Enhancement of the photoelectrochemical properties of Cl-doped ZnO nanowires by tuning their coaxial doping profile

Arrays of vertically aligned ZnO:Cl/ZnO core-shell nanowires were used to demonstrate that the control of the coaxial doping profile in homojunction nanostructures can improve their surface charge carrier transfer while conserving potentially excellent transport properties. It is experimentally shown that the presence of a ZnO shell enhances the photoelectrochemical properties of ZnO:Cl nanowires up to a factor 5. Likewise, the ZnO shell promotes the visible photoluminescence band in highly conducting ZnO:Cl nanowires. These lines of evidence are associated with the increase of the nanowires" surface depletion layer

Direct modulation of electroluminescence from silicon nanocrystals beyond radiative recombination rates

We propose a light emitting transistor based on silicon nanocrystals provided with 200 Mbits/ s built-in modulation. Suppression of electroluminescence from silicon nanocrystals embedded into the gate oxide of a field effect transistor is achieved by fast Auger quenching. In this process, a modulating drain signal causes heating of carriers in the channel and facilitates the charge injection into the nanocrystals. This excess of charge enables fast nonradiative processes that are used to obtain 100% modulation depths at modulating voltages of 1 V.

Rela����o solo-relevo-substrato geol��gico nas restingas da plan��cie costeira do estado de S��o Paulo

Atributos morfol��gicos de 28 pedons, descritos e amostrados em cronossequ��ncias, e data����es absolutas por luminesc��ncia (TL e LOE) e 14C foram utilizados para elucidar os principais fatores envolvidos na forma����o e evolu����o dos Espodossolos nos dep��sitos marinhos quatern��rios da plan��cie costeira do Estado de S��o Paulo. Os solos estudados localizam-se nos munic��pios de Bertioga (Baixada Santista), Cananeia e Ilha Comprida (Litoral Sul). Essa abordagem, pouco comum nos estudos dos ambientes de plan��cie costeira brasileiros, possibilitou as seguintes interpreta����es: (a) o relevo, a din��mica h��drica e o tempo (incluindo as varia����es do n��vel relativo do mar) s��o os principais condicionantes da diferencia����o espacial dos Espodossolos nos terra��os marinhos; (b) os Espodossolos mais antigos e bem drenados, devido ��s condi����es de relevo e rebaixamento do n��vel do len��ol fre��tico, apresentam grande variabilidade e diversidade de seus horizontes e atributos morfol��gicos, diferindo daqueles mal drenados (antigos ou jovens), em que os horizontes s��o mais homog��neos; (c) os Espodossolos mais antigos, quando bem drenados, mostram-se em avan��ado est��dio de degrada����o, enquanto os mal drenados encontram-se bem preservados, indicando que a sua g��nese e perman��ncia na paisagem est��o ligadas ao relevo, que, por sua vez, controla a din��mica h��drica; (d) os Espodossolos mais evolu��dos e antigos, dotados de horizontes cimentados (orstein), podem ser considerados indicadores pedolitoestratigr��ficos dos dep��sitos marinhos pleistoc��nicos da Forma����o Cananeia; e (e) a g��nese do horizonte orstein se deu em condi����es topogr��ficas e hidrol��gicas pret��ritas diferentes das atuais, indicando se tratar de solos poligen��ticos ou paleossolos.

Cation distribution and high field magnetization studies on SrFe12-xCrxO19

With the aim of a better understanding of both cationic distribution and magnetic properties of the uniaxial SrFe12-xCrxO19hexagonal ferrites, M��ssbauer spectroscopy, neutron diffraction and high field magnetization measurements have been carried out. The Cr3+ions occupy the octahedral sites of the M structure with a preference hierarchy within them. The magnetic measurements, together with the deduced cationic distribution, indicate that some sublattices have a random spin canting around the c-axis.

Spin-glass behaviour in mixed metal oxides with a rutile-type structure

We report here on the magnetic properties of compounds of composition Fe1���xCrxSbO4 and Fe1���xGaxSbO4. The introduction of paramagnetic Cr3+ and diamagnetic Ga3+ into the rutile���related iron antimonate lattice does not destroy the antisite atomic ordering which exists in iron antimonate of composition FeSbO4. The initial slope of the Curie temperature dependence on x is similar in both series, indicating that Fe3+���Cr3+ interactions are very small. The magnetic susceptibility measurements recorded from the compounds of composition Fe1���xCrxSbO4, x<0.4, and Fe0.9Ga0.1SbO4 show them to behave as spin glasses at low temperatures. The inhibition of compounds of the type Fe1���xCrxSbO4, x>0.4, and Fe1���xGaxSbO4, x>0.1 to undergo a spin���glass transition above 4.2 K is associated with a dilution effect.

Blue luminescence at room temperature in defective MgO films

Luminescence spectroscopy has been used to characterize MgO films prepared by rf-sputtering. A clear correlation is found between the appearance of an emission peak centered at approximately 460 nm and the detection of ferromagnetic ordering in the samples. We suggest that cationic vacancies are responsible for the blue-light emission by introducing p states into the electronic band-gap. In accordance with this, our results strongly indicate that cationic vacancies are at the heart of the appearance of long-range magnetic ordering in MgO films.

Spatial mapping of exciton lifetimes in single ZnO nanowires

We investigate the spatial dependence of the exciton lifetimes in single ZnO nanowires. We have found that the free exciton and bound exciton lifetimes exhibit a maximum at the center of nanowires, while they decrease by 30% towards the tips. This dependence is explained by considering the cavity-like properties of the nanowires in combination with the Purcell effect. We show that the lifetime of the bound-excitons scales with the localization energy to the power of 3/2, which validates the model of Rashba and Gurgenishvili at the nanoscale.

Up-conversion effect of Er- and Yb-doped ZnO thin films

Visible up-conversion in ZnO:Er and ZnO:Er:Yb thin films deposited by RF magnetron sputtering under different O2-rich atmospheres has been studied. Conventional photoluminescence (325 nm laser source) and up-conversion (980 nm laser source) have been performed in the films before and after an annealing process at 800 ��C. The resulting spectra demonstrate that the thermal treatment, either during or post-deposition, activates optically the Er3+ ions, being the latter process much more efficient. Moreover, the atmosphere during deposition was also found to be an important parameter, as the deposition under O2 flow increases the optical activity of Er+3 ions. In addition, the inclusion of Yb3+ ions into the films has shown an enhancement of the visible up-conversion emission at 660 nm by a factor of 4, which could be associated to either a better energy transfer from the 2F5/2 Yb level to the 4I11/2 Er one, or to the prevention of having Er2O3 clustering in the films.

Current transport and electroluminescence mechanisms in thin SiO2 films containing Si nanocluster-sensitized erbium ions.

We have studied the current transport and electroluminescence properties of metal oxide semiconductor MOS devices in which the oxide layer, which is codoped with silicon nanoclusters and erbium ions, is made by magnetron sputtering. Electrical measurements have allowed us to identify a Poole-Frenkel conduction mechanism. We observe an important contribution of the Si nanoclusters to the conduction in silicon oxide films, and no evidence of Fowler-Nordheim tunneling. The results suggest that the electroluminescence of the erbium ions in these layers is generated by energy transfer from the Si nanoparticles. Finally, we report an electroluminescence power efficiency above 10���3%. �� 2009 American Institute of Physics. doi:10.1063/1.3213386

Bulk silica-based luminescent materials by sol-gel processing of non-conventional precursors

The sol-gel synthesis of bulk silica-based luminescent materials using innocuous hexaethoxydisilane and hexamethoxydisilane monomers, followed by one hour thermal annealing in an inert atmosphere at 950oC-1150oC, is reported. As-synthesized hexamethoxydisilane-derived samples exhibit an intense blue photoluminescence band, whereas thermally treated ones emit stronger photoluminescence radiation peaking below 600 nm. For hexaethoxydisilane-based material, annealed at or above 1000oC, a less intense photoluminescence band, peaking between 780 nm and 850 nm that is attributed to nanocrystalline silicon is observed. Mixtures of both precursors lead to composed spectra, thus envisaging the possibility of obtaining pre-designed spectral behaviors by varying the mixture composition.

1.48 and 1.84 ��m thulium emissions in monoclinic KGd(WO4)2 single crystals

By exciting at 788 nm, we have characterized the near infrared emissions of trivalent thulium ions in monoclinic KGd(WO4)2 single crystals at 1.48 and 1.84 mm as a function of dopant concentration from 0.1% to 10% and temperature from 10 K to room temperature. We used the reciprocity method to calculate the maximum emission cross-section of 3.0310220 cm2 at 1.838 mm for the polarization parallel to the Nm principal optical direction. These results agrees well with the experimental data. Experimental decay times of the 3H4!3F4 and 3F4!3H6 transitions have been measured as a function of thulium concentration.

Ep��puhtauksien vaikutus monokaliumfosfaattikiteen kasvuun

Ty��ss�� on tutkittu ep��puhtauksien vaikutusta kastelulannoitesuolojen monokaliumfosfaatin, kaliumnitraatin ja ureafosfaatin kiteytyksess��. Kirjallisuusosassa on k��sitelty kastelulannoiteprosessit ja ep��puhtauksien vaikutus kastelulannoitteiden valmistuksessa. Kiteytys ja kiteenkasvu on esitetty perusyht��l��in, joissa on otettu ep��puhtauksien vaikutus huomioon. Tarkemmin on perehdytty monokaliumfosfaatin kiteytykseen ja kolmenarvoisten kationeiden, Al3+, Fe3+ ja Cr3+, vaikutukseen kiteiden kasvuun. Kolmenarvoiset metalli-ionit adsorboituvat kiteen pintaan haitaten kiteenkasvua, mik�� vaikuttaa erityisesti kiteen prismapinnan kasvuun. Lis��ksi on esitelty muita kiteenkasvuun vaikuttavia olosuhteita. Lopuksi on k��sitelty kompleksinmuodostajia metalli-ionien haitallisten vaikutusten ehk��isij��in��. Kokeellisessa osassa suoritettiin liukoisuuskokeita monokaliumfosfaatin liukoisuuden selvitt��miseksi eri pH-olosuhteissa. Suoritetuissa yksikidekokeissa tutkittiin pH:n ja kolmenarvoisten kationeiden; Al3+, Fe3+ ja Cr3+, vaikutus monokaliumfosfaattikiteen pituus- ja leveyskasvuun ja kidemuotoon eri ylikyll��isyyksill��. Lis��ksi tutkittiin voidaanko l��mp��tilaa ja pH muuttamalla tai pyrofosfaattia lis����m��ll�� poistaa raudan kasvua inhiboima vaikutus. Kiteytysl��mp��tilaa nostamalla voidaan poistaa raudan haitallinen vaikutus kiteen kasvuun.

L��sers d"upconversion: llum l��ser visible a partir de llum infraroja.

Els l��sers aporten solucions a una gran varietat de problemes de la vida di��ria ...