A scheme is proposed to solve the structure of incommensurate interfaces, starting from high-resolution images of electron microscopy, supplemented by adapted simulation techniques, and complemented by theoretical tools. Direct silicon bonding is a way to produce artificial interfaces, in particular incommensurate ones. We focus on a technology-driven tilt grain boundary in silicon. While the Fibonacci sequence, linked to the golden ratio, is a prototype of the quasicrystalline structures, a silver-ratio sequence allows us to analyze this incommensurate interface. The fourfold coordination of the Si atoms is kept at the interface.

Motivated by recent high resolution results on the inversion domain boundaries (IDB) in gallium nitride, we refine by ab initio DFT calculations the well established atomic model IDB* derived by Northrup et al. This allows us to recover these experimental results obtained by coherent x-ray diffraction and showing small additional shifts of the polarity domains, in particular 8 pm shift along the hexagonal direction. The influence of boundary conditions and electrostatic fields (IDB-IDB and IDB-surface interactions) on the results and the existence of metastable solutions is carefully discussed to stress the accuracy of the method. These results demonstrate a cross-talk between advanced characterization tools and state-of-the-art ab initio calculations that opens perspectives for the structural analysis of defects in the picometer range.

Le riz étuvé local, par ses valeurs nutritionnelles meilleures que celles du riz blanc, est une alternative intéressante pour infléchir la dépendance du Bénin au riz blanc importé. Depuis la crise rizicole de 2008, des efforts ont été consentis pour augmenter la production, améliorer la qualité et faciliter l’accès du riz étuvé local aux marchés urbains. Pourtant, en 2016, ce riz reste quasi introuvable sur les marchés de Cotonou, principale ville du Bénin. Conduit sur un échantillon de 210 acteurs, cet article utilise le paradigme Structure-Comportement-Performance pour identifier les principales raisons de l’indisponibilité du riz étuvé local sur les marchés de Cotonou. Nos résultats indiquent que le riz étuvé local ne représente que 3 % de l’offre de riz, soit cinq fois moins que le riz étuvé importé. La méconnaissance des valeurs nutritionnelles du riz étuvé local par les ménages, son prix supérieur à celui du riz étuvé importé et son indisponibilité tout au long de l’année et chez tous les commerçants sont les principales raisons de la faible demande. Avec des sensibilisations (séances de sensibilisation, des panneaux et spots publicitaires) sur les valeurs nutritionnelles du riz étuvé local, les consommateurs urbains pourraient modifier leur comportement en faveur du riz étuvé local. Cependant, étant donné que le riz étuvé est également importé, nos résultats suggèrent qu’une étude plus fine soit réalisée sur les critères de choix du riz étuvé afin d’éviter la dépendance du Bénin au riz étuvé importé.

Journal Article High Resolution Electron Microscopy of Grain Boundary Motion During Island Grain Shrinkage Get access U Dahmen, U Dahmen National Center for Electron Microscopy, Molecular Foundry, LBNL, Berkeley, CA 94720, USA Search for other works by this author on: Oxford Academic Google Scholar T Radetic, T Radetic National Center for Electron Microscopy, Molecular Foundry, LBNL, Berkeley, CA 94720, USAFaculty of Technology and Metallurgy, University of Belgrade, Belgrade, Serbia Search for other works by this author on: Oxford Academic Google Scholar ML Bowers, ML Bowers National Center for Electron Microscopy, Molecular Foundry, LBNL, Berkeley, CA 94720, USA Search for other works by this author on: Oxford Academic Google Scholar C Ophus, C Ophus National Center for Electron Microscopy, Molecular Foundry, LBNL, Berkeley, CA 94720, USA Search for other works by this author on: Oxford Academic Google Scholar A Gautam, A Gautam National Center for Electron Microscopy, Molecular Foundry, LBNL, Berkeley, CA 94720, USA Search for other works by this author on: Oxford Academic Google Scholar F Lançon F Lançon Laboratoire de Simulation Atomistique (L_Sim), SP2M, INAC, CEA, 38054 Grenoble, France Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 22, Issue S3, 1 July 2016, Pages 1224–1225, https://doi.org/10.1017/S1431927616006966 Published: 25 July 2016

Using extended time series scanning transmission electron microscopy, we investigate structural fluctuations at an incommensurate grain boundary in Au. Atomic-resolution imaging reveals the coalescence of two interfacial steps, or disconnections, of different height via coordinated motion of atoms along close-packed directions. Numerical simulations uncover a transition pathway that involves constriction and expansion of a characteristic stacking fault often associated with grain boundaries in face-centered cubic materials. It is found that local atomic fluctuations by enhanced point defect diffusion may play a critical role in initiating this transition. Our results offer new insights into the collective motion of atoms underlying the lateral advance of steps that control the migration of faceted grain boundaries.

Here, we report on the precise analysis of the atomic structure of inversion domain boundaries (IDBs) in GaN by scanning transmission electron microscopy. IDBs are a common defect in GaN that traps carriers and leads to a slightly modified luminescence wavelength [1,2]. Our analysis of IDBs in MOCVD grown nanowires confirms recent coherent Bragg imaging (CBI) results [3] stating that the atomic structure of this IDB is different or slightly different from the one determined in 1996 by first‐principle calculations (IDB*) [4]. CBI experiments measured a 8 pm shift of the c‐planes of the two domains [3], whereas first‐principle calculations predicted no shift. A previous study by STEM [5] found a shift of “ca. 0.6 Å” (60pm), corresponding roughly to the switch of the Ga and N positions without any additional shift. Here in addition to a shift along c, we show that the interface configuration corresponds qualitatively to the IDB* model (cf. Fig. 1) and that there is a 10 pm dilatation perpendicular to the interface (shown in Fig. 3) in agreement with this model, while CBI did not find a dilatation. To facilitate the measurement of atom positions across the IDB with picometre‐precision, we use HAADF‐STEM to avoid coherent effects leading to artefacts. Scanning and drift artefacts are being suppressed by acquiring series of rapid STEM images and aligning them using the newly developed Zorro code. This algorithm is based on calculating estimated drift positions by correlating every frame to multiple frames and minimizing the error of the overdetermined system to obtain a best estimate for the frame positions relative to each other. The sub‐pixel aligned frames are then averaged and the peak positions are determined via TeMA (template‐matching algorithm). Our quantitative analysis of experimental and simulated STEM images shows that when atomic columns are very close to each other the measured distance can be slightly different from the real value. For instance, when the distance between atomic columns becomes smaller than 0.1 nm, the difference between the measured and real values can account for several picometres. This effect can be well observed when an IDB kinks perpendicular to the observation direction leading to closely projected columns in the overlap region of the two domains as seen in Fig. 2. Electron scattering simulations show that the apparently wider distance between atoms is a channeling effect. These results have provided elements to revisit previous theoretical models of IDBs in GaN.

Grain boundaries in crystalline materials affect many macroscopic properties such as strength, electrical resistivity, and corrosion resistance. Processing techniques to enhance these properties are therefore often aimed at modifying the existing grain boundary content via processes such as recrystallization and grain growth. While much is known about the structural character of grain boundaries and the variables that affect their mobility, the atomic-scale mechanisms of migration are still poorly understood. Recent experimental work suggests that step nucleation and cooperative, string-like motion of many atoms near a boundary may contribute to its advancement [1,2]. Molecular dynamics simulations indicate that these atomic cascade events can be triggered by volume fluctuations at the grain boundary and may occur in the absence of an external driving force [3].

To analyze extended time series of high resolution images, we have employed automated frame-by-frame comparisons that are able to detect dynamic changes in the structure of a grain boundary in Au. Using cumulative averaging of images between events allowed high resolution measurements of the atomic relaxation in the interface with sufficient accuracy for comparison with atomistic models. Cumulative averaging was also used to observe the structural rearrangement of atomic columns at a moving step in the grain boundary. The technique of analyzing changing features in high resolution images by averaging between incidents can be used to deconvolute stochastic events that occur at random intervals and on time scales well beyond that accessible to single-shot imaging.

The 2008 rice-price surge provided ammunition to the opponents of rice trade liberalization in West Africa. However, a comparative analysis of the rice development history and policy changes since the 1980s across selected West African countries shows that neither protectionism nor liberalization had a sustainable impact on West Africa׳s rice import dependency. Both policy options wrongly assume that rice markets are efficient and able to forward price incentives to producers, while they are actually deeply segmented between local and imported rice. Without putting a higher priority on the development of an efficient local rice marketing system, rice development will likely have a limited impact on import dependency level, whatever trade policy options are chosen (protectionism or liberalization).

The Empowering Smallholder Farmers in Markets programme (ESFIM) supported the advocacy capacities of national farmer organisations (NFOs) for improving smallholder market access. The programme gave NFOs in 11 countries the opportunity to contract local experts to strengthen the evidence-base of their advocacy proposals. By means of a participative process, each NFO analysed the key advocacy issues for which research support was most needed. This resulted in a diversity of themes and advocacy processes. These ranged from research and advocacy on the role of cooperatives in Uruguayan innovation policies to a simulation game on market dynamics in a commodity exchange in the Philippines, and from the review of various seed multiplication programmes in Malawi to the legal and administrative hurdles preventing smallholders from selling to government procurement programmes in Peru and Bolivia. We describe the dynamics surrounding research partnerships in each of the 11 countries and reflect on results of this research-for-advocacy. We also assess the impact of the ESFIM programme on the advocacy capacity of the NFOs. We used a self-evaluation technique, in which board members considered a list of statements covering five capacities that are deemed necessary for effective advocacy. We compared their scores with the observations of external stakeholders who were active in the agricultural sector and knowledgeable about the NFOs’ activities. We conclude that ESFIM has helped to increase coherence in advocacy priorities and has influenced decision making on key policy issues, with encouraging results. In most organisations, ESFIM contributed to the advocacy process, together with many other actors and factors defining advocacy. Based on these experiences, we suggest earmarking funds for NFO-led research support, in order to facilitate the participation of smallholders in the design and monitoring of development policies.

The structure of an incommensurate 90°〈1 1 0〉 tilt grain boundary in gold was characterized by atomic resolution aberration-corrected electron microscopy and compared with atomistic simulations. Based on a periodic hyperspace description, the non-periodic structure can be described by the Aubry hull function, which plots atomic relaxations at the core of the boundary relative to an unrelaxed structure, folded into a single repeat unit of the neighboring grain. By measuring the hull functions from atomic resolution images, we were able to make quantitative comparisons of experimental observations with molecular statics simulations of this boundary. The results show good agreement in the pattern of atomic relaxations, replicate features of the hull functions that are characteristic of a boundary with superglide behavior and demonstrate the experimental feasibility of this approach for analysis of interfaces.

By combining high-resolution electron microscopy and atomistic simulations, the atomic structures of several interfaces, {510}, {230} and {810}/{740}, in germanium and in silicon sigma=13 [001] tilt grain boundaries (TGBs) are studied using bicrystals prepared in two different ways from the melt. The interfaces are characterized by either transmission electron microscopy or scanning transmission electron microscopy (STEM). The Si TGB shows only one interface, {150} with one interfacial structure. The Ge TGB contains many facets. In Ge, observations performed in two perpendicular directions, [001] and [50], confirm that the {510} interface has two different structures. One structure, called M-structure, is periodic along [001] and has tetracoordinated atoms. The other structure, called U-structure, is more peculiar as it contains a fixed part surrounding a variable complex core. High-resolution STEM, realised in modern microscopes equipped with a probe Cs-corrector, is a very effective technique for structure determination of grain boundaries (GBs). However, current limitations for high-resolution study of GBs are the structural changes under the electron beam and the limited number of crystallographic axes suitable for atomic-resolution imaging. The structures of GB atomistic models can be ordered according to their calculated energies. It appears that energies calculated using empirical potentials, like Tersoff or Stillinger-Weber potentials, do not give the same classification as ab initio calculations and cannot be used to determine the structure of lowest energy. This structure is the M-structure, the structure observed in the Si bicrystal.

Finite size effects on the exchange-bias properties of patterned antiferromagnetic/ferromagnetic (IrMn/Co) nanodots were investigated experimentally and by numerical simulations. Both the dot lateral dimension (200 x 200 nm(2)-50 x 50 nm(2)) and ferromagnetic layer thickness (2-25 nm) were varied. Two different behaviours were observed: single-shifted loops with coherent reversal for thin Co layers and double-shifted loops via a multidomain state during magnetization reversal for thicker Co. Atomistic simulations including magnetostatic interactions confirmed the existence of these two different regimes. MOKE measurements, focused on few dots, allowed a quantitative evaluation of dot-to-dot exchange-bias variability, which becomes particularly large in the regime of switching via multidomain state formation. This variability issue is of particular importance in the context of MRAM development and of magnetoresistive heads for hard-disk drives.

Extended abstract of a paper presented at Microscopy and Microanalysis 2013 in Indianapolis, Indiana, USA, August 4 – August 8, 2013.

By combining high-resolution electron microscopy and atomistic simulations, the atomic structures of several interfaces, {5 1 0}, {2 3 0} and {8 1 0}/{7 4 0}, in germanium and in silicon Σ = 13 [0 0 1] tilt grain boundaries (TGBs) are studied using bicrystals prepared in two different ways from the melt. The interfaces are characterized by either transmission electron microscopy or scanning transmission electron microscopy (STEM). The Si TGB shows only one interface, {1 5 0} with one interfacial structure. The Ge TGB contains many facets. In Ge, observations performed in two perpendicular directions, [0 0 1] and [ 5 0], confirm that the {5 1 0} interface has two different structures. One structure, called M-structure, is periodic along [0 0 1] and has tetracoordinated atoms. The other structure, called U-structure, is more peculiar as it contains a fixed part surrounding a variable complex core. High-resolution STEM, realised in modern microscopes equipped with a probe Cs-corrector, is a very effective technique for structure determination of grain boundaries (GBs). However, current limitations for high-resolution study of GBs are the structural changes under the electron beam and the limited number of crystallographic axes suitable for atomic-resolution imaging. The structures of GB atomistic models can be ordered according to their calculated energies. It appears that energies calculated using empirical potentials, like Tersoff or Stillinger-Weber potentials, do not give the same classification as ab initio calculations and cannot be used to determine the structure of lowest energy. This structure is the M-structure, the structure observed in the Si bicrystal. Keywords: grain boundaryhigh-resolution electron microscopyatomistic calculationsirradiated solids Acknowledgements This paper is in commemoration of the years between 1985–2004 when Jany Thibault-Pénisson, at that time her name was Jany Thibault, was a researcher at CEA-Grenoble. Substantial parts of electron microscopy described here were carried out using the JEOL 4000EX installed at CEA-Grenoble, which was a wonderful microscope. One of us acknowledges the contribution of Alain Bourret for starting the research project that has led to this work. Models in Figure 9 have been drawn using the free V_Sim software available at [29] D. Caliste. Available at http://inac.cea.fr/L_Sim/V_Sim [Google Scholar]. We thank Sylvie Lartigue for reading and correcting this paper, together with the anonymous referees and the editors of this special issue.

We use extensive first-principles simulations to show the major role played by interfaces in the mechanism of phase separation observed in semiconductor multifunctional materials. We make an analogy with the precipitation sequence observed in oversaturated AlCu alloys, and replace the Guinier-Preston zones in this new context. A class of materials, the alpha phases, is proposed to understand the formation of the coherent precipitates observed in the GeMn system. The interplay between formation and interface energies is analyzed for these phases and for the structures usually considered in the literature. The existence of the a phases is assessed with both theoretical and experimental arguments.

Compared effects of public policies on rice markets and food security in West Africa: Getting beyond the liberalization versus protection debateThe 2007 to 2008 price surges on the world market for cereals have triggered a debate on the ability of trade liberalization to ensure food security in the most food insecure countries. Taking the case of rice in West Africa, the impact of the trade liberalization process is assessed with regards to the changes in the level of West African rice market dependency upon rice imports from the world market. The continuous deterioration of the rice food balance is actually due to a change in diet rather than to declining production, which has, in fact, the same growth rate as that of the population. Comparing the evolution of the per capita production increase across six countries, three of which have widely opened their rice market to imports and three others, which have maintained a higher level of protection, we found that it is not possible to unambiguously correlate rice dependency levels with a specific trade policy option. Beyond the strong debate opposing the proponents of rice trade liberalization in West Africa against the supporters of higher custom tariffs, decision-makers should pay more attention to processes linking local supply to urban demand, as these marketing and processing functions are crucial for materializing the incentives expected from both policy options for strengthening food security.

Extended abstract of a paper presented at Microscopy and Microanalysis 2011 in Nashville, Tennessee, USA, August 7–August 11, 2011.

We study the whole Ge-Mn phase diagram with density functional theory ( DFT) methods. The 16 known phases are described and trends are analyzed. The compounds are then simulated, allowing a complete evaluation of this method in the projector augmented-wave approach within the collinear spin-polarized framework. Structural parameters, as well as magnetic properties, are compared to experimental values. Stability issues are addressed using a thermodynamic approach based on the grand potential, showing good agreement with experimental data. The impact of semicore electrons and the exchange-correlation functional are also discussed. Finally, it is shown that DFT methods are well suited to study this system, provided that the generalized gradient approximation is used, as opposed to the local density approximation, and correlations between structural errors and Mn concentration are taken into account. In addition, the precision achieved when compared to experiments is 40 meV/atom on energy, +/- 3% on the lattice parameter, and 0.2 mu(B)/Mn on magnetic moments. Magnetic orders are mostly well reproduced.

We present microscopic calculations of the cohesive energy of amorphous metallic alloys. Our method is based on a tight-binding and Monte Carlo simulation approaches to calculate the equilibrium atomic structure. The same model tight-binding Hamiltonian is then used to calculate electronic structure and energy using a Bethe-Cluster approximation.