Semiconductor nanowires and nanotubes: growth, properties and applications

• Antonio Polimeni (Dipartimento di Fisica, Sapienza Università di Roma)

Room: Aula Conversi The capability to shape condensed matter, and specifically semiconductors, at the nanometer scale provides the researchers with a unique tool to tailor the physical properties of different materials. In this talk, we focus on two types of nanostructures featuring highly anisotropic electronic and morphological properties: nanowires (NWs) and rolled-up nanotubes (NTs). NWs are filamentary crystals –with diameter of few hundreds of nm or less, and several microns in length– fabricated in a scalable, flexible and well-controlled manner via a vapor–liquid–solid growth mechanism. III-V compounds (e.g., GaAs), III-nitrides (e.g., GaN), oxides (e.g., ZnO), and elemental semiconductors (e.g., Si) may be grown in the NW form. NWs are the smallest dimension structures that allow optical guiding and electrical contacting simultaneously. The large surface to volume ratio of NWs enhances their interaction with the environment, turning them into optimal chemical and biological sensors. Furthermore, NW anisotropic geometry makes their optical and electrical properties dramatically dependent on their orientation. Synthesis, main properties and applications of NWs will be reviewed. Rolled-up NTs are fabricated from strained semiconductor layer systems grown by molecular beam epitaxy. The strain is built up by growing semiconductor materials with a larger lattice constant on top of a material with a smaller lattice constant. By selective wet-chemically etching a sacrificial layer below the strained layers the incorporated strain bends up the layer system and leads to the self-rolling of a nanotube. NTs could act as nanopipelines for fluid transportation and generation of nanodroplets or nanobubbles. They also can be used as optical cavities, whose resonance modes can be effectively coupled with quantum dots. Examples regarding NT applications will be presented. Finally, potential perspectives of these advanced functional materials for high-energy physics will be discussed.

Discussione Room: Aula Conversi

Quantum dot e nanostrutture zero-dimensionali

• Marco Felici (Dip. di Fisica, "Sapienza" Univ. di Roma)

Room: Aula Conversi I quantum dot (QD), noti anche come “atomi artificiali”, sfruttano i disallineamenti esistenti nella struttura a bande di diversi materiali semiconduttori per realizzare sistemi di dimensioni nanometriche, in cui i portatori di carica sono confinati in tre dimensioni. La nascita di livelli discreti nello spettro di energia di tali nanostrutture, dovuta appunto al confinamento dei portatori, fa sì che la radiazione emessa da questi sistemi presenti caratteristiche uniche, potenzialmente utilizzabili per la realizzazione di laser e LED e per applicazioni nel campo dell’informazione quantistica. Il presente seminario tenterà di fornire una panoramica su questi temi, concentrandosi in particolare sugli sforzi fatti nel corso dell’ultimo decennio per controllare la posizione e la lunghezza d’onda di emissione dei QD.

Discussione Room: Aula Conversi

Thermal devices based on Josephson quantum nanocircuits : The era of coherent caloritronics

• Francesco Giazotto (NEST, Istituto Nanoscienze-CNR & Scuola Normale Superiore, Pisa, Italy)

Room: Aula Conversi The Josephson effect [1] represents perhaps the prototype of macroscopic phase coherence and is at the basis of the most widespread interferometer, i.e., the superconducting quantum interference device (SQUID). Yet, in analogy to electric interference, Maki and Griffin [2] predicted in 1965 that thermal current flowing through a temperature-biased Josephson tunnel junction is a stationary periodic function of the quantum phase difference between the superconductors. In this scenario, a temperature-biased SQUID would allow heat currents to interfere thus implementing the thermal version of the electric Josephson interferometer. In this presentation I shall initially report the first experimental realization of a heat interferometer [3]. We investigate heat exchange between two normal metal electrodes kept at different temperatures and tunnel-coupled to each other through a thermal `modulator’ in the form of a DC-SQUID. Heat transport in the system is found to be phase dependent, in agreement with the original prediction. Next, after introducing some basic concepts about diffraction of heat currents I shall present experimental results on the first quantum `diffractor’ for thermal flux [4, 5]. Then, I shall introduce the physics at the basis of thermal rectification with superconducting tunnel junctions, and show the first realization of an ultra-efficient low-temperature hybrid `heat current rectifier’ [6, 7], thermal counterpart of the well-known electric diode. Our design is based on a tunnel junction between two different elements: a normal metal and a superconducting island. Electronic heat current asymmetry in the structure arises from large mismatch between the thermal properties of these two. We demonstrate temperature differences exceeding 60 mK between the forward and reverse thermal bias configurations [8]. This structure offers a remarkably large heat rectification ratio up to about 140 and allows its prompt implementation in true solid-state thermal nanocircuits and general-purpose electronic applications requiring energy harvesting or thermal management and isolation at the nanoscale. Finally, I shall conclude by discussing some possible applications of the Josephson effect in the context of coherent caloritronics as well as for the implementation of new quantum devices for thermal measurements.

Discussione Room: Aula Conversi