Electrical and optical measurements of the bandgap energy of a light-emitting diode

International audienceSemiconductors materials are at the basis of electronics. Most electronic devices are made of semiconductors. The operation of these components is well described by quantum physics which is a little mysterious to students. One of the intrinsic parameters of semiconductors is their bandgap energy Eg. In the case of light-emitting diodes (LEDs) Eg fixes the colour of the light emitted by the diodes. We propose in this article a lab work allowing a comparison of Eg of a green LED obtained by both electrical and optical measurements. The two slightly different results can be explained by the theoritical knowledges of the students on solid physics and electronic devices internal architecture

An introduction to photocatalysis through methylene blue photodegradation

International audienceWe described a simple experimental setup for a lab work on the photocatalytic degradation of methylene blue by T iO 2 nanoparticles. The photocatalysis process can be used for many applications. Treatments for diluted wastewaters industries, air purifying in underground car parks, preventing fouling on glass surfaces, these are some of the things this phenomenon has the ability to do. The described experiment is easy to perform and the interpretation can be easily adapted to different level of students: from high school students demonstrating their interests on a sustainable development, to students who are getting Masters in the science department that want to propose a full explanation of all the phenomenon of the photocatalytic process. Starting by a description of the experimental setup , we analysed the photocatalyst nanoparticles and applied the Langmuir-Hinshelwood model to our experimental data. Finally we discussed shortly on the respective energetic levels of the photocatalyst semiconductor and the methylene blue

Mn5Ge3C0.6/Ge(111) Schottky contacts tuned by a n-type ultra-shallow doping layer

International audienceMn 5 Ge 3 Cxcompound is of great interest for spintronics applications. 10 The various parameters of Au/Mn 5 Ge 3 C 0.6 /Ge(111) and Au/Mn 5 Ge 3 C 0.6 /δ-doped Ge(111) Schottky diodes were measured in the temperature range of 30-300 K by using current-voltage and capacitance-voltage techniques. The Schottky barrier heights and ideality factors were found to be temperature dependent. These anomaly behaviours were explained by Schottky barriers inhomogeneities 15 and interpreted by means of Gaussian distributions model of the Schottky barrier heights. Following this approach we show that the Mn 5 Ge 3 C 0.6 /Ge contact is described with a single Gaussian distribution and a conduction mechanism mainly based on the thermoionic emission. On the other hand the Mn 5 Ge 3 C 0.6 /δ-doped Ge contact is depicted with two Gaussian distributions according to the 20 temperature and a thermionic-field emission process. The differences between the two types of contacts are discussed according to the distinctive features of the growth of heavily doped germanium thin films

Magnetic properties of self-organized Co dimer nanolines on Si/Ag(110)

International audienceWe demonstrate the kinetically controlled growth of one-dimensional Co nanomagnets with a high lateral order on a nanopatterned Ag(110) surface. First, self-organized Si nanoribbons are formed upon submonolayer condensation of Si on the anisotropic Ag(110) surface. Depending on the growth temperature, individual or regular arrays (with a pitch of 2 nm) of Si nanoribbons can be grown. Next, the Si/Ag(110) system is used as a novel one-dimensional Si template to guide the growth of Co dimer nanolines on top of the Si nanoribbons, taking advantage of the fact that the thermally activated process of Co diffusion into the Si layer is efficiently hindered at 220 K. Magnetic characterization of the Co nanolines using X-ray magnetic circular dichroism reveals that the first atomic Co layer directly adsorbed onto the Si nanoribbons presents a weak magnetic response. However, the second Co layer exhibits an enhanced magnetization, strongly suggesting a ferromagnetic ordering with an in-plane easy axis of magnetization, which is perpendicular to the Co nanolines

Ferromagnetic resonance and magnetic damping in C-doped Mn5Ge3

International audience2 X-band ferromagnetic resonance (FMR) was used to investigate static and dynamic magnetic properties of Mn5Ge3 and Carbon-doped Mn5Ge3 (C0.1 and C0.2) thin films grown on Ge(111). The temperature dependence of magnetic anisotropy shows an increased perpendicular magneto-crystalline contribution at low temperature with an in-plane easy axis due to the large shape contribution. We find that our samples show as small as 40Oe FMR linewidth (corresponding Gilbert damping α=0.005), for the out-of-plane direction, certifying of their very good structural quality. The perpendicular linewidth shows a minimum around 200K for all samples, which seems not correlated to the C-doping. The magnetic relaxation parameters have been determined and indicate as main extrinsic contribution the two-magnon scattering. A transition from six-fold to twofold plus fourth-fold in-plane anisotropy is observed in the FMR linewidth of Mn5Ge3C0.2 around 200K

Very low-temperature epitaxial growth of Mn5Ge3 and Mn5Ge3C0.2 films on Ge(111) using molecular beam epitaxy

International audienceC-doped Mn5Ge3 compound is ferromagnetic at temperature up to 430 K. Hence it is a potential spin injector into group-IV semiconductors. Segregation and diffusion of Mn at the Mn5Ge3 /Ge interface could severely hinder the efficiency of the spin injection. To avoid these two phenomena we investigate the growth of Mn5Ge3 and C-doped Mn5Ge3 films on Ge(111) substrates by molecular beam epitaxy at room-temperature. The reactive deposition epitaxy method is used to deposit these films. Reflection high energy electron diffraction, X-ray diffraction analysis, transmission electron microscopy and atomic force microscopy indicate that the crystalline quality is very high. Magnetic characterizations by superconducting quantum interference device and ferromagnetic resonance reinforce the structural analysis results on the thin films quality

Observation of a spontaneous anomalous Hall response in the Mn5Si3 d-wave altermagnet candidate

Phases with spontaneous time-reversal (T ) symmetry breaking are sought after for their anomalous physical properties, low-dissipation electronic and spin responses, and information-technology applications. Recently predicted altermagnetic phase features an unconventional and attractive combination of a strong T -symmetry breaking in the electronic structure and a zero or only weak-relativistic magnetization. In this work, we experimentally observe the anomalous Hall effect, a prominent representative of the T -symmetry breaking responses, in the absence of an external magnetic field in epitaxial thin-film Mn5Si3 with a vanishingly small net magnetic moment. By symmetry analysis and first-principles calculations we demonstrate that the unconventional d-wave altermagnetic phase is consistent with the experimental structural and magnetic characterization of the Mn5Si3 epilayers, and that the theoretical anomalous Hall conductivity generated by the phase is sizable, in agreement with experiment. An analogy with unconventional d-wave superconductivity suggests that our identification of a candidate of unconventional d-wave altermagnetism points towards a new chapter of research and applications of magnetic phases

National records of 3000 European bee and hoverfly species: A contribution to pollinator conservation

peer reviewedPollinators play a crucial role in ecosystems globally, ensuring the seed production of most flowering plants. They are threatened by global changes and knowledge of their distribution at the national and continental levels is needed to implement efficient conservation actions, but this knowledge is still fragmented and/or difficult to access. As a step forward, we provide an updated list of around 3000 European bee and hoverfly species, reflecting their current distributional status at the national level (in the form of present, absent, regionally extinct, possibly extinct or non-native). This work was attainable by incorporating both published and unpublished data, as well as knowledge from a large set of taxonomists and ecologists in both groups. After providing the first National species lists for bees and hoverflies for many countries, we examine the current distributional patterns of these species and designate the countries with highest levels of species richness. We also show that many species are recorded in a single European country, highlighting the importance of articulating European and national conservation strategies. Finally, we discuss how the data provided here can be combined with future trait and Red List data to implement research that will further advance pollinator conservation

Effect of carbon on structural and magnetic properties of Ge1−xMnx\hbox {Ge}_{1-x}\hbox {Mn}_{x} nanocolumns

International audienceWe have investigated the structural and magnetic properties of Ge 0.94 Mn 0.06−δ C δ films (δ = 0.005, 0.01 and 0.02) using reflexion high-energy electron diffraction (RHEED) technique, transmission electron microscopy (TEM) and superconducting quantum interference device magnetometer. All films have been prepared by co-depositing Ge, Mn and C by molecular beam epitaxy. RHEED pattern shows the increase in sample surface roughness when doping carbon into the GeMn films. TEM analyses indicate that adding carbon greatly reduces the surface diffusion of both Ge and Mn elements. Ferromagnetic ordering in samples containing carbon contents of 0.01 and 0.02, persists at temperatures >400 K. An increase in net magnetization is found for carbon-doped samples with increasing carbon content from 0.01 to 0.02. However, we found a decrease in the net magnetization and the Curie temperature of the samples after annealing at 450 and 650°C. The Curie temperature reduces down to about 300 K, which is comparable to the value of the free-carbon sample showing a harmful effect of the post-annealing on the magnetic properties of carbon-doped GeMn nanocolumns