Vertical distribution and migration of planktonic rotifers in Xiangxi Bay of the three Gorges Reservoir, China

We investigated diel vertical migrations (DVM) and distributions of rotifers in summer, 2004 and spring, 2005, in Xiangxi Bay of the Three Gorges Reservoir, China. Water temperature, pH, conductivity, and phytoplankton were closely related to rotifer vertical distribution, while dissolved oxygen had no relationship with the vertical distribution of rotifers. The species composition and population density of rotifers changed significantly between seasons. However, rotifer vertical distributions in both seasons were similar. They aggregated at specific depths in the water column. All the rotifer species inhabited the surface layers (0.5-5 m). Generally, the rotifers did not display DVM except for Polyarthra vulgaris (in summer), which performed reverse migration. The reason that rotifers did not perform DVM may be explained by the low abundance of competitors and predators and the high density of food resources at the surface strata.

Acoustic signals of Chinese alligators (Alligator sinensis): Social communication

This paper reports the first systematic study of acoustic signals during social interactions of the Chinese alligator (Alligator sinensis). Sound pressure level (SPL) measurements revealed that Chinese alligators have an elaborate acoustic communication system with both long-distance signal-bellowing-and short-distance signals that include tooting, bubble blowing, hissing, mooing, head slapping and whining. Bellows have high SPL and appear to play an important role in the alligator's long range intercommunion. Sounds characterized by low SPL are short-distance signals used when alligators are in close spatial proximity to one another. The signal spectrographic analysis showed that the acoustic signals of Chinese alligators have a very low dominant frequency, less than 500 Hz. These frequencies are consistent with adaptation to a habitat with high density vegetation. Low dominant frequency sound attenuates less and could therefore cover a larger spatial range by diffraction in a densely vegetated environment relative to a higher dominant frequency sound. (C) 2007 Acoustical Society of America.

Spatial distribution and morphologic diversity of virioplankton in Lake Donghu, China

The spatial distribution and morphological diversity of virioplankton were determined in Lake Donghu which contains three trophic regions: hypertrophic, eutrophic and mesotrophic region. Virioplankton abundance measured by transmission electron microscope (TEM) ranged from 7.7 x 10(8) to 3.0 x 109 ml(-1), being among the highest observed in any natural aquatic system examined so far. The spatial distribution of virioplankton was correlated significantly with chlorophyll a concentration (r = 0.847; P < 0.01) at the sampling sites in Lake Donghu. 76 morphotypes were observed. Most morphotypes have tails, belonging to Siphoviridae, Myoviridae and Podoviridae. The majority of tailed phages in the lake were Myoviridae. Morphotypes which were rarely reported, such as prolate-headed virus-like particles, lemon-shaped virus-like particle, and viruses resembling Tectiviridae and Corticoviridae were all observed in the lake. It is concluded that the high viral abundance might be associated with high density of phytoplankton including algae and cyanobacteria. There was high viral diversity in this eutrophic shallow lake. In addition, cyanophage represented an important fraction of the virioplankton community in Lake Donghu. (c) 2006 Elsevier SAS. All rights reserved.

Seasonal variation of virioplankton in a eutrophic shallow lake

Lake Donghu is a typical eutrophic freshwater lake in which high abundance of planktonic viruses was recently revealed. In this study, seasonal variation of planktonic viruses were observed at three different trophic sites, hypertrophic, eutrophic, and mesotrophic regions, and the correlation between their abundances and other aquatic environmental components, such as bacterioplankton, chlorophyll a, burst size, pH, dissolved oxygen, and temperature, was analyzed for the period of an year. Virioplankton abundance detected by transmission electron microscope (TEM) ranged from 5.48 x 10(8) to 2.04 x 10(9) ml(-1) in all the sites throughout the study, and the high abundances and seasonal variations of planktonic viruses were related to the trophic status at the sampled sites in Lake Donghu. Their annual mean abundances were, the highest at the hypertrophic site (1.23x10(9) ml(-1)), medium at the eutrophic site (1.19x10(9) ml(-1)), and the lowest at the mesotrophic site (1.02x10(9) ml(-1)). The VBR (virus-to-bacteria ratio) values were high, ranging from 49 to 56 on average at the three sampled sites. The data suggested that the high viral abundance and high VBR values might be associated with high density of phytoplankton including algae and cyanobacteria in this eutrophic shallow lake, and that planktonic viruses are important members of freshwater ecosystems.

Magnetic silicon fullerene

A metal-encapsulating silicon fullerene, Eu@Si-20, has been predicted by density functional theory to be by far the most stable fullerene-like silicon structure. The Eu@Si-20 structure is a dodecahedron with D-2h symmetry in which the europium atom occupies the center site. The calculated results show that the europium atom has a large magnetic moment of nearly 7.0 Bohr magnetons. In addition, it was found that a stable "pearl necklace" nanowire, constructed by concatenating a series of Eu@Si-20 units, with the central europium atom, retains the high spin moment. The magnetic structure of the nanowire indicates potential applications in the fields of spintronics and high-density magnetic storage.

Zn2SiO4/ZnO Core/Shell Coaxial Heterostructure Nanobelts Formed by an Epitaxial Growth

Si-doped ZnO can be synthesized on the surface of the early grown Zn2SiO4 nanostructures and form core/ shell coaxial heterostructure nanobelts with an epitaxial orientation relationship. A parallel interface with a periodicity array of edge dislocations and an inclined interface without dislocations can be formed. The visible green emission is predominant in PL spectra due to carrier localization by high density of deep traps from complexes of impurities and defects. Due to band tail localization induced by composition and defect fluctuation, and high density of free-carriers donated by doping, especially the further dissociation of excitons into free-carriers at high excitation intensity, the near-band-edge emission is dominated by the transition of free-electrons to free-holes, and furthermore, exhibits a significant excitation power-dependent red-shift characteristic. Due to the structure relaxation and the thermalization effects, carrier delocalization takes place in deep traps with increasing excitation density. As a result, the green emission passes through a maximum at 0.25I(0) excitation intensity, and the ratio of the violet to green emission increases monotonously as the excitation laser power density increases. The violet and green emission of ZnO nanostructures can be well tuned by a moderate doping and a variation in the excitation density.

Photoluminescence characterization of nanocrystalline ZnO array

High-density and uniform well-aligned ZnO sub-micron rods are synthesized on the silicon substrate over a large area. The morphology, and structure of the ZnO sub-micron rods are investigated by x-ray diffraction, transmission electron microscopy and Raman spectra. It is found that the ZnO sub-micron rods are of high crystal quality with the diameter in the range of 400-600 nm and the length of several micrometres long. The optical properties were studied bill photoluminescence spectra. The results show that the intensity of the ultraviolet emission at 3.3 eV is rather high, meanwhile the deep level transition centred at about 2.38 eV is weak. The free exciton emission could also be observed at low, temperature, which implies the high optical quality of the ZnO sub-micron rods. This growth technique provides one effective way to fabricate the high crystal quality ZnO nanowires array, which is very important for potential applications in the new-type optoelectronic nanodevices.

Effect of critical thickness on structural and optical properties of InxGa1-xN/GaN multiple quantum wells

InGaN/GaN multiquantum-well (MQW) structures grown by metalorganic chemical-vapor deposition on n-type GaN and capped by p-type GaN were investigated by cross-sectional transmission electron microscopy, double crystal x-ray diffraction, and temperature-dependent photoluminescence. For the sample with strained-layer thicknesses greater than the critical thicknesses, a high density of pure edge type threading dislocations generated from MQW layers and extended to the cap layer was observed. These dislocations result from a relaxation of the strained layers when their thicknesses are beyond the critical thicknesses. Because of indium outdiffusion from the well layers due to the anneal effect of Mg-doped cap layer growth and defects generated from strain relaxation, the PL emission peak was almost depressed by the broad yellow band with an intensity maximum at 2.28 eV. But for the sample with strained-layer thicknesses less than the critical thicknesses, it has no such phenomenon. The measured critical thicknesses are consistent with the calculated values using the model proposed by Fischer, Kuhne, and Richter. (C) 2004 American Institute of Physics.

Synthesis and optical properties of europium-doped ZnS: Long-lasting phosphorescence from aligned nanowires

Quasi-aligned Eu2+-doped wurtzite ZnS nanowires on Au-coated Si wafers have been successfully synthesized by a vapor deposition method under a weakly reducing atmosphere. Compared with the undoped counterpart, incorporation of the dopant gives a modulated composition and crystal structure, which leads to a preferred growth of the nanowires along the [0110] direction and a high density of defects in the nanowire hosts. The ion doping causes intense fluorescence and persistent phosphorescence in ZnS nanowires. The dopant Eu2+ ions form an isoelectronic acceptor level and yield a high density of bound excitions, which contribute to the appearance of the radiative recombination emission of the bound excitons and resonant Raman scattering at higher pumping intensity. Co-dopant Cl- ions can serve not only as donors, producing a donor-acceptor pair transition with the Eu2+ acceptor level, but can also form trap levels together with other defects, capture the photoionization electrons of Eu2+, and yield long-lasting (about 4 min), green phosphorescence. With decreasing synthesis time, the existence of more surface states in the nanowires forms a higher density of trap centers and changes the crystal-field strength around Eu2+. As a result, not only have an enhanced Eu2+ -4f(6)5d(1)-4f(7) intra-ion transition and a prolonged afterglow time been more effectively observed (by decreasing the nanowires' diameters), but also the Eu2+ related emissions are shifted to shorter wavelengths.

Growth of Ge quantum dot mediated by boron on Ge wetting layer

We, report on the influence of boron on the formation of Ge quantum dots. The investigated structure consists of a Ge wetting layer, on which a sub-monolayer boron is deposited and subsequently a Ge top layer. For sufficiently thin Ge top layers, the strain field induced by boron on Ge wetting layer destabilizes the Ge top layer and causes the formation of small Ge quantum dots. However, for thicker Ge top layers, boron on the Ge wetting layer diffuses into Ge layers, compensates partly the strain and delays the evolution of Ge quantum dots. By this method, small Ge quantum dots with high density as well as size uniformity can be formed by optimizing the growth condition. (c) 2005 Elsevier B.V. All rights reserved.

Effects of different modified underlayer surfaces on growth and optical properties of InGaN quantum dots

InGaN/GaN quantum dots were grown on the sapphire (0 0 0 1) substrate in a metalorganic chemical vapor deposition system. The morphologies of QDs deposited on different modified underlayer (GaN) surfaces, including naturally as grown, Ga-mediated, In-mediated, and air-passivated ones, were investigated by atomic force microscopy (AFM). Photo luminescence (PL) method is used to evaluate optical properties. It is shown that InGaN QDs can form directly on the natural GaN layer. However, both the size and distribution show obvious inhomogeneities. Such a heavy fluctuation in size leads to double peaks for QDs with short growth time, and broad peaks for QDs with long growth time in their low-temperature PL spectra. QDs grown on the Ga-mediated GaN underlayer tends to coalesce. Distinct transform takes place from 3D to 2D growth on the In-mediated ones, and thus the formation of QDs is prohibited. Those results clarify Ga and In's surfactant behavior. When the GaN underlayer is passivated in the air, and together with an additional low-temperature-grown seeding layer, however, the island growth mode is enhanced. Subsequently, grown InGaN QDs are characterized by a relatively high density and an improved Gaussian-like distribution in size. Short surface diffusion length at low growth temperature accounts for that result. It is concluded that reduced temperature favors QD's 3D growth and surface passivation can provide another promising way to obtain high-density QDs that especially suits MOCVD system. (c) 2004 Elsevier Ltd. All rights reserved.

Luminescence properties of multi-layer InGaN quantum dots grown on C- and R-plane sapphire substrates

In this study, we report comparative luminescence properties of multi-layer InGaN quantum dots grown on C- and R-plane sapphire substrates by metal organic chemical vapor deposition (MOCVD). High-density InGaN quantum dots (QDs) are formed on GaN templates by decreasing the growth temperature and increasing the adatom hopping-barrier through surface passivation. Atomic force microscopy (AFM) has been employed to estimate the size and height of these dots. Photoluminescence (PL) spectra recorded from (1120) InGaN QDs/(1102) sapphire show much stronger emission intensity compared to spectra recorded from (0001) InGaN QDs/(0001) sapphire. Due to the absence of strong spontaneous polarization and piezoelectric field, such (1150) InGaN QDs in the active layers would lead to high efficiency light emitting devices. (c) 2005 Elsevier B.V. All rights reserved.

Effect of misfit dislocation originated from strained layer on photoluminescence properties of InxGa1-xN/GaN multiple quantum wells

In-x Ga1-xN/GaN multiple quantum well (MQW) samples with strain-layer thickness lager/less than the critical one are investigated by temperature-dependent photoluminescence and transmission electron microscopy, and double crystal x-ray diffraction. For the sample with the strained-layer thickness greater than the critical thickness, we observe a high density of threading dislocations generated at the MQW layers and extended to the cap layer. These dislocations result from relaxation of the strain layer when its thickness is beyond the critical thickness. For the sample with the strained-layer thickness greater than the critical thickness, temperature-dependent photoluminescence measurements give evidence that dislocations generated from the MQW layers due to strain relaxation are main reason of the poor photoluminescence property, and the dominating status change of the main peak with increasing temperature is attributed to the change of the radiative recombination from the areas including dislocations to the ones excluding dislocations.

The effect of a combined InAlAs and GaAs strained buffer layer on the structure and optical property of InAs quantum dots

The character of InAs quantum dots (QD) directly deposited on a combined InAlAs-GaAs (XML) strained buffer layer (SBL) has been investigated. This growth technique realizes high-density QD (5.88 x 10(10) cm(-2)) by changing the thickness of GaAs in InAlAs-GaAs SBL. The dependence of the density and the aspect ratio of QD on the GaAs thickness has been discussed in detail. The photoluminescence (PL) measurements demonstrate an obvious redshift with the increase of GaAs thickness. In addition, the deposition of InAs QDs grown on the combined InAlAs-GaAs SBL has an important effect of the QD properties. The ordered QD array can be observed from the sample deposited by atomic layer epitaxy, of which the PL peak shows an obvious redshift in comparison to the molecular beam epitaxy (MBE) QDs when the GaAs thicknesses are equal. (c) 2004 Elsevier B.V. All rights reserved.

Near-field scanning optical microscopy with an active probe

A near-field scanning optical microscopy (NSOM) system employing a very-small-aperture laser (VSAL) as an active probe is reported in this Letter. The VSAL in our experiment has an aperture size of 300 nmx300 nm and a near-field spot size of about 600 nm. The resolution of the NSOM system with the VSAL can reach about 600 nm, and even 400 nm. Considering the high output power of the VSAL, such a NSOM system is a potentially useful tool for nanodetection, data storage, nanolithography, and nanobiology.