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.

Luminescence characteristics of Ge nanocrystals embedded in SiO2 matrix

Carrier transport mechanism in aluminum nanoparticle embedded AlQ3 structures for organic bistable memory devices

CdS nanowires have been grown by a two-cell chemical method using a porous alumina template. Field-emission scanning and transmission electron microscopy show the growth of nanowires of diameters ranging from 100 to 110 nm and 50 to 60 nm for two different templates. The surface topography of the template filled with CdS has been studied using atomic force microscopy. The microstructural properties of the nanowires have been studied by high resolution x-ray diffraction. UV–visible spectroscopy of nanowires exhibits a clear blue shift due to quantum confinement. The photoluminescence spectra show green emission due to the defect states of the nanowires and the resonance Raman spectra revealed the stoichiometric phase of CdS with good crystallinity.

http://iopscience.iop.org/article/10.1088/0957-4484/18/9/095606/pdf

Characteristics of CdS nanowires grown in a porous alumina template using a two-cell method

CdS/Ge nanowire heterojunction has been grown by chemical deposition of CdS on Ge nanowire templates. Transmission electron micrographs show the growth of core-shell Ge/CdS nanowire radial heterostructures. Raman spectra reveal the confinement of phonons in nanocrystalline CdS shell grown on Ge nanowires. A diodelike behavior in I-V characteristics of Ge/CdS heterojunction is observed due to the formation of rectifying junction between CdS shells and Ge nanowire cores. An improved photocurrent spectrum of Ge/CdS heterojunction nanowires with broadband response from visible to near-IR region is demonstrated.

Enhanced broadband photoresponse of Ge/CdS nanowire radial heterostructures

Unipolar nonvolatile resistive switching memory properties of pulse laser ablated nickel oxide films have been studied. Grazing incidence x-ray diffraction and electron diffraction spectra of the oxide films reveal polycrystalline nature of deposited NiO films. Cross-sectional transmission electron micrograph shows a fairly uniform oxide surface. The rms surface roughnesses of deposited oxides have been studied as a function of annealing temperature using atomic force microscopy. By applying a proper voltage bias and compliance, Pt/NiO/Pt structures exhibited unipolar resistive switching having a very low SET and RESET voltages. The OFF state resistance and SET voltage are found to increase with the increase in annealing temperature. The ratio between the two resistance states can be as high as 1000. The current conduction phenomena at two resistance states have been studied. The switching phenomena have been explained using the rupture and formation of conducting filaments. The effect of postdeposition annealing on the resistance switching properties is discussed.

Samit K. Ray

Papers

Luminescence characteristics of Ge nanocrystals embedded in SiO2 matrix

Carrier transport mechanism in aluminum nanoparticle embedded AlQ3 structures for organic bistable memory devices

Characteristics of CdS nanowires grown in a porous alumina template using a two-cell method

Enhanced broadband photoresponse of Ge/CdS nanowire radial heterostructures

Nonvolatile and unipolar resistive switching characteristics of pulsed laser ablated NiO films