Vijay Kumar

Bio: Vijay Kumar is an academic researcher from National Institute of Technology, Srinagar. The author has contributed to research in topics: Self-healing hydrogels & Irradiation. The author has an hindex of 30, co-authored 113 publications receiving 2870 citations. Previous affiliations of Vijay Kumar include Chandigarh University & Sant Longowal Institute of Engineering and Technology.

Upconversion based temperature sensing ability of Er3+–Yb3+codoped SrWO4: An optical heating phosphor

Abstract: The present paper covers a detail explanation about the laser induced optical heating and temperature sensing ability of intense green upconversion emitting Er 3+ –Yb 3+ codoped SrWO 4 phosphor. The structural studies based on X-ray diffraction and Fourier transform infrared analysis confirmed the phase formation, crystallinity and the presence of vibrational bands of the corresponding host lattice. The optical investigation was performed by recording the diffuse reflectance and upconversion emission spectra upon a 980 nm excitation. Multiphoton upconversion processes were confirmed by pump power dependence study for the various emission bands throughout the visible region. Optical thermometry has been performed using the fluorescence intensity ratio technique in which two thermally coupled levels of the green emission bands were used. A relatively low temperature (518 K) sensor with very high sensor sensitivity (14.98 × 10 −3 K −1 ) is determined from the observed results. The optical heating ability of the synthesized phosphor is also determined by using the results of the optical thermometry and found the heat generation up to ∼417 K. The results obtained may possibly be used in cancer therapy.

The energy transfer phenomena and colour tunability in Y2O2S:Eu3+/Dy3+ micro-fibers for white emission in solid state lighting applications

Abstract: This paper reports on the structural, optical and photometric characterization of an Eu3+/Dy3+ doped yttrium oxysulfide phosphor (Y2O2S:Eu3+/Dy3+) for near white emission in solid state lighting. A series of Y2O2S phosphors doped with Eu3+/Dy3+ were prepared by the hydrothermal method. The microstructures of the as-synthesized phosphors were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The XRD results reveal that the obtained powder phosphors have a single-phase hexagonal structure and also indicate that the incorporation of the dopants/co-dopants did not affect the crystal structure. The SEM images reveal the morphology of the prepared phosphors as an intense interpenetrating network of interconnected micro-fibers with a diameter of about 0.15 μm. The band gap of the phosphors was calculated from diffuse reflectance spectra using the Kubelka–Munk function. The Eu3+, Dy3+ doped and Eu3+/Dy3+ co-doped phosphors illuminated with ultraviolet light showed characteristic red luminescence corresponding to the 5D0→7FJ transitions of Eu3+ and characteristic blue and yellow luminescence corresponding to the 4F9/2→6H15/2 or 4F9/2→6H13/2 transitions of Dy3+. The luminescence spectra, the energy transfer efficiency and the decay curves of the phosphors indicated that there exists a strong energy transfer from Dy3+ to Eu3+ and this was demonstrated to be a resonant type via a dipole–quadrupole reaction. Furthermore, the critical distance of the Eu3+ and Dy3+ ions have also been calculated. By utilizing the principle of energy transfer it was also demonstrated that with an appropriate tuning of the activator content the Y2O2S:Eu3+/Dy3+ phosphors can exhibit a great potential to act as single-emitting component phosphors for white light emission in solid state lighting technology.

Magical Allotropes of Carbon: Prospects and Applications

Abstract: The invention of carbon and its allotropes have transformed the electronic and optoelectronic industry due to their encouraging properties in a large spectrum of applications. The interesting characteristic of carbon is its ability to form many allotropes due to its valency. In recent decades, various allotropes and forms of carbon have been invented, including fullerenes, carbon nanotubes (CNTs), and graphene (GR). Since the inception of nanotechnology, carbon allotropes-based nanocomposites have become a leading sector of research and advancement due to their unique bonding properties. Fullerenes and CNTs-based polymer nanocomposites have attracted significant research interest due to their vast applications in every sphere of science and technology. Current research impetus reveals that carbon and its allotropes have revolutionized the industry and academia due to their fascinated properties. Recent advances in various aspects of graphene, CNTs, graphene nanoribbons, fullerenes, carbon encapsul...

Enhanced upconversion and temperature sensing study of Er3+–Yb3+ codoped tungsten–tellurite glass

Abstract: This article reports on the visible upconversion and temperature sensing behaviour of Er3+–Yb3+ doped/codoped TeO2–WO3 glasses prepared by a melting and quenching method. The upconversion emissions have been observed throughout the visible and infrared region upon 980 nm and 808 nm excitations and assigned suitably from different transitions of the Er3+ ion. The emission intensities of the most intense green band have been enhanced on the co-doping with theYb3+ ions with about 5.92 and 3.99 times upon 980 and 808 nm excitations, respectively. The reason behind this improvement is explained on the basis of a power dependence study and an energy level structure. The colour tunability behaviour of the codoped glass has also been investigated upon both excitations. The temperature sensing performance of the prepared glass has been studied by using the fluorescence intensity ratio technique up to 745 K. The results suggested that the present glass is a suitable candidate for making a high temperature sensor up to 690 K with a sensitivity of about 28.72 × 10−4 K−1.

Harrison's Principles of Internal Medicine

Abstract: The 11th edition of Harrison's Principles of Internal Medicine welcomes Anthony Fauci to its editorial staff, in addition to more than 85 new contributors. While the organization of the book is similar to previous editions, major emphasis has been placed on disorders that affect multiple organ systems. Important advances in genetics, immunology, and oncology are emphasized. Many chapters of the book have been rewritten and describe major advances in internal medicine. Subjects that received only a paragraph or two of attention in previous editions are now covered in entire chapters. Among the chapters that have been extensively revised are the chapters on infections in the compromised host, on skin rashes in infections, on many of the viral infections, including cytomegalovirus and Epstein-Barr virus, on sexually transmitted diseases, on diabetes mellitus, on disorders of bone and mineral metabolism, and on lymphadenopathy and splenomegaly. The major revisions in these chapters and many

Intrinsic ripples in graphene

Abstract: The stability of two-dimensional (2D) layers and membranes is subject of a long standing theoretical debate. According to the so called Mermin-Wagner theorem, long wavelength fluctuations destroy the long-range order for 2D crystals. Similarly, 2D membranes embedded in a 3D space have a tendency to be crumpled. These dangerous fluctuations can, however, be suppressed by anharmonic coupling between bending and stretching modes making that a two-dimensional membrane can exist but should present strong height fluctuations. The discovery of graphene, the first truly 2D crystal and the recent experimental observation of ripples in freely hanging graphene makes these issues especially important. Beside the academic interest, understanding the mechanisms of stability of graphene is crucial for understanding electronic transport in this material that is attracting so much interest for its unusual Dirac spectrum and electronic properties. Here we address the nature of these height fluctuations by means of straightforward atomistic Monte Carlo simulations based on a very accurate many-body interatomic potential for carbon. We find that ripples spontaneously appear due to thermal fluctuations with a size distribution peaked around 70 \AA which is compatible with experimental findings (50-100 \AA) but not with the current understanding of stability of flexible membranes. This unexpected result seems to be due to the multiplicity of chemical bonding in carbon.

Solution Combustion Synthesis of Nanoscale Materials

Abstract: Solution combustion is an exciting phenomenon, which involves propagation of self-sustained exothermic reactions along an aqueous or sol–gel media. This process allows for the synthesis of a variety of nanoscale materials, including oxides, metals, alloys, and sulfides. This Review focuses on the analysis of new approaches and results in the field of solution combustion synthesis (SCS) obtained during recent years. Thermodynamics and kinetics of reactive solutions used in different chemical routes are considered, and the role of process parameters is discussed, emphasizing the chemical mechanisms that are responsible for rapid self-sustained combustion reactions. The basic principles for controlling the composition, structure, and nanostructure of SCS products, and routes to regulate the size and morphology of the nanoscale materials are also reviewed. Recently developed systems that lead to the formation of novel materials and unique structures (e.g., thin films and two-dimensional crystals) with unusual...

Optical temperature sensing of rare-earth ion doped phosphors

Abstract: Accurate and reliable temperature measurement of many special inaccessible objects is a challenging task. Optical temperature sensing is a promising method to achieve it. The current status of optical thermometry of rare-earth ion doped phosphors is reviewed in detail. Based on the mechanisms of optical temperature sensing of different phosphors, temperature dependent luminescence spectra, the fluorescence intensity ratio technique in the data fitting process, and errors of the energy difference between thermally coupled levels, we describe the recent developments in the use of optical thermometry materials. The most important results obtained in each case are summarized, and the main challenges that we need to overcome are discussed. Research in the field of phosphor sensors has shown that they have significant advantages compared to conventional sensors in terms of their properties like greater sensitivity, freedom from electromagnetic interference, long path monitoring, and independence of compatibility with electronic devices.