Atomic layer deposition: an overview.

High-performance gas sensors prepared using p-type oxide semiconductors such as NiO, CuO, Cr2O3, Co3O4, and Mn3O4 were reviewed. The ionized adsorption of oxygen on p-type oxide semiconductors leads to the formation of hole-accumulation layers (HALs), and conduction occurs mainly along the near-surface HAL. Thus, the chemoresistive variations of undoped p-type oxide semiconductors are lower than those induced at the electron-depletion layers of n-type oxide semiconductors. However, highly sensitive and selective p-type oxide-semiconductor-based gas sensors can be designed either by controlling the carrier concentration through aliovalent doping or by promoting the sensing reaction of a specific gas through doping/loading the sensor material with oxide or noble metal catalysts. The junction between p- and n-type oxide semiconductors fabricated with different contact configurations can provide new strategies for designing gas sensors. p-Type oxide semiconductors with distinctive surface reactivity and oxygen adsorption are also advantageous for enhancing gas selectivity, decreasing the humidity dependence of sensor signals to negligible levels, and improving recovery speed. Accordingly, p-type oxide semiconductors are excellent materials not only for fabricating highly sensitive and selective gas sensors but also valuable additives that provide new functionality in gas sensors, which will enable the development of high-performance gas sensors.

Highly sensitive and selective gas sensors using p-type oxide semiconductors: Overview

Atomic layer deposition (ALD) is gaining attention as a thin film deposition method, uniquely suitable for depositing uniform and conformal films on complex three-dimensional topographies. The deposition of a film of a given material by ALD relies on the successive, separated, and self-terminating gas–solid reactions of typically two gaseous reactants. Hundreds of ALD chemistries have been found for depositing a variety of materials during the past decades, mostly for inorganic materials but lately also for organic and inorganic–organic hybrid compounds. One factor that often dictates the properties of ALD films in actual applications is the crystallinity of the grown film: Is the material amorphous or, if it is crystalline, which phase(s) is (are) present. In this thematic review, we first describe the basics of ALD, summarize the two-reactant ALD processes to grow inorganic materials developed to-date, updating the information of an earlier review on ALD [R. L. Puurunen, J. Appl. Phys. 97, 121301 (2005)], and give an overview of the status of processing ternary compounds by ALD. We then proceed to analyze the published experimental data for information on the crystallinity and phase of inorganic materials deposited by ALD from different reactants at different temperatures. The data are collected for films in their as-deposited state and tabulated for easy reference. Case studies are presented to illustrate the effect of different process parameters on crystallinity for representative materials: aluminium oxide, zirconium oxide, zinc oxide, titanium nitride, zinc zulfide, and ruthenium. Finally, we discuss the general trends in the development of film crystallinity as function of ALD process parameters. The authors hope that this review will help newcomers to ALD to familiarize themselves with the complex world of crystalline ALD films and, at the same time, serve for the expert as a handbook-type reference source on ALD processes and film crystallinity.

Crystallinity of inorganic films grown by atomic layer deposition: Overview and general trends

ZnS nanostructures: From synthesis to applications

Aim This paper presents a tool for long-term global change studies; it is an update of the History Database of the Global Environment (HYDE) with estimates of some of the underlying demographic and agricultural driving factors.

Methods  Historical population, cropland and pasture statistics are combined with satellite information and specific allocation algorithms (which change over time) to create spatially explicit maps, which are fully consistent on a 5′ longitude/latitude grid resolution, and cover the period 10,000 bc to ad 2000.

Results  Cropland occupied roughly less than 1% of the global ice-free land area for a long time until ad 1000, similar to the area used for pasture. In the centuries that followed, the share of global cropland increased to 2% in ad 1700 (c. 3 million km2) and 11% in ad 2000 (15 million km2), while the share of pasture area grew from 2% in ad 1700 to 24% in ad 2000 (34 million km2) These profound land-use changes have had, and will continue to have, quite considerable consequences for global biogeochemical cycles, and subsequently global climate change.

Main conclusions  Some researchers suggest that humans have shifted from living in the Holocene (emergence of agriculture) into the Anthropocene (humans capable of changing the Earth's atmosphere) since the start of the Industrial Revolution. But in the light of the sheer size and magnitude of some historical land-use changes (e.g. as result of the depopulation of Europe due to the Black Death in the 14th century and the aftermath of the colonization of the Americas in the 16th century) we believe that this point might have occurred earlier in time. While there are still many uncertainties and gaps in our knowledge about the importance of land use (change) in the global biogeochemical cycle, we hope that this database can help global (climate) change modellers to close parts of this gap.

The HYDE 3.1 spatially explicit database of human‐induced global land‐use change over the past 12,000 years

Size-Dependent Photoconductance in SnO2 Nanowires

Iron oxide nanotubes of 50−150 nm outer diameter and 2−20 nm wall thickness are prepared in ordered arrays. Atomic layer deposition (ALD) of Fe2O3 from the precursor iron(III) tert-butoxide at 130−180 °C yields very smooth coverage of the pore walls of anodic alumina templates, with thickness growth of 0.26(±0.04) A per cycle. The reduced Fe3O4 tubes are hard ferromagnets, and variations of the wall thickness dw have marked consequences on the magnetic response of the tube arrays. For 50 nm outer diameter, tubes of dw = 13 nm yield the largest coercive field (Hc > 750 Oe), whereas lower coercivities are observed on both the thinner and thicker sides of this optimum.

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Ordered Iron Oxide Nanotube Arrays of Controlled Geometry and Tunable Magnetism by Atomic Layer Deposition

Nanometer-sized zinc aluminate (ZnAl2O4) particles were synthesized from heterometal alkoxides, [ZnAl2(OR)8], possessing an ideal cation stoichiometry for the ZnAl2O4 spinel. ZnAl2O4 is formed at 400°C, which is the lowest temperature reported for the formation of monophasic ZnAl2O4. 27Al magic-angle spinning nuclear magnetic resonance spectroscopy revealed that ZnAl2O4 possesses an inverse structure at <900°C, while the normal spinel phase is observed at higher temperatures. The homogeneity of the in-depth composition and Zn:Al stoichiometry (1:2) was confirmed by electron spectroscopy for chemical analysis. Evaluation of the valence-band spectra of ZnAl2O4 and ZnS suggested that the hybridization of O 2p and Zn 3d orbitals is responsible for lowering the bandgap in the latter. The average crystallite size showed an exponential relationship to the calcination temperature (X-ray diffractometry and transmission electron microscopy data). The optical spectra of different spinel powders (average particle sizes, 20–250 nm) showed that the absorption edge exhibits a blue shift as particle size decreases.

Single-Source Sol-Gel Synthesis of Nanocrystalline ZnAl2O4: Structural and Optical Properties

Evaluation of phase, composition, microstructure and properties in TiC/a-C:H thin films deposited by magnetron sputtering

High-yield synthesis of germanium nanowires (NWs) and core−shell structures is achieved by the chemical vapor deposition (CVD) of dicyclopentadienyl germanium ([Ge(C5H5)2]). The one-dimensional (1D) nanostructures are formed on an iron substrate following a base-growth model in which an Fe−Ge epilayer functions as a catalytic bed. The wire growth is selective and no catalyst particles are observed at the tip of the NWs, which is contrary to the characteristic feature of a 1D growth based on the vapor−liquid−solid (VLS) mechanism. The diameter and length of the NWs were in the ranges 15−20 nm and 25−40 μm, respectively, as found by high-resolution electron microscopy. Both axial and radial dimensions of the NWs can be controlled by adjusting the precursor feedstock, deposition temperature, and size of alloy nuclei in the Fe−Ge epilayer. High precursor flux produced coaxial heterostructures where single-crystalline Ge cores are covered with an overlayer of nanocrystalline Ge. Single-crystal Ge nanowires exh...

Hao Shen

Papers

Size-Dependent Photoconductance in SnO2 Nanowires

Ordered Iron Oxide Nanotube Arrays of Controlled Geometry and Tunable Magnetism by Atomic Layer Deposition

Single-Source Sol-Gel Synthesis of Nanocrystalline ZnAl2O4: Structural and Optical Properties

Evaluation of phase, composition, microstructure and properties in TiC/a-C:H thin films deposited by magnetron sputtering

Germanium Nanowires and Core−Shell Nanostructures by Chemical Vapor Deposition of [Ge(C5H5)2]