Rajkumar Patra

Vertically aligned cerium hexaboride nanorods with enhanced field emission properties

Cerium hexaboride is a well-known material for the filaments of electron microscopes due to its field emission properties (high brightness electron source) and its long service life. The synthesis of cerium hexaboride (and most other borides) normally requires very high temperatures (1500 °C–1700 °C) and high pressures. Thus, the low temperature synthesis of cerium hexaboride at ambient pressure is a challenge. The present study highlights the synthesis of vertically aligned cerium hexaboride nanorods which offer better field emission properties with the highest field enhancement factor reported so far. The optimization of the process for obtaining vertically aligned cerium hexaboride nanorods involves three different stages. First, the low temperature synthesis of polycrystalline cerium hexaboride; second, the fabrication of cerium hexaboride films having vertically oriented nanorods (by spin coating and slow evaporation) and third, the enhancement of the field emission properties. The synthesis of cerium hexaboride nanorods has been carried out by a low temperature borothermal reduction process using a cerium precursor (synthesized via a reverse micellar route and a hydrothermal route) and boron as the starting materials. The borothermal reduction of the cerium precursors has been carried out at low temperature (∼1300 °C) and ambient pressure in an inert atmosphere. The field emission studies of the vertically aligned nanorods of diameter 30 nm and 200 nm show a field enhancement factor of 3863 and 3658, respectively, which is nearly seven-fold higher than the maximum field enhancement factor known so far.

Enhanced field emission from cerium hexaboride coated multiwalled carbon nanotube composite films: A potential material for next generation electron sources

Intensified field emission (FE) current from temporally stable cerium hexaboride (CeB6) coated carbon nanotubes (CNTs) on Si substrate is reported aiming to propose the new composite material as a potential candidate for future generation electron sources. The film was synthesized by a combination of chemical and physical deposition processes. A remarkable increase in maximum current density, field enhancement factor, and a reduction in turn-on field and threshold field with comparable temporal current stability are observed in CeB6-coated CNT film when compared to pristine CeB6 film. The elemental composition and surface morphology of the films, as examined by scanning electron microscopy, transmission electron microscopy, and energy dispersive X-ray measurements, show decoration of CeB6 nanoparticles on top and walls of CNTs. Chemical functionalization of CNTs by the incorporation of CeB6 nanoparticles is evident by a remarkable increase in intensity of the 2D band in Raman spectrum of coated films as c...

Enhanced field emission from lanthanum hexaboride coated multiwalled carbon nanotubes: Correlation with physical properties

Detailed results from field emission studies of lanthanum hexaboride (LaB6) coated multiwalled carbon nanotube (MWCNT) films, pristine LaB6 films, and pristine MWCNT films are reported. The films have been synthesized by a combination of chemical and physical deposition processes. An impressive increase in field enhancement factor and temporal stability as well as a reduction in turn-on field and threshold field are observed in LaB6-coated MWCNTs compared to pristine MWCNT and pristine LaB6 films. Surface morphology of the films has been examined by scanning electron microscopy. Introduction of LaB6 nanoparticles on the outer walls of CNTs LaB6-coated MWCNTs films is confirmed by transmission electron microscopy. The presence of LaB6 was confirmed by X-ray photoelectron spectroscopy results and further validated by the Raman spectra. Raman spectroscopy also shows 67% increase in defect concentration in MWCNTs upon coating with LaB6 and an upshift in the 2D band that could be attributed to p-type doping. U...

Enhancement in field emission current density of Ni nanoparticles embedded in thin silica matrix by swift heavy ion irradiation

The field emission (FE) properties of nickel nanoparticles embedded in thin silica matrix irradiated with 100 MeV Au+7 ions at various fluences are studied here. A large increase in FE current density is observed in the irradiated films as compared to their as deposited counterpart. The dependence of FE properties on irradiation fluence is correlated with surface roughness, density of states of valence band and size distribution of nanoparticles as examined with atomic force microscope, X-ray photoelectron spectroscopy, and grazing incidence small angle x-ray scattering. A current density as high as 0.48 mA/cm2 at an applied field 15 V/μm has been found for the first time for planar field emitters in the film irradiated with fluence of 5.0 × 1013 ions/cm2. This significant enhancement in the current density is attributed to an optimized size distribution along with highest surface roughness of the same. This new member of field emission family meets most of the requirements of cold cathodes for vacuum mic...

Manipulating the anisotropy and field emission of lanthanum hexaboride nanorods

The present study describes the controlled synthesis of lanthanum hexaboride nanostructures with efficient field emission properties. The synthesis is mediated by a nanostructured lanthanum hydroxide precursor, which is controlled by varying the capping agent and pH using a hydrothermal route. The effect of charge on the capping agent (surfactant) strongly affects the shape and size of the precursor (neutral surfactants lead to the formation of nanorods while a cationic surfactant results in the formation of particles). This precursor mediated route leads to lanthanum hexaboride nanostructures at much lower temperatures (∼500 °C lower than the conventional solid state route) and allows for variation of morphology of nanostructured films. Vertically aligned nanorods (30 nm × 200–400 nm), nanoparticles (25 nm) and sub-micron particles (0.2–0.25 microns) could be precisely obtained. Field emission studies of these vertically aligned nanorods show a very high field enhancement factor (4191), which is required for an efficient field emitter.

Highly enhanced and temporally stable field emission from MWCNTs grown on aluminum coated silicon substrate

In this work, a detailed field emission study of multi-walled carbon nanotubes (MWCNTs) grown on Si and Al coated Si substrates is reported. Morphological and microstructural studies of the films show higher entanglement of CNTs in the case of CNT/Si film as compared to CNT/Al/Si film. Raman studies show that the defect mediated peak (D) is substantially suppressed as compared to graphitic peak (G) resulting in significant reduction in ID/IG value in CNT/Al/Si film. Field emission (FE) current density of CNT/Al/Si film (∼25 mA/cm2) is significantly higher as compared to that of CNT/Si film (∼1.6 mA/cm2). A substantial improvement in temporal stability is also observed in CNT/Al/Si film. This enhancement in field emission current is attributed to strong adhesion between substrate and CNTs, low work function, high local field enhancement factor at the CNT tips and less entanglement of CNTs grown on Al/Si. The temporally stable CNT/Al/Si cold cathode can be a potential candidate to replace conventional elect...

Thouless length and valley degeneracy factor of ZnMnO thin films with anisotropic, highly conductive surface layers

Isothermal magnetoresistance (MR) of n-type conducting Zn1–xMnxO thin films on a sapphire substrate with a Mn content of 5 at. % has been studied in in-plane and out-of-plane magnetic fields up to 6 T in the temperature range of 5 K to 300 K. During pulsed laser deposition of the ZnMnO thin films, we controlled the thickness and roughness of a highly conductive ZnMnO surface layer. The measured MR has been modeled with constant s-d exchange (0.2 eV in ZnMnO) and electron spin (S = 5/2 for Mn2+) for samples with a single two dimensional (2D) ZnMnO layer, a single three dimensional (3D) ZnMnO layer, or a 2D and 3D (2D + 3D) ZnMnO layer in parallel. The temperature dependence of modeled Thouless length LTh (LTh ∼ T−0.5) is in good agreement with the theory [Andrearczyk et al., Phys. Rev. B 72, 121309(R) (2005)]. The superimposed positive and negative MR model for ZnCoO thin films [Xu et al., Phys. Rev. B 76, 134417 (2007)] has been extended in order to account for the increase in the density of states close ...