Nanocrystals (NCs) discussed in this Review are tiny crystals of metals, semiconductors, and magnetic material consisting of hundreds to a few thousand atoms each. Their size ranges from 2-3 to about 20 nm. What is special about this size regime that placed NCs among the hottest research topics of the last decades? The quantum mechanical coupling * To whom correspondence should be addressed. E-mail: dvtalapin@uchicago.edu. † The University of Chicago. ‡ Argonne National Lab. Chem. Rev. 2010, 110, 389–458 389

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Prospects of Colloidal Nanocrystals for Electronic and Optoelectronic Applications

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Principles Of Fluorescence Spectroscopy

Metal oxide-based resistive-type gas sensors are solid-state devices which are widely used in a number of applications from health and safety to energy efficiency and emission control. Nanomaterials such as nanowires, nanorods, and nanoparticles have dominated the research focus in this field due to their large number of surface sites facilitating surface reactions. Previous studies have shown that incorporating two or more metal oxides to form a heterojunction interface can have drastic effects on gas sensor performance, especially the selectivity. Recently, these effects have been amplified by designing heterojunctions on the nano-scale. These designs have evolved from mixed commercial powders and bi-layer films to finely-tuned core–shell and hierarchical brush-like nanocomposites. This review details the various morphological classes currently available for nanostructured metal-oxide based heterojunctions and then presents the dominant electronic and chemical mechanisms that influence the performance of these materials as resistive-type gas sensors. Mechanisms explored include p–n and n–n potential barrier manipulation, n–p–n response type inversions, spill-over effects, synergistic catalytic behavior, and microstructure enhancement. Tables are presented summarizing these works specifically for SnO2, ZnO, TiO2, In2O3, Fe2O3, MoO3, Co3O4, and CdO-based nanocomposites. Recent developments are highlighted and likely future trends are explored.

Nanoscale metal oxide-based heterojunctions for gas sensing: A review

It is clear that the above can lead to confusion when scientists of different countries are trying to communicate with each other. Another example is the burrowing rodent called a gopher found throughout the western United States. In the southeastern United States the term gopher refers to a burrowing turtle very similar to the desert tortoise found in the American southwest. One final example; two North American mammals known as the elk and the caribou are known in Europe as the reindeer and the elk. We never sing “Rudolph the Red-nosed elk”! Confused? This was the reason for creating an internationally recognized system of naming organisms. To avoid confusion, living organisms are assigned a scientific name based on Latin or Latinized words. The English sparrow is Passer domesticus or Passer domesticus (italics or underlining these two names is the official written representation of a scientific name). Using a uniform naming system allows scientists from all over the world to recognize exactly which life form a scientist is referring to. The naming process is called the binomial system of nomenclature. Passer is comparable to a surname and is called the genus, while domesticus is the specific or species name (like your given name) of the English sparrow. Now scientists can give all sparrow-like birds the genus Passer but the species name will vary. All similar genera (plural for genus) can be grouped into another, “higher” category (see below). Study the following for a more through understanding of taxonomy. Taxonomy Analogy Kingdom: Animalia Country

Diversity of life

Graphene-based materials are gaining heightened attention as novel materials for environmental applications The unique physicochemical properties of graphene, notably its exceptionally high surface area, electron mobility, thermal conductivity, and mechanical strength, can lead to novel or improved technologies to address the pressing global environmental challenges This critical review assesses the recent developments in the use of graphene-based materials as sorbent or photocatalytic materials for environmental decontamination, as building blocks for next generation water treatment and desalination membranes, and as electrode materials for contaminant monitoring or removal The most promising areas of research are highlighted, with a discussion of the main challenges that we need to overcome in order to fully realize the exceptional properties of graphene in environmental applications

Environmental applications of graphene-based nanomaterials.

Temperature- and Time-Dependent Shape Transformation of ZnO Nanostructures Grown by Vapor–Solid Method

Capacitance studies of Si/Si1?xGex/Si heterostructures with three Si1?xGex layers selectively doped with boron are presented. A surface charge and carrier distributions in Si1?xGex/Si heterostructures were found as a function of Ge content. The large positive charge observed at the surface of p-Si/SiGe/Si heterostructures was shown to result from the surface segregation of Ge during the epitaxial growth. The effects of native oxide and additional annealing in hydrogen ambient on this charge were also found. It was found that population of quantum wells strongly increased after exposure of the SiGe/Si structures under voltage.

Capacitance study of selectively doped SiGe/Si heterostructures

The recharging of quantum confinement levels in SiGe quantum wells (QWs) was studied by charge deep-level transient spectroscopy (Q-DLTS) for Si/SiGe/Si structures with different Ge contents in the SiGe layer. The set of levels were observed as the different slopes in the Arrhenius plots for the same Q-DLTS peak in different temperature ranges. These activation energies were compared to the energies of quantum confinement levels in the QW calculated in frames of six-band model taking into account spin-orbit interaction and attributed to a thermally activated tunneling of holes from the SiGe QW.

Deep-level spectroscopy studies of confinement levels in SiGe quantum wells

Using vapour-liquid-solid mechanism, we have grown Ge and composition modulated Ge:SnO2 nanowires on gold coated silicon substrates with diameter distribution ranging from 30-100 nm, and length more than tens of micrometers. The outer sheath of pure Ge nanowires consists of GeO2 as revealed from the XRD and TEM analyses. Raman spectra of the as-grown nanowires exhibit a blue shift, which is attributed to the compressive strain in the wires. The formed T-junction Ge:SnO2 axial nanowire heterostructure is attractive for novel optical and electronic devices.

Formation of Ge-based nanowires for nanoelectronic applications by vapor-liquid-solid mechanism

Careful tuning of formation (calcination) temperature of Sr 2C doped BiFeO3 multiferroic ceramics results in tailorable particle morphologies ranging from spherical to pillar-like. Based on the minimization of Gibb’s free energy approach, the dominant homogeneous mechanism for particle growth is suggested. The chemical substitution of a trivalent ion (Bi 3C ) by a divalent ion (Sr 2C ) causes the transformation of certain fraction of Fe 3C to Fe 4C and/or the appearance of oxygen vacancies. This has been respectively proved by the analysis of XPS and refinement of neutron diffraction data. Although significant modification in the particle morphology is observed, the crystal unit cell remains rhombohedral with a R3c space group but interesting variations in physical properties are achieved. O-vacancies induced strong and sharp photoluminescence activity in the IR region, similar to ZnO, is reported for the first time. This observation opens up a new application for multiferroic ceramics. SQUID M‐H data confirms the straightening of the canted spin structure of BiFeO3, which in turn results in magnetism similar to ferromagnetic materials. Findings of the magneto-dielectric effect are also discussed to claim the multiferroic nature of the sample. (Some figures may appear in colour only in the online journal)

Samit K. Ray

Papers

Temperature- and Time-Dependent Shape Transformation of ZnO Nanostructures Grown by Vapor–Solid Method

Capacitance study of selectively doped SiGe/Si heterostructures

Deep-level spectroscopy studies of confinement levels in SiGe quantum wells

Formation of Ge-based nanowires for nanoelectronic applications by vapor-liquid-solid mechanism

multifunctional ceramics with tunable photoluminescence and magnetic properties