Sonal Sahai

Valence band and core-level analysis of highly luminescent ZnO nanocrystals for designing ultrafast optical sensors

The detailed surface analysis such as survey scan, core-level, and valence band spectra of highly luminescent ZnO:Na nanocrystals were studied using the x-ray photoelectron spectroscopy to establish the performed presence of Na+ ions. The observed increase in band gap from 3.30 (bulk) to 4.16 eV (nano), is attributed to the quantum confinement of the motion of electron and holes in all three directions. The photoluminescence and decay measurements have complemented and supported our study to design an efficient and ultrafast responsive optical sensing device.

Investigation of confinement effects in ZnO quantum dots

We report a simple method for the synthesis of Na(+) doped and stable zinc oxide quantum dots, using the quantum confinement atom method. An intense broad green photoluminescence (PL) was observed with a maximum located at approximately 535 nm when excited by UV radiation of 332 nm. The PL peak intensity is found to be highly dependent on the size of the quantum dots (QDs). Electron microscopy observation revealed that the radius of the QD was approximately 1 nm, which clearly indicated that the QDs are in the strong quantum confinement region (exciton Bohr radius, r(B), for bulk ZnO is 1.8 nm). Phase purity of ZnO and the presence of Na(+) was confirmed by x-ray diffraction (XRD) and atomic absorption spectroscopy (AAS), respectively. The results are well incremented by x-ray photoelectron spectroscopy (XPS) studies. Intentional ageing of QDs for several days under controlled experimental conditions such as temperature, relative humidity and pH etc, facilitated the formation of various nanostructures with a slight red shift in the PL peak position. Time resolved emission spectroscopy measurements indicated that PL decay time changes from 35 ns for QDs to 1660 micros for nanocrystals. The observed high-intensity and stable green PL emissions have been analyzed and thoroughly discussed.

Facile synthesis and step by step enhancement of blue photoluminescence from Ag-doped ZnS quantum dots

Our results pertaining to the step by step enhancement of photoluminescence (PL) intensity from ZnS:Ag,Al quantum dots (QDs) are presented. Initially, these QDs were synthesized using a simple co-precipitation technique involving a surfactant, polyvinylpyrrolidone (PVP), in de-ionised water. It was observed that the blue PL originated from ZnS:Ag,Al QDs was considerably weak and not suitable for any practical display application. Upon UV (365 nm) photolysis, the PL intensity augmented to ~170% and attained a saturation value after ~100 min of exposure. This is attributed to the photo-corrosion mechanism exerted by high-flux UV light on ZnS:Ag,Al QDs. Auxiliary enhancement of PL intensity to 250% has been evidenced by subjecting the QDs to high temperatures (200 °C) and pressures (~120 bars) in a sulphur-rich atmosphere, which is due to the improvement in crystallanity of ZnS QDs. The origin of the bright-blue PL has been discussed. The results were supported by X-ray phase analysis, high-resolution electron microscopy and compositional evaluation.

Tuning of emission colors in zinc oxide quantum dots

High brightness zinc oxide quantum dots were made with intentional alkali metal doping using quantum-confined atom model. The tuning of emission spectrum in the range of 480–562nm was achieved by dispersing them in solvents with varying index of refraction. The observed emission bands are quite distinct from the nonstructured green emission of zinc oxide at 2.4eV (515nm) but are attributed to donor-acceptor recombination involving the zinc vacancy and Li+∕Na+, or the modifications assigned to the surface states by the surrounding medium. The photoluminescence shifts are found to be sensitive to refractive index term n2−1∕2n2+1 useful for practical applications.

Fabrication and electro-optic properties of a MWCNT driven novel electroluminescent lamp

We present a novel, cost-effective and facile technique, wherein multi-walled carbon nanotubes (CNTs) were used to transform a photoluminescent material to exhibit stable and efficient electroluminescence (EL) at low voltages. As a case study, a commercially available ZnS:Cu phosphor (P-22G having a quantum yield of 65 ± 5%) was combined with a very low (~0.01 wt%) concentration of CNTs dispersed in ethanol and its alternating current driven electroluminescence (AC-EL) is demonstrated. The role of CNTs has been understood as a local electric field enhancer and facilitator in the hot carrier injection inside the ZnS crystal to produce EL in the hybrid material. The mechanism of EL is discussed using an internal field emission model, intra-CNT impact excitation and the recombination of electrons and holes through the impurity states.

Effective Doping of Rare-earth Ions in Silica Gel: A Novel Approach to Design Active Electronic Devices

Eu3+ luminescence spectroscopy has been used to investigate the effective doping of alkoxide-based silica (SiO2) gels using a novel pressure-assisted sol-gel method. Our results pertaining to intense photoluminescence (PL) from gel nanospheres can be directly attributed to the high specific surface area and remarkable decrease in unsaturated dangling bonds of the gel nanospheres under pressure. An increased dehydroxylation in an autoclave resulted in enhanced red (∼611 nm) PL emission from europium and is almost ten times brighter than the SiO2 gel made at atmospheric pressure and ∼50°C using conventional Stöber-Fink-Bohn process. The presented results are entirely different from those reported earlier for SiO2:Eu3+ gel nanospheres and the origin of the enhanced PL have been discussed thoroughly.

Highly efficient, tunable and bright photoluminescence from hydrophobic silica gel nanoparticles

For the first time, we report a highly efficient, tunable and bright photoluminescence with external quantum yield up to ∼34% under 350 nm excitation from hydrophobic SiO2 gel nanoparticles. This is achieved by suitably modifying the relative surface states of the gel network with trivalent (Al3+/Ga3+) ions and emission from rare-earth (Eu3+) dopant. This attractive property arises from the intrinsic separation of absorption and the emission energies due to a large Stokes shift and offers opportunities for the design of novel nanophosphor based white light-emitting devices.