinsic ferromagnetism. First principles calculations identify the beneficial role of low oxygen partial pressure in the realization of uniform carrier-mediated ferromagnetism. This work demonstrates that Co:TiO_{2} is an intrinsic diluted magnetic semiconductor.

The manipulation of atomic-scale magnetization is important from both a fundamental and a practical perspective. Using first-principles density-functional-theory calculations within the hybrid functional approach, we systematically study spin-lattice coupling effects for isolated $3{d}^{4}--3{d}^{7}$ transition-metal dopants in a nonmagnetic, ferroelectric ${\mathrm{PbTiO}}_{3}$ host material. When present at the B-site, a low-spin (or intermediate-spin) to high-spin crossover induces marked ferroelectric-like distortions in the local geometry, characterized by a shift of the dopant ion with respect to the surrounding ${\mathrm{O}}_{6}$ octahedral cage. The origins of this microscopic multiferroic effect are discussed in terms of the pseudo-Jahn-Teller theory for ferroelectricity. The possibility to exploit this phenomenon to achieve strong magnetoelectric coupling, including controlled spin switching, is also investigated. These results provide a further understanding of ferroelectricity and multiferroicity in perovskite oxides, and they suggest a possible pathway to manipulate single atomic spins in semiconductor solid solutions.

Interstitially doped Cu 3 N represents a model system to study “enclosed atoms” in a cuboctahedral environment. Based on density functional theory calculations using the generalized gradient approximation, we report a systematic study of 3d-transition metals (TM), as well as Li-, H-, and Pd-doped Cu 3 N , whose stabilities and magnetic properties are investigated. The interposition of 3d-TM atoms leads to mechanically stable yet brittle structures, with Sc, Mn, Ni, Cu, Zn possessing relatively small positive (endothermic) formation energies ( 0.12 ∼ 0.54 eV / TM ) , suggesting it may be easier to realize them experimentally than other 3d-TM elements. Li-, H-, Pd-doping in Cu 3 N are exothermic, while Ti, V, Cr, Fe, and Co have higher formation energy (0.93∼1.39 eV/TM) at a doping concentration 3.7 %. The fully 3d-TM doped Cu 3 N systems exhibit a wide spectrum of magnetic properties, ranging from weak antiferromagnetic (Sc-), antiferromagnetic (Ti-, V-, Cr-) to ferromagnetic (Mn-, Fe-, Co-) and non-magnetic (Ni-, Cu-, Zn-) behaviour. In particular, Ti : Cu 3 N exhibits weak itinerant magnetic properties with a large positive magnetovolume effect. All the 3d-TM atom intercalations into cubic Cu 3 N lead to a semiconductor-to-metal transition for both 100% and 3.7% doping, with the exception of Ni : Cu 3 N exhibiting a weak metallic or narrow semiconducting behaviour depending on the doping concentration.

We report the fabrication of single crystal kinked ZnO [001] and [100] nanowires (NWs). From the detailed morphology and microstructure analyzed by transmission electron microscopy, we discuss the growth characteristics, including nucleation and kinking, from a growth process point of view. We have discovered that the crystallographic property of the substrate surface played a critical role in determining the growth directions, which in turn determine the characteristics of single crystal kinking. A mechanism based on the unbalanced facet growth was proposed to explain the kinking of the single crystal kinked nanowires. Photoluminescence spectra confirm that the optical property of the ZnO NWs was not altered by the single crystal kinking.

Electric fields are regarded as a promising means of graphene oxide reduction, but previous studies have only focused on pristine graphene. Here, based on first principles density functional theory calculations, we report on electric field mediated reduction of neutral and charged O and hydroxyl groups from both pristine and defective graphene sheets. The critical electric field strengths for different species are determined in facilitating a progressive and selective graphene oxide reduction. Our results demonstrate that the presence of vacancy defects significantly inhibits the effectiveness of electric fields as a means of reduction of O and OH functionals, due to the complexities that arise between the functional group and vacancy edge atoms in the presence of an applied electric field.

Understanding the band structure evolution of (AlxGa1−x)2O3 alloys is of fundamental importance for developing Ga2O3-based power electronic devices and vacuum ultraviolet super-radiation hard detectors. Here, we report on the bandgap engineering of β-(AlxGa1−x)2O3 thin films and the identification of compositionally dependent electronic band structures by a combination of absorption spectra analyses and density functional theory calculations. Single-monoclinic β-phase (AlxGa1−x)2O3 (0 ≤ x ≤ 0.54) films with a preferred (−201) orientation were grown by laser molecular beam epitaxy with tunable bandgap ranging from 4.5 to 5.5 eV. The excellent fitting of absorption spectra by the relation of (αhν)1/2 ∝ (hν-E) unambiguously identifies that β-(AlxGa1−x)2O3 alloys are indirect bandgap semiconductors. Theoretical calculations predict that the indirect nature of β-(AlxGa1−x)2O3 becomes more pronounced with increased Al composition due to the increased eigenvalue energy gap between M and Г points in the valence band. The experimentally determined indirect bandgap exhibits almost a linear relationship with Al composition, which is consistent with the theoretical calculation and indicates a small bowing effect and a good miscibility. The identification and modulation of (AlxGa1−x)2O3 band structures allows rational design of ultra-wide bandgap oxide heterostructures for the applications in power electronics and solar-blind or X-ray detection.

The microscopic mechanism of the tunneling magnetoresistance (TMR) in Cr-doped GaN/AlN/GaN (0 0 0 1) trilayer junctions is studied based on density functional theory calculations. For enhanced performance, we propose δ -Cr-layer doping in GaN, close to the GaN/AlN interfaces. Depending on the doping concentration, Cr dopants produce local metallic (1 ML) or half-metallic ( 1 2 and 1 4 ML) states surrounded by the host semiconductor materials. Very thin AlN barriers are predicted to yield a low TMR effect. These results help explain existing experimental results and are expected to be valuable with regard to the practical fabrication of improved pure semiconductor spintronic devices.

We propose a new functionality for diamondoids in nanoelectronics. Based on the nonequilibrium Green's function formalism and density functional theory, we reveal that when attached to gold electrodes, the pentamantane-cumulene molecular junction exhibits large and oscillatory rectification and negative differential resistance (NDR) - depending on the number of carbon atoms in cumulene (Cn). When n is odd rectification is greatly enhanced where the rectification ratio can reach ∼180 and a large negative differential resistance peak current of ∼3 μA. This oscillatory behavior is well rationalised in terms of the occupancy of the carbon 2p states in Cn. Interestingly, different layers of C atoms in the pentamantane molecule have different contributions to transmission. The first and third layers of C atoms in pentamantane have a slight contribution to rectification, and the fifth and sixth layers have a stronger contribution to both rectification and NDR. Thus, our results suggest potential avenues for controlling their functions by chemically manipulating various parts of the diamondoid molecule, thus extending the applications of diamondoids in nanoscale integrated circuits.

Modulation of material physical and chemical properties through selective surface engineering is currently one of the most active research fields, aimed at optimizing functional performance for applications. The activity of exposed crystal planes determines the catalytic, sensory, photocatalytic and electrochemical behavior of a material. In the research on nanomagnets, it opens up new perspectives in the fields of nanoelectronics, spintronics and quantum computation. Herein, we demonstrate controllable magnetic modulation of α-MnO2 nanowires, which displayed surface ferromagnetism or antiferromagnetism, depending on the exposed plane. First-principles density functional theory calculations confirm that both Mn- and O-terminated α-MnO2 (1 1 0) surfaces exhibit ferromagnetic ordering. The investigation of surface-controlled magnetic particles will lead to significant progress in our fundamental understanding of functional aspects of magnetism on the nanoscale, facilitating rational design of nanomagnets. Moreover, we approved that the facet engineering pave the way on designing semiconductors possessing unique properties for novel energy applications, owing to that the bandgap and the electronic transport of the semiconductor can be tailored via exposed surface modulations.

Based on first-principles density functional calculations we propose a novel Al vacancy induced ferromagnetism occurring at the LaAlO(3) surface of SrTiO(3)/LaAlO(3) bilayers. Magnetism at cation vacancies away from the surface is quenched due to charge compensation. Magnetic surface Al vacancies are stabilized due to the built-in electric field inside the LaAlO(3) region that raises the energy of the defect level, making charge compensation unfavorable. Surface Al vacancies prefer to form clusters and exhibit two-dimensional ferromagnetic alignment mediated by a long-range magnetic interaction. These results are discussed in light of recent experimental observations.

A complete phase diagram and its corresponding physical properties are essential prerequisites to understand the underlying mechanism of iron-based superconductivity. For the structurally simplest 11 (FeSeTe) system, earlier attempts using bulk samples have not been able to do so due to the fabrication difficulties. Here, thin FeSe(x)Te(1-x) films with the Se content covering the full range (0 ≤ x ≤ 1) were fabricated by using pulsed laser deposition method. Crystal structure analysis shows that all films retain the tetragonal structure in room temperature. Significantly, the highest superconducting transition temperature (T(C) = 20 K) occurs in the newly discovered domain, i.e., 0.6 ≤ x ≤ 0.8. The single-phased superconducting dome for the full Se doping range is the first of its kind in iron chalcogenide superconductors. Our results present a new avenue to explore novel physics as well as to optimize superconductors.

The effects of iron deficiency in FexSe0.5Te0.5 thin films (0.8, Comment: 15 pages, 4 figures

Structural stability and hydrogen adsorption capacity are two key quantities in evaluating the potential of metal-adatom decorated graphene for hydrogen storage and related devices. We have carried out extensive density functional theory calculations for the adsorption of hydrogen molecules on 12 different adatom (Ag, Au, Ca, Li, Mg, Pd, Pt, Sc, Sr, Ti, Y, and Zr) decorated graphene surfaces where the adatoms are found to be stabilized on double carbon vacancies, thus overcoming the ``clustering problem'' that occurs for adatoms on pristine graphene. Ca and Sr are predicted to bind the greatest number, namely six, of H${}_{2}$ molecules. We find an interesting correlation between the hydrogen capacity and the change of charge distribution with increasing H${}_{2}$ adsorption, where Ca, Li, Mg, Sc, Ti, Y, Sr, and Zr adatoms are partial electron donors and Ag, Au, Pd, and Pt are partial electron acceptors. The ``18-electron rule'' for predicting maximum hydrogen capacity is found not to be a reliable indicator for these systems.

Using first-principles calculations, we investigate the band offsets, built-in electric fields, and band gaps of (0001)-oriented wurtzite ZnO/Mg${}_{0.25}$Zn${}_{0.75}$O superlattices, including the dependence on superlattice geometry and strain. Significant built-in electric fields form inside the quantum-well region that are found to be tunable over the range $0.24\phantom{\rule{4pt}{0ex}}\mathrm{MV}/\mathrm{cm}\ensuremath{\le}{E}_{w}\ensuremath{\le}0.63\phantom{\rule{4pt}{0ex}}\mathrm{MV}/\mathrm{cm}$, and potentially up to $1\phantom{\rule{4pt}{0ex}}\mathrm{MV}/\mathrm{cm}$ by varying the relative width of the well and barrier regions. The valence band offset at the ZnO/Mg${}_{0.25}$Zn${}_{0.75}$O interface is calculated to be 0.25--0.26 eV which, in contrast to the ``common anion rule,'' is a significant portion of the total band offset, and this is in support of recent experiment. Calculated values for the valence band offset were found to be insensitive to variations in superlattice geometry and strain. The band gap of the superlattice is determined by the competing effects of quantum confinement and the quantum-confined Stark effect, with the former being more dominant for the systems investigated. These findings will be useful in the design and optimization of ZnO/Mg${}_{x}$Zn${}_{1\ensuremath{-}x}$O superlattices for electronics and optoelectronics applications.

In order to unravel the magnetism of Co-doped ZnO films, we have performed rigorous experiments on Co-doped ZnO grown on O-polar ZnO (0001̄) substrates by molecular beam epitaxy. We find that the ZnO:Co with Co composition less than 20% is paramagnetic even at low temperatures, whereas that with Co composition of 20% shows ferromagnetism at room temperature. Although an additional n-type doping with Ga increases the magnitude of magnetization, the origin of the observed ferromagnetism is not carrier induced, as confirmed by electric-field effect measurements. Three-dimensional atom probe tomography shows that Co ions are randomly distributed, indicating that Co clustering or spinodal decomposition is not the origin of the ferromagnetism either. One possible mechanism for the ferromagnetism is hydrogen-facilitated interaction, which is supported experimentally by magnetic measurements on hydrogen-treated ZnO:Co as well as theoretically by first-principles calculation. © 2012 American Physical Society.

Density functional theory calculations are performed to evaluate the hardness of various cubic metal nitrides: rocksalt TiN, VN, ZrN, NbN, AlN, and SiN; zincblende AlN and BN; and diamond C for comparison. The isotropic elastic stiffness constants ${c}_{ij}$, bulk modulus $K$, shear modulus $G$, Young's modulus $E$, and isotropic Poisson's ratio $\overline{\ensuremath{\nu}}$ are calculated. From simulated uniaxial stress-strain curves, ideal strength values ${\ensuremath{\sigma}}_{\mathrm{max}}$ in the [100], [110], and [111] directions are also evaluated for all systems. In particular, rocksalt AlN is found to possess both high elastic moduli and ideal strength. These quantities are then compared for correlations with existing experimental Vicker's hardness data. The bulk modulus is found to be a poor indicator of hardness, while $E$, $G$, $1/\overline{\ensuremath{\nu}}$, and ${\ensuremath{\sigma}}_{\mathrm{max}}$ all exhibit stronger correlations. With a view to circumvent the need to run computationally expensive relaxation steps, different methodologies for approximating uniaxial stress-strain curves are introduced. Utilizing the anisotropic Poisson's ratio to approximate the relaxed transverse lattice parameters at a given axial strain is a good approximation to stress-strain curves, and the ideal strengths obtained in this way exhibit strong correlations to experimental Vicker's hardness values.