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The polar nature of the wurtzite crystalline structure of GaN and ZnO results in the existence of a spontaneous electric polarization within these materials and their associated alloys (Ga,Al,In)N and (Zn,Mg,Cd)O. The polarity has also important consequences on the stability of the different crystallographic surfaces, and this becomes especially important when considering epitaxial growth. Furthermore, the internal polarization fields may adversely affect the properties of optoelectronic devices but is also used as a potential advantage for advanced electronic devices. In this article, polarity-related issues in GaN and ZnO are reviewed, going from theoretical considerations to electronic and optoelectronic devices, through thin film, and nanostructure growth. The necessary theoretical background is first introduced and the stability of the cation and anion polarity surfaces is discussed. For assessing the polarity, one has to make use of specific characterization methods, which are described in detail. Subsequently, the nucleation and growth mechanisms of thin films and nanostructures, including nanowires, are presented, reviewing the specific growth conditions that allow controlling the polarity of such objects. Eventually, the demonstrated and/or expected effects of polarity on the properties and performances of optoelectronic and electronic devices are reported. The present review is intended to yield an indepth view of some of the hot topics related to polarity in GaN and ZnO, a fast growing subject over the last decade. [ABSTRACT FROM AUTHOR]

Atmospheric contamination with organic compounds is undesired in industry and in society because of odor nuisance or potential toxicity. Resistive gas sensors made of semiconducting metal oxides are effective in the detection of gases even at low concentration. Major drawbacks are low selectivity and missing sensitivity toward a targeted compound. Acetaldehyde is selected due to its high relevance in chemical industry and its toxic character. Considering the similarity between gas-sensing and heterogeneous catalysis (surface reactions, activity, selectivity), it is tempting to transfer concepts. A question of importance is how doping and the resulting change in electronic properties of a metal-oxide support with semiconducting properties alters reactivity of the surfaces and the functionality in gas-sensing and in heterogeneous catalysis. A gas-phase synthesis method is employed for aerogel-like zinc oxide materials with a defined content of aluminum (n-doping), which were then used for the assembly of gas sensors. It is shown that only Al-doped ZnO represents an effective sensor material that is sensitive down to very low concentrations (<350 ppb). The advance in properties relates to a catalytic effect for the doped semiconductor nanomaterial. [ABSTRACT FROM AUTHOR]

The benefits and promise of nanoscale dimensions for the properties of (ceramic) semiconductors are widely known. 1-D nanostructures in particular have proven to be of extraordinary relevance due to their applicability in future electronic and optoelectronic devices. Key to successful technological implementation of semiconductor nanostructures is the control of their electronic properties via doping. Despite its tremendous importance, precise chemical doping of defined nano-objects has been addressed rarely so far. Frequent problems are the creation of secondary defects and related undesired property changes by incorporation of hetero-elements, and the difficulty in ensuring a uniform and precise positioning of the dopant in the nanocrystal lattice. Here, we present the synthesis of Li-doped zinc oxide nanorods, which possess excellent (single-crystal) quality. The method is based on a novel non-classical crystallization mechanism, comprising an unusually oriented disassembly step. Afterwards, the nanorods are incorporated into mesoporous layers using colloidal self-assembly. Proof-of-principle gas sensing measurements with these novel materials demonstrate the beneficial role of Li-doping, indicating not only better conductivity but also the occurrence of catalytic effects. [ABSTRACT FROM AUTHOR]

The advanced application of wide-band gap semiconductorsin areaslike photovoltaics, optoelectronics, or photocatalysis requires aprecise control over electronic properties. Zinc oxide is favorablefor large-scale technological applications now and in the future becauseof the large, natural abundance of the involved, chemical elements.Often it is important that the band gap can be controlled precisely.While a blue-shift of the band gap can be reached quite easily usingthe quantum-size effect, it is still very difficult to achieve a red-shift.We present a powerful method for the band gap engineering of ZnO viathe incorporation of sulfur as a solid solutions. The reduction ofthe energy gap is controlled by ZnO1–xSxcomposition, whereas the latteris adjusted via special organometallic precursor molecules. The materialcan be supplied in a continuous fashion and in a more refined morphology,for instance spherical ZnO1–xSxcolloids with sizes below λvis/2 (≈ 200 nm). As a concrete application of contemporary importancefirst steps toward the full inorganic UV protection are made. [ABSTRACT FROM AUTHOR]