Atomically Thin Resonant Tunnel Diodes built from Synthetic van der Waals Heterostructures

Vertical integration of two-dimensional van der Waals materials is predicted to lead to novel electronic and optical properties not found in the constituent layers. Here, we present the direct synthesis of two unique, atomically thin, multi-junction heterostructures by combining graphene with the monolayer transition-metal dichalocogenides: MoS2, MoSe2, and WSe2.The realization of MoS2-WSe2-Graphene and WSe2-MoSe2-Graphene heterostructures leads toresonant tunneling in an atomically thin stack with spectrally narrow room temperature negative differential resistance characteristics

Effect of MoO3 in the cathode buffer layer on the behaviour of layered organic solar cells

The behaviour of small-molecule organic solar cells based on coper-phthalocyanine/fullerene with different cathode buffer layer is investigated as a function of air exposure duration. We present the study of the effect of MoO3 on the properties of photovoltaic solar cells (OPVCs) when it is introduced in the cathode buffer layer (CBL). Photovoltaic performances were measured as a function of time of air exposure. During the first days of air exposure the efficiency of the OPVCs with MoO3 in their CBL increases significantly, while it decreases immediately after air exposure in the case of reference OPVCs, i.e. without MoO3 in the CBL. Nevertheless, the lifetime of the OPVCs with MoO3 in their CBL is around 60 days, while it is only 10 days in the case of reference OPVCs. The initial increase of the OPVC with MoO3 in their CBL is attributed to the slow decrease of the work function of MoO3 due to progressive contamination. Then, the progressive degradation of the OPVCs efficiency is due water vapour and oxygen contamination of the organic layers. The use of a double CBL, Alq3/MoO3, allows to interrupt the growth of pinholes, defects and increases the path of permeating gas. Also it can prevent the contamination of the organic layer by Al. All this results in significant increase of the lifetime of the OPVCs

First attempts for predicting future Salinization in coastal irrigated areas

Salinity contamination of irrigation ground water is a serious worldwide problem. The Mnasra zone, which has an agricultural land area that represents 70% of the total area and its agricultural production reaches12% of the national production, is threatened by a Salinization of underground waters. The ground water available in this region allows the supply of a population of 145 000 inhabitants. We have developed a simple mechanistic simulation model for predicting future Salinization in this ground water. The method adopted to predict the variation of salinity pollution in ground water is the same as the population based on an increase (ordecrease) geometry (geometric series). This work allows preparing for and coping with the environmental pollution that threatens not only humans, but also the fauna and flora of this region. According to this study, we predict an area of 8730 hectares where water will be polluted by the salinity in 2021. These results will have a negative impact at medium and long terms on the agricultural production of this region

The surface science of quasicrystals

The surfaces of quasicrystals have been extensively studied since about 1990. In this paper we review work on the structure and morphology of clean surfaces, and their electronic and phonon structure. We also describe progress in adsorption and epitaxy studies. The paper is illustrated throughout with examples from the literature. We offer some reflections on the wider impact of this body of work and anticipate areas for future development. (Some figures in this article are in colour only in the electronic version

Incidence de pluies exceptionnelles sur un aquifère hétérogène côtier (M’nasra, Maroc)

Les eaux souterraines qui représentent les seules ressources en eau de la M’nasra côtière, sont minéralisées : intrusion marine, évaporation, recyclage des eaux d’irrigation chargées en sels et l’utilisation abusive des engrais. L’évolution spatio-temporelle de la pollution nitrique de cette nappe phréatique a été suivie sur 159 puits, objective en trois périodes de campagne éclatées : 2003, 2007 et 2010 qui inclue une période exceptionnellement pluvieuses. Une comparaison par apport à l’état de la nappe en 1993, date de début de la surexploitation des eaux souterraines, a été faite. Les suivis ont montré qu’à la suite des Pluiesimportantes de la période de campagne 2009/2010, on a pu observer une dégradation de la qualité de la nappe aussi bien au niveau ponctuel qu’au niveau spatial, en particulier dans la partie Nord de la zone étudiée (aunord de la ligne de cordonnée Y = 430 000 m). Les résultats de cette étude sont présentés sous forme de cartes thématiques par le SIG.  Mots clés: Aquifère côtier, inondation, pollution nitrique, nappe phréatique, M’nasra, Maro

Évaluation de la pollution nitrique de la nappe phréatique de la

La pollution nitrique de la nappe phréatique de la zone côtière du Gharb du Maroc (M’nasra) a été révélée depuis 1993 par l’Office Régional de Mise en Valeur Agricole du Gharb. En effet, il a été procédé en 2003 à la prospection de 161 puits dans la zone côtière correspondant aux même puits prospectés en 1993 et dont les cordonnées Lambert sont bien identifiés. Cette prospection avait pour objectif l’analyse des nitrates, pour appréhender l’évolution de la pollution nitrique. La nappe de M’nasra qui a fait l’objet de notre étude est soumise à une activité agricole très intense, qui risquent de compromettre l’avenir de sa qualité. Les principaux résultats montrent que les moyennes générales de la concentration des nitrates en 1993 et 2003 sont respectivement de 106,4 mg/L et 119.6 mg/L ; soit une augmentation de 13,2 mg/L. Si en 1993, 44 puits des 161 puits prospectés (27,3 %) répondaient aux normes de potabilité et dosaient moins de 50 mg/L, en 2003 seulement 28 puits parmi les 161 puits prospectés (17,4 %) répondaient aux normes de potabilité. L’analyse spatiale de la pollution nitrique de la zone M’nasra, montre qu’il y a une certaine amélioration de la qualité de la nappe entre 1993 et 2003 au Sud de la zone étudiée (au Sud de la ligne de coordonnée Y = 430 000 m), en revanche la qualité de la nappe a été dégradée dans la moitié Nord de cette zone

Topological degree and application to a parabolic variational inequality problem

We are interested in constructing a topological degree for operators of the form F=L+A+S, where L is a linear densely defined maximal monotone map, A is a bounded maximal monotone operators, and S is a bounded demicontinuous map of class (S+) with respect to the domain of L. By means of this topological degree we prove an existence result that will be applied to give a new formulation of a parabolic variational inequality problem

On the solvability of a variational inequality problem and application to a problem of two membranes

The purpose of this work is to give a continuous convex function, for which we can characterize the subdifferential, in order to reformulate a variational inequality problem: find u=(u1,u2)∈K such that for all v=(v1,v2)∈K, ∫Ω∇u1∇(v1−u1)+∫Ω∇u2∇(v2−u2)+(f,v−u)≥0 as a system of independent equations, where f belongs to L2(Ω)×L2(Ω) and K={v∈H01(Ω)×H01(Ω):v1≥v2  a.e. in Ω}

Risk of cervical abnormality after age 50 in women with previously negative smears

There is discussion over the benefit of continuing cervical screening in women over the age of 50 with a history of negative cytology. We aimed to determine the risk of abnormal cytology in such women. Screening history data from 1985 to 2003 were obtained for a cohort of 2 million women from the NHS cervical screening programme from four Health Authorities in England. The 57 651 women in the cohort who reached age 40 between 1 January 1985 and 31 December 1990 and had at least one routine or opportunistic smear between ages 50 and 54 were included in the analysis. Exposure groups (negative cytology history, negative but including inadequate smears, and positive history) were defined on the basis of screening histories from ages 40 to 49. Sixty-four percent (134/206) (95% CI: 57–71%) of the moderate dyskaryosis or worse lesions at ages over 50 were detected from women in the negative smear history group. After allowance for time since last negative smear, the relative risk for the first primary smear over the age of 50 having moderate dyskaryosis or worse decreased from 0.60 (95% CI: 0.41–0.84) for two negative smear episodes to 0.25 (95% CI: 0.10–0.56) for four negative smear episodes, compared with the positive history group. If screening were discontinued for all women over 50 with a negative history, the majority of cytological abnormalities now being detected at these ages that lead directly to referral to colposcopy would be missed