We analyze theoretically the optical properties of ideal semiconductor crystallites so small that they show quantum confinement in all three dimensions [quantum dots (QD's)]. In the limit of a QD much smaller than the bulk exciton size, the linear spectrum will be a series of lines, and we consider the phonon broadening of these lines. The lowest interband transition will saturate like a two-level system, without exchange and Coulomb screening. Depending on the broadening, the absorption and the changes in absorption and refractive index resulting from saturation can become very large, and the local-field effects can become so strong as to give optical bistability without external feedback. The small QD limit is more readily achieved with narrow-band-gap semiconductors.

Theory of the linear and nonlinear optical properties of semiconductor microcrystallites

The magnetic properties of materials only receive a satisfactory microscopic explanation by reference to the quantum structure of the constituent atoms and molecules. Quantum states of electrons are always dynamic in nature and they can involve surprisingly large internal electric currents. Bohr’s theory of the hydrogen atom, for example, describes an electron in its lowest energy state as orbiting the atomic nucleus with a period, T, of about 10−16 s. Transport of electronic charge, of magnitude e, around a circuit in this time corresponds to an average electric current of I = e/T ~ 1 mA. Taking into account the very small radius of this orbit (r ~ 1/2 A = 0.5 × 10−10 m) gives a much more modest figure for the magnetic dipole moment, m, of the current loop, namely m = I πr2 ~ 10−23 A m2.

Magnetic properties of materials

Structural characterization and photoluminescence properties of zinc oxide nano particles synthesized by chemical route method

Zinc oxide thin films were deposited on glass slide from aqueous solution of ZnCl2 and NaOH by chemical bath deposition method. The films of various thicknesses have been obtained by varying the concentration of TEA (1 M−0.01 M). Optical properties, surface morphology and particle size of the deposited thin film have been studied by U.V spectrophotometer (Varian) SEM and XRD. The optical band gap of the ZnO thin film was found in the range of 2.59–3.57 ev. Optical constant is such as refractive index, extinction coefficient, real and imaginary parts of dielectric constant were evaluated from reflectance, transmittance and absorbance curve. The film show high transmittance in the visible/near infrared region. Reflectance (10%–20%), refractive index (2–2.6), extinction coefficient (0.04–0.075), real (4–7) and imaginary parts (0.2–.0.3) of dielectric constant are obtained in visible/near infrared region. SEM studies shown plate and powder like morphology of sample. Particles size is obtained in nano range.

Synthesis and optical properties of chemical bath deposited ZnO thin film

Photoluminescence behavior of ZrO2: Eu3+ with variable concentration of Eu3+ doped phosphor

Multiferroic bulk YMnO3 sample was prepared through the solid state reaction method. After characterizing the sample structurally, a systematic investigation of magnetization and specific heat has been undertaken over a temperature range 2–300 K under different magnetic fields. Based on these studies, it has been found that the sample exhibited a paramagnetic to ferrimagnetic phase transition of spin glass type at ~42 K that could be attributed to spin cantering. The magnetic transition peak seen in the magnetic entropy change versus temperature curves became broader with increasing magnetic field. A large magnetic entropy change of ~1 J mol−1 K−1 was obtained under a magnetic field change of 0–10 T.

Specific heat and magnetocaloric effect studies in multiferroic YMnO3

The present paper reports thermoluminescence (TL) glow curves of Eu3+-doped Y2O3 phosphor with different ultraviolet (UV) exposure times. The glow peak shows second-order kinetics of Eu3+-doped Y2O3, and corresponding kinetic parameters were evaluated using the peak shape method. Calculations of trap depth were carried out using different methods. The kinetics order, activation energy, and frequency factor were calculated. The recorded glow curve shifts towards higher intensity with longer UV exposure. The heating rate used for recording TL was 3.0 °C s−1. Particle size and structure were verified by X-ray diffraction (XRD) pattern and morphology by scanning electron microscopy (SEM) imaging.

Thermoluminescence studies of UV-irradiated Y2O3:Eu3+ doped phosphor

A Rectangular
Microstrip Patch Antenna model is proposed using air as a substrate to study
the characteristics of designed antenna. The dimensions of designed antenna are 17 mm × 16.66 mm with
substrate at frequency 3.525 GHz. In this paper,
the simulation is performed by using software Computer Simulation Technology
(CST) Microwave studio based on finite difference time domain technique. The
characterization of the designed antenna was analyzed in terms of return loss, bandwidth, directivity, gain, radiation
pattern, VSWR.

/pdf/rectangular-microstrip-patch-antenna-using-air-as-substrate-2xi6i791nu.pdf

Rectangular Microstrip Patch Antenna Using Air as Substrate for S-Band Communication

The structural, magnetic, thermodynamic, and transport properties of polycrystalline sample of A-site disordered Nd0.3Sm0.2Sr0.5MnO3 were investigated in an attempt to understand the effect of substitution of Sm over well-studied parent compound Nd0.5Sr0.5MnO3. Samples were prepared by the solid-state ceramic route method and characterized by XRD and standard iodometric titration. DC magnetization of the sample was measured by a SQUID Magnetometer. Electrical resistivity of the sample was measured by the standard four probe method. Results showed that doping with Sm allowed first-order phase transition from ferromagnetic metal to antiferromagnetic insulator state and a charge ordering at 150 K was also observed. Magnetocaloric effect studies were carried out from the specific heat data.

Structural, magnetic, thermodynamic, and transport properties of A-site disordered Nd 0.3 Sm 0.2 Sr 0.5 MnO 3

In this paper we have studied the variation of normalized per-atom pair cohesive (binding) energy and melting temperature with size and number of atom-pairs in the BN nano-crystal by simple model approach. In small nano-particles, large proportions of their atoms reside either at or near the surface, and those in clusters are basically all on the surface. So, we have to study the surface configuration in detail that leads the constancy of the crystals. Even for bulk materials, parameters that ensure this constancy involving normalized cohesive energy, surface reconstruction, iconicity, bulk structure, hybridization and charge balance, melting point. It is worthwhile to say that many of these factors are quite interdependent. These all factors influence the whole cohesive energy of crystals and give important information about the deflections from inverse size dependence. The per-atom-pair binding energy and melting temperature of BN nano-crystal is a quadratic function of the inverse of the crystal size. The binding energy and melting temperature comes near their bulk value with increasing the crystal size and same as the bulk material when the crystal size is above than 100 nm.

https://www.worldscientific.com/doi/pdf/10.1142/S2010194513010611

N. Kumar Swamy

Papers

Specific heat and magnetocaloric effect studies in multiferroic YMnO3

Thermoluminescence studies of UV-irradiated Y2O3:Eu3+ doped phosphor

Rectangular Microstrip Patch Antenna Using Air as Substrate for S-Band Communication

Structural, magnetic, thermodynamic, and transport properties of A-site disordered Nd 0.3 Sm 0.2 Sr 0.5 MnO 3

Dependency of thermo-physical properties on surface bonding of bn nano-crystal