Enhanced near band edge luminescence of Ti/ZnO nanorod heterostructures due to the surface diffusion of Ti

Information on the mechanistic differences in the luminescence properties of Ti/ZnO nanorods (NRs) has been obtained through the preparation of heterostructures by (a) varying the thickness of Ti from 1 nm to 20 nm keeping the substrate temperature at 400 °C, (b) varying the substrate temperature from room temperature (RT) to 500 °C while keeping the metal thickness constant at 10 nm and (c) annealing the RT Ti sputtered NRs at temperatures of 400 °C and 500 °C. The photoluminescence (PL) spectra show that the near band edge luminescence of ZnO in the ultraviolet (UV) region is enhanced as the thickness of Ti increases up to 5 nm and, thereafter, it falls. Sputtering of Ti on ZnO NRs at RT does not cause any UV enhancement but when sputtered at and above 400 °C, the UV intensity is enhanced. Annealing of RT Ti sputtered NRs at and above 400 °C also results in the enhancement of the UV peak, although with a lesser magnitude. Analysis of the PL results, supported by X-ray diffraction, field emission scanning electron microscopy, elemental mapping, high resolution transmission electron microscopy, Fourier transform infrared spectroscopy and electrical I-V measurement results, show a clear indication that the surface diffusion of Ti causes a reduction in the surface defects.

Is dye mixture more suitable rather than single dye to fabricate dye sensitized solar cell?

The steady state and time resolved spectroscopic studies reveal that two xanthene dyes Rhodamine 6G (R6G) and Rhodamine B (RB), used in the present investigations, form ground state hydrogen -bonded complexes with meso-tetrakis(4-carboxyphenyl) porphyrin (TCPP). However, it is apparent that upon photoexcitation the H-bonding complexes formed in the ground state decompose into the individual reacting components. This presumption was confirmed from the observation of the presence of only static quenching mode in the steady state fluorescence of the dyes in presence of porphyrin. The photoelectrochemical properties of the free dyes and the mixtures of each dye with porphyrin are investigated by measuring incident photon-to-current conversion efficiency (IPCE) using ZnO electrode and also with TiO2 electrode. It is seen that Rhodamine B-porphyrin mixture has attained maximum IPCE among the four samples studied at approximately 550 nm using ZnO electrode. Using TiO2 electrode, slight improvement in the value of IPCE was found for the same mixture. Therefore Rhodamine B-porphyrin mixture may act as a good sensitizer for converting solar energy to electrical energy.

Pd-nanoparticle-decorated ZnO nanowires: ultraviolet photosensitivity and photoluminescence properties

ZnO nanowires (NWs) have been decorated with Pd nanoparticles of sizes less than 10 nm (Pd-ZnO NWs) via a chemical solution route. The microstructural characterizations have been done using field emission scanning electron and high-resolution transmission electron microscopes. The effects of attaching Pd nanoparticles to the walls of ZnO NWs have been investigated by studying the ultraviolet (UV) photosensitivity and photoluminescence (PL) properties. The surface-modified NWs show a UV photosensitivity more than double and a response seven times faster compared to the bare NWs. The photocarrier relaxation under the steady UV illumination condition is quite different in Pd-ZnO NWs. The higher and faster photosensitivity has been explained on the basis of photocarrier transfer from the conduction band of ZnO to the Fermi level of Pd and subsequent electron trapping by the adsorbed O(2) molecules on the NWs' surface, which have been presented through a proposed model. The PL spectrum of Pd-ZnO NWs shows that the intensities of the band-edge and defect-related emissions decrease and increase, respectively, due to Pd anchoring, the effect being pronounced as the density of Pd nanoparticles increases.

Core-shell TiO2@ZnO nanorods for efficient ultraviolet photodetection

Core-shell TiO(2)@ZnO nanorods (NRs) have been fabricated by a simple two step method: growth of ZnO NRs' array by an aqueous chemical technique and then coating of the NRs with a solution of titanium isopropoxide [Ti(OC(3)H(7))(4)] followed by a heating step to form the shell. The core-shell nanocomposites are composed of single-crystalline ZnO NRs, coated with a thin TiO(2) shell layer obtained by varying the number of coatings (one, three and five times). The ultraviolet (UV) emission intensity of the nanocomposite is largely quenched due to an efficient electron-hole separation reducing the band-to-band recombinations. The UV photoconductivity of the core-shell structure with three times TiO(2) coating has been largely enhanced due to photoelectron transfer between the core and the shell. The UV photosensitivity of the nanocomposite becomes four times larger while the photocurrent decay during steady UV illumination has been decreased almost by 7 times compared to the as-grown ZnO NRs indicating high efficiency of these core-shell structures as UV sensors.

Enhanced photoluminescence and photoconductivity of ZnO nanowires with sputtered Zn

We have sputtered Zn onto quasi-one-dimensional ZnO nanowires (NWs) in order to investigate the effect of Zn diffusion on the photoluminescence and photoconduction properties of ZnO NWs. Elemental mapping clearly indicates higher Zn concentration in the NWs due to diffusion of Zn. The Zn-sputtered NWs show an enhanced ultraviolet emission with 7 nm red shift. Since the ionization energy of Zni is 51 meV, the enhanced PL emission with a red shift is correlated to the coupling between free exciton and zinc interstitials (Zni) defects. The photocurrent transients show almost 20 times more photocurrent generation in Zn/ZnO NWs compared to the as-grown NWs. In contrast, the thin film shows no significant change in the photoluminescence and photoconductivity. Based on the photoconductivity and photoluminescence results, we predict that Zn diffusion in the NWs occurs easily compared to the films because of the smaller dimensions of the NWs.

Highly efficient ultraviolet photodetection in nanocolumnar RF sputtered ZnO films: a comparison between sputtered, sol-gel and aqueous chemically grown nanostructures

Highly efficient ultraviolet (UV) photodetection has been realized in nanocolumnar radio frequency (RF) sputtered ZnO thin films as compared to sol-gel nanocrystalline films and aqueous chemically grown nanowire (NW) arrays. The photo-to-dark current ratio in the columnar films reaches a value as high as 10(6) which is at least 2-4 orders of magnitude higher than that of the sol-gel films and NWs. Such a high value is attributed to a dense compact columnar morphology of the sputtered films with higher surface-related trap states. Within 2 s of UV illumination, five orders of change in the photocurrent have occurred in the sputtered film in contrast to three orders of change in the NWs. The photocurrent decay under steady UV illumination in the sputtered films is only 23%, compared to 50% in the NWs. However, the fall in current under the UV 'off' condition in the sputtered film takes a longer time than in the NWs. These changes are correlated to the deep defect levels of the nanocolumnar films.

Photoluminescence and photoconductivity of ZnS-coated ZnO nanowires

ZnO nanowires (NWs) with a ZnS coating are synthesized in order to modify the surface without changing the diameter of the NWs. They have the wurtzite ZnO at the core and a cubic ZnS at the outer layer. The NWs show a sharp ultraviolet and a broad visible emission of the photoluminescence spectra. Surface modification has led to a change in the position of the maxima of the visible emission in ZnO-ZnS NWs. The photocarrier relaxation under steady UV illumination occurs in ZnO NW arrays but is absent in ZnO-ZnS NW arrays. The dark current value for both type of NWs are similar, whereas the photocurrent value is much higher in the surface-modified NWs. Higher photocurrent value indicates a transport of the photogenerated carriers from the ZnS layer to ZnO during UV illumination. The carrier transport mechanism is proposed through a model.

A novel and simple method to grow beaded nanochains of ZnO with superior photocatalytic activity

We have demonstrated a novel and simple low-cost method to grow beaded nanochains of ZnO using an aqueous chemical growth method. Whatman filter paper (40) is used as the template. The filter paper is generally made up of cellulose fibers along which the growth of beaded ZnO nanoparticles (NPs) is initiated. When the filter paper is burnt at 700 degrees C temperature, the NPs appear as a beaded nanochain morphology while those synthesized without the filter paper form lumped nanostructures without any regular shape and size. A model has been proposed to explain the growth mechanism. A sharp and strong green emission has been observed for the template-grown sample in contrast to a broad and less intense hump of the without template-grown sample. The beaded nanochains shows 64% photocatalytic degradation of methyl orange (MO) under UV irradiation, which is much superior to a value of only 22% shown by the lumped sample. Not only can this low-cost simple template-based synthesis be applied to grow other nanostructures of similar morphology but is also promising for enhancing the properties in the multifunctional materials.

Encapsulation of 2-3-nm-sized ZnO quantum dots in a SiO2 matrix and observation of negative photoconductivity

Quantum dots (QDs) of ZnO of 2-4 nm size have been encapsulated within a SiO(2) matrix using aqueous chemically grown ZnO nanoparticles in a precursor of tetraethyl orthosilicate. The microstructure shows almost a uniform embedment of the QDs in the SiO(2) matrix, resulting in a ZnO QDs-SiO(2) composite structure. The photocurrent transients of the composite show an instant fall in the current followed by an exponential decay under ultraviolet (UV) illumination, causing negative photoconductivity (NPC), in contrast to the positive photoconductivity in only ZnO nanoparticles. The interface defect states due to the presence of the SiO(2) network around ZnO act as charge trap centers for the photoexcited electrons and are responsible for the NPC. The presence of interface-trapped charges under UV illumination has been further confirmed from capacitance-voltage measurements.

Effect of surface capping with poly(vinyl alcohol) on the photocarrier relaxation of ZnO nanowires

The effect of surface capping with poly(vinyl alcohol) (PVA) on the photocarrier relaxation of the aqueous chemically grown ZnO nanowires (NWs) has been investigated. The decay in the photocurrent during steady ultraviolet illumination due to the photocarrier relaxation has been reduced in the capped NWs, as evidenced from a decrease in the photocurrent only by 12% of its maximum value under steady illumination for 15 min and a decrease in the photocurrent by 49% of its maximum value during the same interval of time in the as-grown NWs. The surface modification is confirmed from the FESEM, HRTEM, and FTIR results. The photoluminescence spectrum shows an enhanced ultraviolet emission and a reduced defect-related emission in the capped ZnO NWs compared to bare ZnO.

The fabrication of a ZnO nanowire/La0.65Sr0.35MnO3 heterojunction and characterization of its rectifying behavior

We have fabricated a ZnO nanowire (NW)/La(0.65)Sr(0.35)MnO(3) (LSMO) p-n heterojunction by growing the NWs by an easy aqueous chemical route on a pressed powdered pellet of LSMO. The NWs have hexagonal wurtzite structure with good optical emission properties. The current-voltage (I-V) curves of a single NW junction measured by a scanning tunneling microscope (STM) probe show excellent rectifying behavior with rectification ratio approximately 40, which is comparable to the characteristics of the junction made by large area NW array junctions. Different voltage-dependent current transport mechanisms have been found, which are explained through the use of a band diagram.

A novel and simple approach to enhance ultraviolet photosensitivity: activated-carbon-assisted growth of ZnO nanoparticles

An activated-carbon (AC) assisted route is developed to synthesize a ZnO nanoparticle network. The route involves simple addition of AC to the solution containing the zinc salt and finally removing them by burning at higher temperature to form a sponge-like porous ZnO nanoparticles. The surface area measurements show that AC-assisted ZnO nanoparticles (AC-ZnO) have a higher surface area than those synthesized without AC (B-ZnO), which is further confirmed by the field emission scanning electron microscope (FESEM) and high resolution transmission electron microscope (HRTEM) images. Ultraviolet (UV) absorbance results show that the optical quality remains almost unchanged for both types of nanoparticles. Enhanced and faster UV photosensitivity has been observed for the AC-ZnO. The change in the UV photosensing properties demonstrated here provides a new approach to synthesizing other high surface area materials for novel physical and chemical properties.

Posterior urethral valves: the scenario in a developing center

We have reviewed 233 patients with posterior urethral valves treated in a single center in Calcutta, India, over the last 20 years: 37 were neonates, 75 were between 1 and 12 months, 88 were between 1 and 5 years, and 33 were more than 5 years old when first seen. The clinical presentation and methods employed in diagnosis and assessment are described. Primary endoscopic valve ablation was performed in 140 patients (60%). One or other form of diversion was done in 100 (43%), 93 before and 7 either during or after valve ablation. The short- and long-term results have been studied. Eleven patients died during the initial hospitalization, 3 died subsequently, 15 are in end-stage renal disease, 17 are in poor health, and 18 have been totally lost to follow-up. The remaining 169 have been in good health for periods between 1 and 20 years. While our results of primary valve ablation in low-risk patients with responsible parents are as good as anywhere else in the world, we are concerned at our relatively high diversion rate and relatively poor long-term follow up; the methods being adopted to reduce these problems are discussed.

Penile fracture: differential diagnosis, management and outcome

OBJECTIVE

To determine the value of a diagnostic and therapeutic approach for patients presenting with acute penis and the effects of this management on the outcome.

PATIENTS AND METHODS

The study comprises 21 consecutive patients (mean age 33.3 years, range 19-53) who presented with acute penis (acute pain and swelling during and soon after intercourse). The mean interval between the accident and the treatment was 6 h (range 2-20). All patients initially underwent cavernosography, followed by surgical exploration.

RESULTS

Cavernosography showed extravasation of the contrast medium, indicating a corporal tear, in 19 of 21 patients. Surgical exploration revealed rupture of the deep dorsal vein in two patients whose radiological examinations were normal. In the remaining 19 patients, corporeal tears were repaired. The mean follow-up was 26.7 months; all patients regained penile function (potency). Penile curvature was the only adverse effect in three of the 21 patients, all of whom presented for medical treatment relatively late after the initial accident.

CONCLUSION

In the management of the acute penis, cavernosography should be performed first and the treatment policy should be determined from the radiological findings. Conservative therapy should be chosen only when the corporeal bodies are intact.

Renal hemodynamics in patients with obstructive uropathy evaluated by color Doppler sonography

OBJECTIVES

Prior animal studies have shown through direct pressure measurements that there is a definite rise in the renal vascular resistance in obstruction. Therefore intrarenal hemodynamic changes can be determined by the vascular impedance expressed as pulsatility and resistive indexes (PI and RI) obtained from the Doppler waveforms of intrarenal arteries. We investigated whether various degrees of obstruction result in different hemodynamic responses according to color Doppler sonography.

METHODS

22 kidneys with varied degrees of hydronephrosis and 19 normal kidneys were examined. The pulsatility and resistive index of Doppler waveforms from interlobar arteries were obtained. The grade of hydronephrosis was based upon the width of parenchyma.

RESULTS

The mean resistive index in the study and controls were 0.70 +/- 0.07 and 0.60 +/- 0.03, respectively. The difference was significant (p < 0.001). Pulsatility indexes in the study and controls were 1.07 +/- 0.34 and 0.98 +/- 0.23, respectively. The difference was not significant (p > 0.005). Significant difference was also noticed in the parenchymal widths between both groups (p < 0.001).

CONCLUSION

It appeared that intrarenal color Doppler sonography can provide physiologic information reflecting renal vascular resistance status by means of an easily obtained parameter: resistive index.