Structure and optical properties of Cd substituted ZnO (Zn1-xCdxO) nanostructures synthesized by high pressure solution route

We report synthesis of Cd substituted ZnO nanostructures (Zn1-xCdxO with x upto \approx .09) by high pressure solution growth method. The synthesized nanostructures comprise of nanocrystals that are both particles (~ 10-15 nm) and rods which grow along (002) direction as established by Transmission electron microscope (TEM) and X-ray diffraction (XRD) analysis. Rietveld analysis of the XRD data shows monotonous increase of the unit cell volume with the increase of Cd concentration. The optical absorption as well as the photoluminescence (PL) shows red shift on Cd substitution. The line width of the PL spectrum is related to the strain inhomogenity and it peaks in the region where the CdO phase separates from the Zn1-xCdxO nanostructures. The time resolved photoemission showed a long lived (~10ns) component. We propose that the PL behavior of the Zn1-xCdxO is dominated by strain in the sample with the redshift of the PL linked to the expansion of the unit cell volume on Cd substitution

Electric field induced reversible control of visible photoluminescence from ZnO nanoparticles

Reversible control of the photoluminescence of ZnO occurring in the visible range, has been achieved by application of a few volts (< 5V) to a device consisting of nanostructured ZnO film sandwiched between Indium Tin Oxide electrode and polyethylene oxide-lithium perchlorate, a solid polymer electrolyte. The photoluminescence intensity shows nearly 100% modulation with a response time less than 30 seconds, when the bias is applied at the electrolyte-electrode. A model is proposed for the observed effect that is based on defect states of ZnO and the band bending at the ZnO-electrolyte interface that can be changed by the applied bias

Structural and Optical properties of Zn(1-x)MgxO nanocrystals obtained by low temperature method

In this paper we report structural and optical properties of Magnesium substituted Zinc Oxide (Zn1-xMgxO) nanocrystals (~10-12nm) synthesized by low temperature route. In the low temperature synthesis route it was possible to reach x = 0.17 without segregation of Mg rich phase. The exact chemical composition has been established by quantitative analysis. Rietveld analysis of the XRD data confirms the Wurzite structure and a continuous compaction of the lattice (in particular the c-axis parameter) as x increases. There is an enhancement of the strain in the lattice as the Mg is substituted. The bandgap also gets enhanced as x is increased and reaches a value of 4eV for x = 0.17. From the TEM and the XRD data it has been concluded that when there is a phase segregation for x > 0.17, there is a shell of Mg(OH)2 on the ZnO. The absorption also shows persistence of the excitoinc absorption on Mg substitution. The nanocrystals show near band edge photo luminescence (PL) at room temperature which shows blue shift on Mg incorporation. In addition to the near band edge emission the ZnO and Zn1-xMg xO alloy nanocrystals show considerable emission in the blue-green region at wavelength of ~550 nm. We find that the relative intensity of the green emission increases with the Mg concentration for very low x (upto x = 0.05) and on further increase of the Mg concentration there is a sharp decrease of relative intensity of the green emission eventually leading to a complete quenching of blue emission. It is concluded that due to phase segregation (for x \geq 0.20), the formation of the shell of Mg(OH)2 on the ZnO leads to quenching of the green emission .However, this shell formation does not have much effect on the near band edge PL

Ionic environment control of visible photo-luminescence from ZnO nanoparticles

We report a novel effect that the visible photoluminescence (in the blue-green band) from ZnO anoparticles can be controlled by changing the ionic or polar nature of the medium in which the nanoparticles are dispersed. We find that the presence of sufficient amount of electrolytes can even quench the emission. We propose an explanation based on surface charge of the ZnO nanoparticles which control the band bending in the depletion layer at the surface of the anoparticles. The band bending in turn, decides the predominant nature of the visible emission. The explanation is validated by establishing a direct correlation between the visible emission and the zeta potential.Comment: 3 pages including 4 figure

Growth of compact arrays of optical quality single crystalline ZnO nanorods by low temperature method

We report the synthesis and optical properties of compact and aligned ZnO nanorod arrays (dia, ∼50-200 nm) grown on a glass substrate with varying seed particle density. The suspension of ZnO nanoparticles (size, ∼15 nm) of various concentrations are used as seed layer for the growth of nanorod arrays via selfassembly of ZnO from solution. We studied the effect of various growth parameters (such as seeding density, microstructure of the seed layer) as well as the growth time on the growth and alignment of the nanorods. We find that the growth, areal density and alignment of the nanorods depend on the density of seed particles which can be controlled. It is observed that there is a critical density of the seed particles at which nanorod arrays show maximum preferred orientation along [002] direction. The minimum and maximum radius of the aligned nanorods synthesized by this method lie in the range 50-220 nm which depend on the seeding density and time of growth. These nanorods have a bandgap of 3.3 eV as in the case of bulk crystals and show emission in the UV region of the spectrum (∼400 nm) due to excitonic recombination and defect related emission in the visible region

Majorana Dark Matter and Neutrino mass in a singlet-doublet extension of the Standard Model

A minimal extension of the Standard Model (SM) by a vector-like fermion doublet and three right handed (RH) singlet neutrinos is proposed in order to explain dark matter and tiny neutrino mass simultaneously. The DM arises as a mixture of the neutral component of the fermion doublet and one of the RH neutrinos, both assumed to be odd under an imposed $\mathcal{Z}_2$ symmetry. Being Majorana in nature, the DM escapes from $Z$-mediated direct search constraints to mark a significant difference from singlet-doublet Dirac DM. The other two $\mathcal{Z}_2$ even heavy RH neutrinos give rise masses and mixing of light neutrinos via Type-I Seesaw mechanism. Relic density and direct search allowed parameter space for the model is investigated through detailed numerical scan.Comment: DAE-BRNS-HEP symposium 2020 Procceding

Description of accretion induced outflows from ultra-luminous sources to under-luminous AGNs

We study the energetics of the accretion-induced outflow and then plausible jet around black holes/compact objects using a newly developed disc-outflow coupled model. Inter-connecting dynamics of outflow and accretion essentially upholds the conservation laws. The energetics depend strongly on the viscosity parameter \alpha and the cooling factor f which exhibit several interesting features. The bolometric luminosities of ultra-luminous X-ray binaries (e.g. SS433) and family of highly luminous AGNs and quasars can be reproduced by the model under the super-Eddington accretion flows. Under appropriate conditions, low-luminous AGNs (e.g. Sagittarius A^*) also fit reasonably well with the luminosity corresponding to a sub-Eddington accretion flow with f\to 1.Comment: 16 pages including 3 figures; to appear in New Astronom

Mechanistic Understanding of Stability and Photocatalytic Efficiency of Titanium Dioxide Nanomaterials in Aquatic Media: A Sol-Gel Approach

Titanium dioxide (TiO2) nanoparticles enhance the intrinsic value of commercial products like various cosmetics, paints, self-cleaning products, etc. Several research on the fabrication of TiO2, stabilization of TiO2 to retain its nanometric scale and increasing the inherent property of the material (i.e., photocatalytic) is ongoing for the last few decades. Still, the synthesis of highly efficient, stable, reproducible and cost-effective TiO2 nanoparticles remains a grand challenge for the researchers and scientific community. Further research is needed to develop an in-depth understanding of synthesis, aggregation kinetics and efficiency to improve the performance of TiO2 nanomaterial for the degradation of persistent organic pollutants (POPs). In this book chapter, we have summarized the synthesis process using the sol-gel pathway followed by its stability behavior and photocatalytic activity in the aqueous solutions. This study also highlighted the effects of various process variables such as pH, catalyst concentration, inorganic species etc. in the photocatalytic performance of TiO2 nanoparticles. Finally, we have reviewed various strategies that have been performed for increasing the photocatalytic efficiency of TiO2 by overcoming its limitations

Historical Geography of Forestry and Forest Culture in Sub-Himalayan West Bengal, 1757-2015

Re-examining a range of archival data and information regarding colonial forestry in the state of West Bengal, India, it has been found that the colonial period was the clear-cut turning point of the forest landscape in terms of environmental changes of the state. In West Bengal, the British colonial rulers were in a highly dominating position; and they had implemented one single model of forest management throughout the Indian subcontinent including the Bengal Presidency. The primary objective of the present study is to understand the changing nature of colonial forest landscape through plantation (Sal, Tea, and Cinchona) activities and the establishment of forest villages through Taungya process in the sub-Himalayan West Bengal from 1757 to 1947. In particular, this study aims to examine as to how the colonial plantation activities changed the forest landscape of this particular region. It also examines civil society movement based on forest resource rights and problems in the implementation of the Forest Right Act (2006) in the sub-Himalayan West Bengal till 2015, since independence. The colonial forest management authority was more interested in the plantation and cultivation of trees with high timber values compared to the natural forest. For this purpose, the saplings of Teak, Mahogany, Sisso were supplied to different parts of Bengal province to increase the timber productions. In 1886, for the first time in the Indian forest history, an area of about 15,5,399.29 sq.km was demarcated as Reserved Forest which included the whole western Dooars region located on the right bank of the river Teesta in the northern part of West Bengal. It was the starting point of 'scientific' forest management in the then Bengal Presidency or present West Bengal. The Taungya system of scientific forest management was first initiated in the Bengal Presidency in colonial India after colonial Burma (Myanmar). The system has changed the traditional cultivation practice within the forest land. Due to the Taungya system, 168 forest villages were established in the Himalayan foothills of Bengal. And since India’s independence in 1947, West Bengal has witnessed a number of civil society movements linked to the welfare of poor forest villagers demanding the forest resources rights, for example, the Jangal Mahal movement. The Forest Right Act (2007) has also created conflicts in different parts of the study area. In a nutshell, rapid exploitation of forest resources along with trading monopoly of forest management by the state Forest Department has done historical injustice to the people of sub-Himalayan West Bengal