1,721,012 research outputs found
Attitudes towards Physics: Developing an instrument to measure the Physics learning improvement in Italian high school
No full textTo assess students’ attitudes towards Physics, teachers and researchers need an instrument focusing on the learning difficulties arising from different causes. As is well known in the literature, these difficulties depend on a number of factors deeply involved in the learning process and related with the teaching strategy and methodologies. Considering these main factors as referred to three-dimensional model, we have developed an attitude scale according to the cognitive, affective and behavioral components. The reliability test on a sample of about 500 students in a scientific high school of Trieste (Italy) retained an excellent Cronbach-Alpha coefficient. A brief description of the statistical analysis is reported focusing on the main results from the collected data. The use of this tool is proposed for a more extensive investigation in order to quantitatively highlight which attitudes emerge and encourage the promotion of curricular and orientation activities better calibrated to develop a positive attitude towards Physics
Moiré patterns generated by stacked 2D lattices: A general algorithm to identify primitive coincidence cells
Full text linkTwo-dimensional materials on metallic surfaces or stacked one on top of the other can form a variety of moiré superstructures depending on the possible parameter and symmetry mismatch and misorientation angle. In most cases, such as incommensurate lattices or identical lattices but with a small twist angle, the common periodicity may be very large, thus making numerical simulations prohibitive. We propose here a general procedure to determine the minimal simulation cell which approximates, within a certain tolerance and a certain size, the primitive cell of the common superlattice, given the two interfacing lattices and the relative orientation angle. As case studies to validate our procedure, we report two applications of particular interest: the case of misaligned hexagonal/hexagonal identical lattices, describing a twisted graphene bilayer or a graphene monolayer grown on Ni(111), and the case of hexagonal/square lattices, describing for instance a graphene monolayer grown on Ni(100) surface. The first one, which has also analytic solutions, constitutes a solid benchmark for the algorithm; the second one shows that a very nice description of the experimental observations can be obtained also using the resulting relatively small coincidence cells
Role of defects in the electronic properties of amorphous/crystalline Si interface
No full textThe mechanism determining the band alignment of amorphous/crystalline Si heterostructures is addressed with direct atomistic simulations of the interface performed using a hierarchical combination of various computational schemes ranging from classical model-potential molecular dynamics to ab initio methods. We found that in coordination defect-free samples the band alignment is almost vanishing and independent of interface details. In defect-rich samples, instead, the band alignment is sizably different with respect to the defect-free case, but, remarkably, almost independent of the concentration of defects. We rationalize these findings within the theory of semiconductor interfaces
Band offsets and stability of BeTe/ZnSe (100) heterojunctions
No full textWe present ab initio studies of band offsets, formation energy, and stability of (100) heterojunctions between (Zn,Be)(Se,Te) zinc-blende compounds, and in particular of the lattice-matched BeTe/ZnSe interface. Equal band offsets are found at Be/Se and Zn/Te abrupt interfaces, as well as at mixed interfaces, in agreement with the established understanding of band offsets at isovalent heterojunctions. Thermodynamical arguments suggest that islands of non-nominal composition may form at the interface, causing offset variations over similar to0.8 eV depending on growth conditions. Our findings reconcile recent experiments on BeTe/ZnSe with the accepted theoretical description
Band offsets engineering at semiconductor heterojunctions
No full textIn the present paper we discuss the electronic properties of semiconductor hetero junctions, focussing on the band offset problem. We address interface-specific phenomena, where the conditions of growth-including controlled contamination and strain effects-may significantly alter the properties of the junction. We study the effects of ultrathin intralayers (i.e. heterovalen-t implantation) both at homojunctions (such as GaAs/Ge/GaAs) and heterojunctions (such as GaAs/Si/AlAs). In both cases, our theory demonstrates how the intralayers control the band offset: this result is confirmed by recent experimental observation in several systems. We then consider the band-offset engineering at lattice-mismatched heterojunctions, studying the paradigmatic case GaAs/Si. We evaluate the band-offset for several fully relaxed inequivalent interfaces. Both macroscopic strain and microscopic morphology effects are considered
First-principles study of nickel reactivity under two-dimensional cover: Ni2 C formation at rotated graphene/Ni(111) interface
Full text linkRecent experiments indicate that the reactivity of metal surfaces changes profoundly when they are covered with two-dimensional (2D) materials. Nickel, the widespread catalyst choice for graphene (G) growth, exhibits complex surface restructuring even after the G sheet is fully grown. In particular, due to excess carbon segregation from bulk nickel to surface upon cooling, a nickel carbide (Ni2C) phase is detected under rotated graphene (RG) but not under epitaxial graphene (EG). Motivated by this experimental evidence, we construct different G/Ni(111) interface models accounting for the two types of G domains. Then, by applying density functional theory, we illuminate the microscopic mechanisms governing the structural changes of nickel surface induced by carbon segregation. A high concentration of subsurface carbon reduces the structural stability of Ni(111) surface and gives rise to the formation of thermodynamically advantageous Ni2C monolayer. We show the restructuring of the nickel surface under RG cover and reveal the essential role of G rotation in enabling high density of favorable C binding sites in the Ni(111) subsurface. As opposed to RG, the EG cover locks the majority of favorable C binding sites preventing the build-up of subsurface carbon density to a phase transition threshold. Therefore we confirm that the conversion of C-rich Ni surface to Ni2C takes place exclusively under RG cover, in line with the strong experimental evidence
Bulk and Interfacial Strain in Si/Ge heterostructures
No full textThe structural properties of pseudomorphic Si/Ge(001) heterostructures are investigated using a newly developed model in which the interface is viewed as a perturbation with respect to an infinite periodic system (the virtual crystal). The use of density-functional perturbation theory allows us to treat composition profiles of virtually any complexity at a negligible numerical cost and with an accuracy which is very close to that of state-of-the-art self-consistent calculations. We find that-at variance with recent measurements from x-ray photoelectron diffraction and ion scattering experiments-the macroscopic theory of elasticity accurately predicts the interplanar spacings of the heterostructure starting already at 2-3 planes away from the interface whereas a sizeable overstrain occurs in the region close to the interface. We suggest that the discrepancy between our findings and the experimental results could be due to atomic interdiffusion across the interface which would affect the composition of nominally pure Si (Ge) slabs, thus reducing the stress acting on the epilayer
The role of strain in bad offsets engineering at lattice-mismatched semiconductor heterojuctions
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