Munisamy Subramanian

Transparent and flexible field electron emitters based on the conical nanocarbon structures

The fabrication of conical nanocarbon structures (CNCSs) on a transparent and flexible nafion substrate at room temperature using an ion irradiation technique and their application toward field emission displays (FEDs) have been demonstrated. The main advantage of this technique is that CNCSs can be fabricated directly on the transparent substrate while retaining the transparency of the substrate. A scanning electron microscopy (SEM) image revealed that the sputtered surface was entirely covered with CNCSs with a calculated numerical density of 6 x 10(6) /mm(2). Such nafion based CNCSs have proved to be an effective electron emitter with turn-on and threshold fields of 6.1 and 9.5 V/mum, respectively. The field enhancement factor was estimated to be 1020 from the Fowler-Nordheim (F-N) plot. Thus the room temperature fabricated CNCSs based on transparent and flexible nafion substrate would be very promising for future flexible (roll-up) and transparent FEDs.

Intrinsic ferromagnetism and magnetic anisotropy in Gd-doped ZnO thin films synthesized by pulsed spray pyrolysis method

This paper reports an in-depth study on nanocrystalline Gd-doped ZnO thin films synthesized using versatile pulsed spray pyrolysis method and exhibit room temperature ferromagnetism. The detailed structural and microstructural studies confirm that the doped Gd ions occupy Zn sites and the peak shift can be elucidated by charge neutrality. Optical investigation shows that the Gd doping in ZnO lattice leads to a decrease in the near band edge position due to the introduction of new unoccupied states by Gd 4f electrons. The electronic structure of the Zn1−xGdxO at the O K edge shows the evolution of pre-edge spectral features similar to cuprates and manganites, and also confirms the strong hybridization of O 2p–Gd 4f/5d states. Furthermore, the Gd M5 edge provides evidence that Gd ions are in the trivalent state. Hysteresis measurements demonstrate that the Gd-doped ZnO films are magnetically anisotropic and exhibit intrinsic ferromagnetic behavior at room temperature. Higher magnetization in 3 kOe values is...

Investigations on the structural, optical and electronic properties of Nd doped ZnO thin films

We report the synthesis and characterization of Nd doped ZnO thin films grown on Si (1 0 0) substrates by the spray pyrolysis method. The surface morphology of these thin films was investigated by scanning electron microscopy and shows the presence of randomly distributed structures of nanorods. Grazing angle x-ray diffraction studies confirm that the doped Nd ions occupied Zn sites and these samples exhibited a wurtzite hexagonal-like crystal structure similar to that of the parent compound, ZnO. The micro-photoluminescence measurement shows a decrease in the near band edge position with Nd doping in the ZnO matrix due to the impurity levels. The near-edge x-ray absorption fine structure (NEXAFS) measurements at the O K edge clearly exhibit a pre-edge spectral feature which evolves with Nd doping, suggesting incorporation of more charge carriers in the ZnO system and the presence of strong hybridization between O 2p‐Nd 5d orbitals. The Nd M5 edge NEXAFS spectra reveal that the Nd ions are in the trivalent state. (Some figures in this article are in colour only in the electronic version)

Ferromagnetism in Cu-doped AlN films

AlN films doped with 3.2–8.2 at. % Cu were deposited by helicon magnetron sputtering. The films exhibited ferromagnetism with a Curie temperature above 360 K. The observed magnetic anisotropy and exclusion of ferromagnetic contamination indicated that the ferromagnetism was the intrinsic property of Cu-doped AlN films. Room-temperature (RT) saturation magnetization (Ms) tended to decrease with increasing Cu concentration and the maximum RT Ms obtained was about 8.5 emu/cm3 (0.6 μB/Cu). Vacuum annealing increased the RT Ms values and meanwhile reduced ratio of N/(Al+Cu), which suggested that Al interstitial and N vacancy might contribute to the observed ferromagnetic behavior.

Highly transparent and conducting C:ZnO thin film for field emission displays

Incorporation of carbon into a zinc oxide (ZnO) thin film has led to a potential new application as a transparent and conductive oxide thin film. Here, we fabricated carbon doped zinc oxide (C:ZnO) films by RF sputtering at a low temperature, aiming to produce a transparent and conductive screen for transparent field emission displays. The incorporation of highly inert carbon atoms into ZnO thin films can tune the intrinsic defect sites. A co-sputtering technique was used for carbon doping into ZnO, where the dopant amount was controlled by varying the number, size and position of the graphite plates used as the carbon source. A sheet resistance of 37 Ω □−1 and a transmittance of 84% at a wavelength of 550 nm were achieved for the C:ZnO thin film with a nominal carbon concentration of 2.7%. The bonding of carbon with ZnO was analyzed using de-convolution of XPS peak C1s. We also demonstrated the light emission properties of the C:ZnO thin films by fabricating a field emission device. A high current density of 1 mA cm−2 was obtained with a lower applied electric field of 5 V μm−1. Our findings showed that the C:ZnO thin films can be a promising transparent and conducting phosphor screen for a field emission display.

Magnetic and optical property studies on controlled low-temperature fabricated one-dimensional Cr doped ZnO nanorods

Fabrications of one-dimensional (1D) nanostructures and magnetic/semiconducting heterostructures have recently been the limelight of study to establish functional nanodevices with their size and dimensionality influencing the material properties of a system. In this regard, Cr substituted ZnO nanorods were prepared under low-temperature conditions and their optical property studies have been investigated. X-Ray diffraction and magnetic measurements revealed the specimens to be composed of wurtzite phase and ferromagnetic character, respectively. Morphological features of the ZnO: Cr magnetic semiconductors have been studied to be of rod forms from the field emission scanning electron microscopy and transmission electron microscopy results. UV-vis diffused mode absorbance measurements provides enough evidence to tune the band gap of ZnO based nanomaterials through Cr incorporation, with a typical blue shift in the spectrum due to the increase in carrier concentration. Modifications in the intensity of E2 (high) mode and sub-band edge emissions provides enough evidence for the existence of intrinsic defects associated with the O atoms, that has been reasoned for the observed ferromagnetism.

Bamboo-shaped aligned CNx nanotubes synthesized using single feedstock at different temperatures and study of their field electron emission

The present investigation deals with the synthesis of N-doped aligned bamboo-shaped carbon nanotubes (CNx) by chemical vapour deposition of monoethanolamine/ferrocene mixture on quartz substrate over the temperature range 700–900 °C. All nanotubes exhibited bamboo-like morphology with transverse carbon bridges forming compartments in this temperature range. The concentration of nitrogen content has been controlled in the range 4.8–5.8 at% by controlling the deposition temperature. Scanning electron microscopy, transmission electron microscopy (TEM)/high-resolution transmission electron microscopy (HRTEM), x-ray photoelectron spectroscopy (XPS), Raman spectroscopy and thermogravimetric analysis were used to characterize the composition and structural analysis of the as-grown nanotubes. HRTEM revealed that nanotubes prepared at high temperature with low nitrogen content have high graphitization degree. XPS analysis indicated that N 1s signal was split into three peaks. The decomposition of monoethanolamine at reaction temperature produces ammonia which may provide an efficient route of higher nitrogen content on carbon nanotubes. Field emission measurements indicated that N-doped carbon nanotubes prepared at 800 °C showed nice field emission performance with turn-on and threshold field of 1.6 and 2.3 V µm−1, respectively.

Band Gap Variation of Mn Doped ZnO Films Prepared by Spray Pyrolysis Technique

Undoped and manganese doped zinc oxide thin films were prepared on Si(100) substrate using spray pyrolysis technique. Nanorods having random distribution were observed on the ZnO films by scanning electron microscopy. Grazing angle X-ray diffraction and Raman spectroscopy confirmed successful growth of the wurtzite structure Zn1-xMnxO films without any secondary phase. From Raman spectra, the lattice defect was found to increase with increasing Mn concentration. The isothermal magnetization hysteresis measurements at room temperature show that saturation of magnetization decreased with increasing Mn concentration. Optical gap energy determined by the optical reflectance measurement showed a maximum at Mn concentration 10 mol %. It was also found that the strain parameter increases up to 10 mol % and then decreases at 15 mol %. These tendencies seem to be related to the coexistence of Mn3+ and Mn4+ (Mn3+/Mn4+) ions along with the Mn2+ ions in the Zn1-xMnxO films.

Luminescence and high temperature ferromagnetism in YAlO nanophosphors: materials for efficient next generation LEDs and spintronic applications

Current research on environment friendly and efficient luminescent materials for white LEDs, envisaged as next generation energy efficient lighting devices, has spurred research into the different forms of yttrium-aluminum-oxide (YAlO) (YAG: garnet structured YAlO and YAM: monoclinic structured YAlO), including the discovery of a new phase of tetragonal structured YAlO named YAT. Appropriate luminescence in the visible-yellow region has been realized by doping rare earth (Ce) in the YAlO host. Interestingly, this work indicates that all the studied YAlO nanophosphors (NPs) exhibit dilute ferromagnetism up to 600 K. The observed magnetic character of YAlO NPs is attributed to the oxygen vacancies and remains unperturbed even after Ce doping, thereby establishing the independent nature of the two phenomena. This non-interfering luminescence and magnetic character of the YAlO NPs is anticipated to have a wide application potential in high temperature spintronics, LEDs and other biological applications.

Raman Spectra and Magnetic Property Analysis of Nd-Doped ZnO Thin Films

The present investigation is a detailed study aiming to understand the origin of the ferromagnetism of the well-characterized Nd-doped ZnO films. Raman scattering shows that the E2high phonon line of Zn1-xNdxO (x = 0, 0.05, 0.10, and 0.15) is broadened asymmetrically and shifts to a lower frequency, suggesting microscopic structural disorder induced by Nd3+ substitutions. Zn1-xNdxO films exhibit room-temperature ferromagnetism with a Curie temperature higher than 350 K. In addition, the films revealed magnetic anisotropy with higher magnetization in 3 kOe when the field was applied in the perpendicular direction to the sample surface than when applied in an in-plane direction.