Band Gap Engineering via Doping: A Predictive Approach

We employ an extension of Harrison\u27s theory at the tight binding level of approximation to develop a predictive approach for band gap engineering involving isovalent doping of wide band gap semiconductors. Our results indicate that reasonably accurate predictions can be achieved at qualitative as well as quantitative levels. The predictive results were checked against ab initio ones obtained at the level of DFT/SGGA + U approximation. The minor disagreements between predicted and ab initio results can be attributed to the electronic processes not incorporated in Harrison\u27s theory. These include processes such as the conduction band anticrossing [Shan et al., Phys. Rev. Lett. 82, 1221 (1999); Walukiewicz et al., Phys. Rev. Lett. 85, 1552 (2000)] and valence band anticrossing [Alberi et al., Phys. Rev. B 77, 073202 (2008); Appl. Phys. Lett. 92, 162105 (2008); Appl. Phys. Lett. 91, 051909 (2007); Phys. Rev. B 75, 045203 (2007)], as well as the multiorbital rehybridization. Another cause of disagreement between the results of our predictive approach and the ab initio ones is shown to be the result of the shift of Fermi energy within the impurity band formed at the edge of the valence band maximum due to rehybridization. The validity of our approach is demonstrated with example applications for the systems GaN1− x Sbx , GaP1− x Sbx , AlSb1− x Px , AlP1− xSbx , and InP1− xSbx

Prediction of a New Graphenelike Si\u3csub\u3e2\u3c/sub\u3e BN Solid

While the possibility to create a single-atom-thick two-dimensional layer from any material remains, only a few such structures have been obtained other than graphene and a monolayer of boron nitride. Here, based upon ab initio theoretical simulations, we propose a new stable graphenelike single-atomic-layer Si2 BN structure that has all of its atoms with sp2 bonding with no out-of-plane buckling. The structure is found to be metallic with a finite density of states at the Fermi level. This structure can be rolled into nanotubes in a manner similar to graphene. Combining first- and second-row elements in the Periodic Table to form a one-atom-thick material that is also flat opens up the possibility for studying new physics beyond graphene. The presence of Si will make the surface more reactive and therefore a promising candidate for hydrogen storage

Porosity-moderated ultrafast electron transport in Au nanowire networks

We demonstrate for first time the ultrafast properties of a newly formed porous Au nanostructure. The properties of the porous nanostructure are compared with those of a solid gold film using time-resolved optical spectroscopy.The experiments suggest that under the same excitation conditions the relaxation dynamics are slower in the former. Our observations are evaluated by simulations based on a phenomenological rate equation model. The impeded dynamics has been attributed to the porous nature of the structure in the networks, which results in reduced efficiency during the dissipation of the laser-deposited energy. Importantly,the porosity of the complex three-dimensional nanostructure is introduced as a geometrical control parameter of its ultrafast electron transport

Switching and Rectification in Carbon-Nanotube Junctions

Multi-terminal carbon-nanotube junctions are under investigation as candidate components of nanoscale electronic devices and circuits. Three-terminal "Y" junctions of carbon nanotubes (see Figure 1) have proven to be especially interesting because (1) it is now possible to synthesize them in high yield in a controlled manner and (2) results of preliminary experimental and theoretical studies suggest that such junctions could exhibit switching and rectification properties. Following the preliminary studies, current-versus-voltage characteristics of a number of different "Y" junctions of single-wall carbon nanotubes connected to metal wires were computed. Both semiconducting and metallic nanotubes of various chiralities were considered. Most of the junctions considered were symmetric. These computations involved modeling of the quantum electrical conductivity of the carbon nanotubes and junctions, taking account of such complicating factors as the topological defects (pentagons, heptagons, and octagons) present in the hexagonal molecular structures at the junctions, and the effects of the nanotube/wire interfaces. A major component of the computational approach was the use of an efficient Green s function embedding scheme. The results of these computations showed that symmetric junctions could be expected to support both rectification and switching. The results also showed that rectification and switching properties of a junction could be expected to depend strongly on its symmetry and, to a lesser degree, on the chirality of the nanotubes. In particular, it was found that a zigzag nanotube branching at a symmetric "Y" junction could exhibit either perfect rectification or partial rectification (asymmetric current-versus-voltage characteristic, as in the example of Figure 2). It was also found that an asymmetric "Y" junction would not exhibit rectification

Coupled and Implicit Relationships of the d‑Band Center of the Magnetic Dopants in Diluted Magnetic Semiconductors and Transition Metal Oxides

Recently, we have extended the single parameter predictive model based on the d-band center, <i>d</i>_{<i>c</i>,<i>TM</i>}, of the adsorbent transition metal (TM) atom and proposed a multidescriptor predictive model for the adsorption and binding properties of catalytic surfaces. In addition to that, we have also demonstrated that <i>d</i>_{<i>c</i>,<i>TM</i>–<i>dop</i>} of TM-dopants in diluted magnetic semiconductors (DMSs) and transition metal oxides (TMOs) correlates quite well with the magnetic and other electronic properties of both DMSs and doped TMOs. In the present work we revisit the issue of <i>d</i>_{<i>c</i>,<i>TM</i>–<i>dop</i>} as a suitable descriptor for magnetic systems. In particular, we analyze <i>ab initio</i> results obtained for nine host materials (DMSs and TMOs) (i.e., ZnO, GaN, GaP, TiO₂, SnO₂, Sn₃N₄, MoS₂, ZnS, and CdS) codoped with TM atoms of the whole 3d-series. Our results indicate coupled and implicit correlations among the various features of the codoped systems, namely, the magnetic moment of the dopant in a particular host, the dopant’s d-band center, as well as the p-band center of the host’s anions and the band gap of the doped system. It is also demonstrated that this set of features, complimented by an additional set of secondary descriptors (crystal field and spin–orbit coupling splittings, point group symmetry of the dopant sites, induced gap states, heterovalency, and heteroelectronegativity between host and dopant constituent atoms), could constitute a valuable set of descriptors suitable for developing statistical predictive theories for a much larger class of magnetic materials

Tourism in an Era of Uncertainty Rhodes Island, Greece 27–30 April 2011

The twenty-first century has brought new challenges for tourism. Climate change, infectious diseases, economic crises, terrorism, globalisation, and entrenched social problems pose threats and create an uncertain tourism environment that affects destinations and their populations in virtually every corner of the contemporary world. To keep pace with these challenges, governments, businesses, communities, and tourism organisations need to continuously assess environmental factors, to identify future trends and to examine various response policies and management techniques to decrease risk and uncertainty. Bearing all these in mind, ICOT 2011 aims to contribute to the debate on tourism in relation to different aspects of uncertainty, by stimulating discussion and exchange of ideas between tourism professionals, academics, researchers, policy-makers, consultants, practitioners, government officials and postgraduate students from tourismrelated fields

Nanotechnology Based Materials and Devices for Health Care

This viewgraph presentation provides information on trends in NASA nanotechnology research and development, and future biotechnological applications for that nanotechnology. The presentation covers nanoelectronics, nanosensors, and nanomaterials, biomimetics, devices and materials for health care, carbon nanotubes, biosensors for astrobiology, solid-state nanopores for DNA sequencing, and protein nanotubes