196,288 research outputs found
I caratteri e le criticità fisico-ambientali del territorio
No full textI territori agroforestali stanno ritrovando una nuova vitalità, soprattutto nelle aree ad alta urbanizzazione. Chiamati «spazi aperti» senza ulteriore specificazione, semplicemente per contrapporli agli spazi costruiti, essi sono stati a lungo considerati puri «spazi vuoti», abbandonati o in attesa di urbanizzazione. Al territorio rurale periurbano e alle aree verdi interstiziali (parchi, giardini, orti ecc.) è oggi invece riconosciuto un ruolo decisivo nella fornitura di servizi essenziali alla popolazione, in termini di qualità della vita e di resilienza insediativa.
Riflettendo sulle attività di ricerca del progetto Coltivare con l’Arno. Parco agricolo perifluviale – un laboratorio sperimentale
di innovazione finanziato dall’Autorità Regionale Toscana per la Partecipazione, dalla Città Metropolitana di Firenze (ente capofila) e dai Comuni di Firenze, Scandicci e Lastra a Signa –, questo
libro propone una visione bioregionale del progetto di territorio, individuando nella struttura ecologica, nella dotazione patrimoniale e identitaria di lunga durata (centri storici, viabilità fondativa, paesaggi rurali, fiumi ecc.), e soprattutto nella capacità della comunità locale di prendersi cura dei propri luoghi e di governare
il proprio futuro, gli elementi vitali per la rinascita delle aree metropolitane.
Con scritti di Francesco Alberti, Giovanni Belletti, Marco Benvenuti, Elisa Butelli, Maria Rita Gisotti, Leonardo Lombardi, Stefano Morelli, Alexander Palummo, Maddalena Rossi, Adalgisa Rubino, Alessio Tanganelli, Alessandro Trivisonno
Silicon−Germanium Nanowires: Chemistry and Physics in Play, from Basic Principles to Advanced Applications
No full textCONTENTS
1. Introduction 1371
2. Growth Techniques, Morphology, and Structural
Properties 1373
2.1. Alloyed Nanowires 1373
2.2. Axial Heterostructures 1375
2.3. Radial Heterostructures 1377
3. Chemical and Physical Properties 1379
3.1. Electronic Properties 1379
3.1.1. Modulation of the Electronic Properties
by Composition Control 1379
3.1.2. Interfaces at Work: Strain, Band-Offset,
and Carrier Gases 1381
3.1.3. Doped Nanowires 1384
3.2. Thermal and Thermoelectric Properties 1385
4. Theoretical Modeling 1389
4.1. Electronic Structure 1390
4.1.1. Quantum Confinement Effect and Band
Offset 1391
4.1.2. Size Effects 1393
4.1.3. Alloying and Interface Effects 1394
4.1.4. Strain Effects 1395
4.1.5. Addition of Impurities 1395
4.1.6. Electronic Transport 1396
4.1.7. Optical Properties 1397
4.2. Phonons and Thermal Conductivity 1398
4.2.1. Breakdown of Fourier’s Law at Nanoscale 1398
4.2.2. Numerical Simulations of Thermal Properties
1398
5. Devices and Applications 1402
5.1. High-Performance Nanoelectronic Components
1403
5.1.1. Si1−xGex Alloy Nanowire Transistor 1403
5.1.2. Si-Shell Ge-Core Nanowire Transistor 1404
5.2. From Quantum Transport to Superconductivity:
SiGe Nanowires As Platforms for
Fundamental Physics Studies 1405
6. Conclusions and Perspectives 1405
Author Information 1406
Corresponding Authors 1406
Notes 1406
Biographies 1407
Acknowledgments 1408
References 140
Band structure analysis in SiGe nanowires
No full textOne of the main challenges for Silicon-Germanium nanowires (SiGe NWs) electronics is the possibility to modulate and engine their electronic properties in an easy way, in order to obtain a material with the desired electronic features. Diameter and composition constitute two crucial ways for the modification of the band gap and of the band structure of SiGe NWs. Within the framework of density functional theory we present results of ab initio calculations regarding the band structure dependence of SiGe NWs on diameter and composition. We point out the main differences with respect to the case of pure Si and Ge wires and we discuss the particular features of SiGe NWs that are useful for future technological application
First Principles simulations
No full textIn this paper we outline the major features of the “ab-initio” simulation scheme of Car and
Parrinello, focusing on the physical ideas and computational details at the basis of its efficiency
and success. We briefly review the main applications of the method. We discuss the limitations
of the standard scheme, as well as recent developments proposed in order to extend the reach
of the method. Moreover, we consider more in detail two specific subjects. First, we describe a simple improvement
(Gradient Corrections) on the basic approximation of the "ab-initio" simulation, ie the
Local Density Approximation. These corrections can be easily and efficiently included in the
Car-Parrinello code, bringing computed structural and cohesive properties significantly closer
to their experimental values. Finally, we discuss the choice of the pseudopotential, with special
attention to the possibilities and limitations of the last generation of soft pseudopotentials
Optical absorption modulation by selective codoping of SiGe core-shell nanowires
No full textFirst-principles calculations on the structural, electronic, and optical properties of B-P codoped SiGe core-shell nanowires are discussed. We show that the simultaneous addition of B and P impurities into the wire can be energetically favored with respect to the single-doping. We demonstrate that impurities energetic levels in the band gap are dependent by the Si/Ge band offset, as well as by their location in the wire (i.e., core or shell region). This electronic tunability results in a significant optical modulation, as demonstrated by the red-shift of the first optical peak when B and P locations are switched in the wir
Segregation, quantum confinement effect and band offset for [110] SiGe NWs
No full textResults of first-principles DFT simulations provide strongevidence that, at zero temperature, for [110] oriented SiGenanowires (NWs), the segregated structure is favoured withrespect to the mixed ones; for this observation two differentschemes of calculations are presented and discussed. Moreoverthe segregation strongly influences the NWs electronic properties,inducing a reduced quantum confined effect (RQCE). Weshow here that it depends on the effect of strain in the planenormal to the direction of growth and not on the choice of latticeparameter in the direction of growth.Aqualitative evaluation ofthe band offset between Si and Ge for SiGe NWs is alsopresented
SiGe nanowires for thermoelectric applications
No full textThe possibility to reduce the thermal conductivity leaving essentially
unaltered the electron transport makes semiconducting nanowires ideal materials for
the engineering of high-efficiency thermoelectric devices. A simple and appealing
route to achieve these goals is bringing together Si and Ge, giving rise to Si1 x Gex
alloy nanowires with tunable Ge concentration, core–shell structures and multiple
axial junctions, i.e. superlattices. In this chapter we review the most recent pro-
gresses in this field
Ab-initio opto-electronic properties of SiGe nanowires: role of many-body effects Phys
No full textThe self-energy and electron-hole interaction corrections to the one-particle approximation for SiGe nanowireshave been calculated for different geometries and diameters. We show that, at fixed nanowire diameterand orientation, the self-energy corrections for the SiGe nanowires can be obtained as a weighted average, onthe relative composition of one type of atom with respect to the total numbers of atoms in the unit cell, of thecorrections for the pure Si and Ge nanowires, thus circumventing cumbersome computations and allowing adirect and practical determination of the electronic band gap. Moreover we show that particular geometricalconfigurations are at the origin of an enhancement of the optical oscillator strength that should be important foroptoelectronic applications
Novel optoelectronic properties of simultaneously n- and p-doped silicon nanostructures
No full textDoping control at the nanoscale can be used to modify optical and electronic properties thus inducing interesting effects that cannot be observed in pure systems. For instance, it has been shown that luminescence energies in silicon nanocrystals can be tuned by properly controlling the impurities, for example by boron (B) and phosphorus (P) codoping. Starting from hydrogen-terminated silicon nanoclusters, we have previously calculated from first-principles that codoping results are always energetically favored with respect to single B- or P-doping and that the two impurities tend to occupy nearest neighbor sites near the surface. The codoped Si nanoclusters present band-edge states localized on the impurities which are responsible for the red-shift of the absorption thresholds with respect to that of pure undoped Si nanoclusters. Here we investigate how the properties of the codoped nanoclusters are influenced by adding one or two more impurities. Moreover we study also the effect of B- and P-codoping on the electronic and optical properties of Si nanowires, thus investigating the role of dimensionality, 0-versus 1-dimensionality, of the systems. (C) 2007 Elsevier Ltd. All rights reserved
Reduced quantum confinement effect and electron-hole separation in SiGe nanowires
No full textUsing first-principles methods, we investigate the structural and electronic properties of SiGe nanowiresbasedheterostructures, whose lattice contains the same number of Si and Ge atoms but arranged in a different manner. Our results demonstrate that the wires with a clear interface between Si and Ge regions not only form the most stable structures but show a strongly reduced quantum confinement effect. Moreover, we, with the inclusion of many-body effects, prove that these nanowires—under optical excitation—display a clear electron-holeseparation property which can have relevant technological applications
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